Class 9 Physical Science

Chapter 7 Sources Of Sound And Propagation Of Sound Synopsis

1. Sound is a type of longitudinal elastic wave which is produced due to the vibration of a body. Sound can propagate through any material medium i.e., solid, liquid or gaseous medium.
2. The sound produced due to the vibration of the two elastic vocal chords present in the larynx of human beings is known as voice. The air coming from the lungs passes through the larynx and generates vibration in the vocal chords. If there is a change in the tension of the vocal chords or a change of air flow during respiration, then different types of sound are produced.
3. Sound waves are elastic waves, hence need a material medium for their propagation.
4. If a body traverses the same path repeatedly at fixed intervals of time or its motion undergoes repetition, then the motion of the body is known as periodic motion.
5. The motion of a particle is called vibration or oscillation when it follows a periodic motion along the same path and in opposite directions, repeatedly.

Some Quantities Related To Vibration:

Amplitude:

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

The maximum displacement of a particle in a vibratory motion from its equilibrium position.

Time Period:

The time required to complete one oscillation.

Frequency:

The number of complete oscillations performed per unit of time.

Relation Between Time Period And Frequency:

If time period of a vibrating particle is T s. Then,

Frequency of the particle n = \(\frac{1}{t}\)

or, nT = 1

The turbulence created due to the combined vibration of the particles of a material medium, which transmits energy from one place to another without moving the particles, is known as a mechanical wave.

Production And Propagation Of Mechanical Waves Depend On The Following Three Properties Of The Material Medium:

1. Elasticity,
2. Inertia and
3. Cohesion

Waves Are Of Following Two Types:

1. Longitudinal waves and
2. Transverse waves.
3. If a wave propagates in a direction parallel to the motion of the vibrating particles of a material medium, the wave is known as a longitudinal wave.
4. If a wave propagates in a direction perpendicular to the motion of the vibrating particles of a material medium, the wave is known as a transverse wave.
5. When a wave propagates through a material medium, then the maximum displacement of any vibrating particle of the medium from its position of equilibrium is called its amplitude (a).
6. During propagation of a wave, the distance traversed by the wave during the time when a particle of the medium undergoes a complete vibration is called its
   wavelength (λ).
7. If the velocity, wavelength, and frequency of a wave are V, λ, and n respectively, then the relationship between them is V = nλ.
8. Among solid, liquid, and gaseous mediums, only the solid medium can oppose or resist any change of shape. Hence, transverse waves can propagate only through a solid medium.
9. Among solid, liquid and gaseous mediums, all the three can oppose or resist any change of volume. Hence, longitudinal waves can propagate through all of them.
10. Light wave is an electromagnetic wave. So, a light wave can also propagate through vacuum.
11. Alternate compressions and rarefactions are generated during the propagation of a longitudinal wave.
12. Alternate crests and troughs are generated during the propagation of a transverse wave.
13. In case of longitudinal waves, the summation of one compression and one rarefaction gives us the definition of wavelength. In case of transverse waves, wavelength is defined as the distance between two consecutive crests or two consecutive troughs.
14. In a solid medium, the velocity of a longitudinal wave is greater than the velocity of a transverse wave.

Chapter 7 Sources Of Sound And Propagation Of Sound Short And Long Answer Type Questions

Question 1. What is sound?

Answer:

Sound:

Sound is a type of longitudinal elastic wave which is produced due to the vibration of a body. It can propagate through any material medium, i.e., solid, liquid, or gaseous medium. If the frequency of sound is within the range of 20 Hz, it produces a sensation of nearing in human ears and is known as audible sound.

Question 2. Sound does not propagate in the absence of a material medium. Why?

Answer:

Sound Does Not Propagate In The Absence Of A Material Medium:-

Sound wave is an elastic wave. A material medium is necessary for the propagation of elastic waves. That is why sound does not propagate in the absence of a material medium.

Question 3. Show by experiments that sound produced due to vibration of a body.

Answer:

Sound Produced Due To Vibration Of A Body Is Shown Below:-

Experiment: A bell is hung by a rope. Now, the bell is struck by a hammer.

Observation: A loud sound is heard after the bell is struck by the hammer and the bell starts vibrating. Now if the’ bell is touched slowly, vibration of the bell can be felt and the sound of bell gradually stops.

Conclusion: Sound is emanated from the bell only because it vibrates.

Question 4. Explain the working principle of production of sound in the vocal chords of a human being with examples.

Answer:

Working Principle Of Production Of Sound In The Vocal Chords Of A Human Being With Examples:-

Narrow strips of two thin papers of the same type are taken. By keeping one of the papers on the other, the papers are stretched by both hands and air is blown heavily in the gap between the two papers. It is found that sound is produced.

Sound is produced in our vocal chords in a similar way. The vibration of the elastic vocal chords produces sound which we hear as our voice. Air from the lungs passes through the larynx and produces vibrations in the vocal chords. During each vibration of the vocal chord, free air enters the pharynx and initiates a sound.

As this sound progresses more through the oral cavity, it becomes louder. In this way, we can make different types of sound through our mouth by producing vibration of air.

Question 5. How different types of sound are produced in our vocal chords? Which muscle controls the elasticity of the two vocal chords?

Answer:

1. If there is a change in the tension of the vocal chords or a change of airflow during respiration, then different types of sound are produced.
2. Elasticity of the two vocal chords is controlled by thyroarytenoid muscles.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 6. Demonstrate with the help of an experiment that propagation of sound requires a material medium.

Answer:

Propagation of sound requires a material medium

A bell jar is kept on the upper platform of an air- extraction pump. A rubber stopper is fixed in the opening of the bell jar. Two wires are inserted through the stopper and connected to a calling bell in such a way that the bell remains inside the jar.

A battery and a switch is connected with the wires which come out of the rubber stopper. Vaseline is applied at the junction of the bell-jar and the platform to make the jar air tight. The calling bell starts ringing when the switch is turned on.

Now, air is slowly pumped out of the jar through the air-extraction pump and it is observed that the sound of the bell is reduced slowly and finally no sound can be heard anymore.

Again, if air is pumped into the jar slowly and the switch is turned on, sound of the calling bell can be heard again. We may conclude from the experiment that sound requires a material medium for its propagation. Sound cannot propagate through vacuum.

Question 7. Why is a walkie-talkie necessary for two people to talk standing side by side on the surface of the moon?

Answer:

There is no atmosphere on the surface of the moon. Entire space is a vacuum. Sound cannot propagate through vacuum. So, two people cannot hear each other even if they stand side by side. This is the reason why a walkie-talkie is necessary for two people to talk on the surface of the moon.

Question 8. Why does not the sound of an’explosion in the sun reach the earth?

Answer:

There is no material medium in the space between the sun and the earth. It is simply vacuum. As sound requires a material medium for its propagation, sound of an explosion in the sun does not reach the earth.

Question 9. If sound is made at one end of a long iron pipe, two sounds can be heard at the other end. Why?

Answer:

If sound is made at one end of a long iron pipe, two sounds can be heard at the other end.

If sound is made at one end of a long iron pipe, sound reaches the other side through both the mediums, iron and air. Velocity of sound in iron is far greater than the velocity of sound in air.

So, sound takes less time to travel through iron than it takes through air. As a result, the first sound one hears comes through iron and the second one that is heard comes through air.

Question 10. Explain with the help of simple experiments that sound may propagate through solid and liquid mediums.

Answer:

Sound may propagate through solid and liquid mediums

Suppose two boys are sitting on two sides of a table [Fig. 4]. If one of the boys put his ear on the table and the other boy knocks gently on the table at the other end, the first boy hears the sound very loudly. Table, made up of wood is a solid material. Hence, sound is propagated through this solid medium.

Sound can also propagate through liquid medium. A bell is immersed in a big bucket filled with water and vibrated. Now, if the ear is touched very carefully to the surface of the water, the ringing of the bell can be heard very clearly. Here, sound is propagated through water. So, sound can propagate through liquid medium.

Question 11. What is periodic motion? Give examples.

Answer:

Periodic Motion:

1. If a body traverses the same path repeatedly at fixed intervals of time or its motion undergoes repetition, then the motion of the body is known as periodic motion.
2. Motion of the earth around the sun, motion of the hands of a clock, and motion of a simple pendulum are examples of periodic motion.

Question 12. What is vibration? Explain with examples.

Answer:

Vibration

1. When a particle makes very fast movements on the two sides of its normal position, then the particle is said to undergo vibration.
2. A thin string or wire fixed at its two ends and stretched firmly may be considered as a summation of many particles on the same line. If the mid-point of the wire is pulled and then released, it starts vibrating on its two sides.
3. At that time, every point or particle of the wire undergoes vibration. Every particle is also said to undergo periodic motion.
4. Thus the motion of a particle is called vibration or oscillation when it follows a periodic motion along the same path and in opposite directions.

Question 13. What do you mean by the amplitude, time period, and frequency of a vibrating particle?

Answer:

Amplitude, Time Period, And Frequency Of A Vibrating Particle:-

Amplitude: The maximum displacement of a vibrating particle from its position of equilibrium is called its amplitude. Suppose, O is the equilibrium position of a vibrating particle.

The article is vibrating within the straight line AB. Then the maximum displacement of the particle in the right-hand side of its equilibrium position is OB and that in the left-hand side is OA. OA is always equal to OB.

Time period: The total time taken by a vibrating particle to execute one complete oscillation is called its time period. Time period T of the particle means that the particle starts from A reaches B and again comes back to A in T seconds.

Frequency: The frequency of a vibrating particle is the number of complete oscillations that the particle undergoes in one second.

Question 14. Establish a relationship between time period and frequency.

Answer:

Relationship Between Time Period And Frequency:-

If T is the time period of a vibrating particle, then one oscillation of the particle takes place in T seconds.

In one second, the particle undergoes \(\frac{1}{T}\) complete oscillation.

∴ frequency, n = \(\frac{1}{T}\)

Question 15. Explain with an example, what do you mean by waves.

Answer:

Waves:-

Wave is a type of turbulence that transmits energy from one place to another place due to collective vibration of the particles of a material • medium without displacement of the particles.

If a stone is thrown in still water of a pond, it is found that a turbulence is set in the water. This turbulence does not remain restricted to a single point. Centred around the point where the stone is struck, this turbulence spreads in a circular form. This is called a wave.

Question 16. Explain the production of a longitudinal wave by using a spring. What is a longitudinal wave?

Answer:

Longitudinal Wave And Production Of A Longitudinal Wave By Using A Spring

1. A long spring is attached to a hook and a small weight is hung at the lower end. The spring is pulled a little in the downward direction and then released. It is seen that the weight attached to the spring oscillates up and down. As a result, a wave proceeds from top to bottom through the spring by means of contraction and expansion.
2. This wave is longitudinal in nature. At any moment, contraction of the spring is called compression of the medium, and expansion of the spring is called rarefaction of the medium.
3. A longitudinal wave is defined as the wave moving parallel to the motion of the vibrating particles of a material medium.

Question 17. What is a transverse wave? Explain the generation of a transverse wave by using a string.

Answer:

Transverse Wave And The Generation Of A Transverse Wave By Using A String:-

1. If a wave moves in a direction perpendicular to the motion of the vibrating particles of a material medium, the wave is called a transverse wave.
2. One side of a thin and long rope is attached to a firm support and the other side is kept horizontal and is jerked several times at right angle to the rope. It is seen that a wave is produced in the rope. Here, each particle vibrates at right angle to the length of the wire. This type of wave is called transverse wave.

1. A transverse wave produced in the wire at any particular moment is shown. At a particular moment, the displacements of points A and C are highest on the upper side, so these are called crests of the wave. At that moment, displacements of point B and D are lowest on the lower side, so these are called troughs of the wave.

Question 18. Define

1. Amplitude and
2. Wavelength.

Answer:

Amplitude And Wavelength:-

1. When a wave is transmitted through a material medium, then the maximum displacement of any vibrating particle from its equilibrium position is called the amplitude (a) of the wave.
2. During the propagation of a wave, the distance traversed by the wave during the time when a particle of the medium undergoes a complete vibration is called its wavelength (λ).

Question 19. What do you mean by the frequency and velocity of a wave?

Answer:

Frequency And Velocity Of A Wave:-

Frequency: The frequency (n) is the number of complete vibrations made by a particle of the medium in one second during the propagation of a wave.

Velocity Of A Wave: Velocity (V) of a wave is the distance traversed by the wave through the medium in unit time.

Question 20. Establish the equation V = nλ, where V is the velocity of the wave, λ is its wavelength and n is its frequency.

Answer:

Suppose, the wavelength of a wave = λ, frequency = n, and velocity of the wave = V.

So, the wave traverses a distance A during the time when a particle of the medium makes one complete vibration. Therefore, the wave traverses a distance nλ during the time when the particle executes n complete vibrations.

The particle takes one second to complete n number of complete vibrations.

∴ The wave progresses a distance of nλ through the medium in one second.

∴ Velocity of the wave, V = nλ.

Question 21. Longitudinal waves can propagate through solid, liquid and gaseous mediums whereas transverse waves can propagate only through solid medium. Explain with reasons.

Answer:

Longitudinal waves can propagate only through that medium that opposes or resists the change of its volume or stops the change itself. As this property is a common feature of solid, liquid and gaseous mediums, longitudinal waves can propagate through all these three.

Again, transverse waves can propagate through that medium which opposes the change of its shape or stops the change itself. Liquid and gaseous mediums have no shape or form, unlike solid mediums. So, transverse waves can propagate only through a solid medium.

Question 22. Is a light wave a longitudinal wave?

Answer:

Longitudinal wave requires a material medium for its propagation. That is, a longitudinal wave propagates only through a material medium. As a light wave does not require any solid medium for its propagation (light wave can pass through vacuum also), it cannot be called a longitudinal wave. It is a type of electromagnetic wave.

Question 23. What are the differences between a longitudinal wave and a transverse wave?

Answer:

The differences between a longitudinal wave and a transverse wave:

Question 24. Which event proves that sound wave is an elastic wave?

Answer:

An elastic wave requires a material medium for its propagation. Similarly, the propagation of a sound Wave also requires a material medium. This proves that sound wave is an elastic wave.

Question 25. Describe in brief, how sound propagates through air medium.

Answer:

Sound Propagates Through Air Medium

Air medium may be considered as a sum of several equidistant parallel layers in the absence of any sound wave in the medium. The source of sound is one arm of a vibrating tuning fork. The arm of the fork (B) vibrates in the direction AC. When it goes from A to C in its vibrating state, it compresses the layers of air. As a result, that layer condenses and its pressure is increased.

This is called compression. Again, the gaseous medium opposes the change of volume. Due to application of pressure on that layer of air, particles of that region are displaced from their equilibrium position (displacement is different for different positions). As a result of displacement, elastic stress is originated in that area which tries to restore the particles to its to its equilibrium position.

Hence, that layer of air applies pressure on the next layer which is compressed and energy is transmitted. Next, when the arm B of the tuning fork goes from C to A, then pressure on the right-hand side of point C is reduced. So, a partial vacuum is created and expansion of the layer of air at that place takes place. This is called rarefaction.

One full vibration takes place when the arm of the tuning fork goes from A to C and again from C to A. Thus, due to vibration (half-vibration) of the arm of the tuning fork from A to C, a compression of air adjoining the arm is produced and due to vibration (half-vibration) of the arm of the tuning fork from C to A, a rarefaction of air adjoining the arm is produced.

An long as the vibration of the tuning fork continues, compressions and rarefactions are created periodically and these move through the medium. In this way, sound propagates through air.

Chapter 7 Sources Of Sound And Propagation Of Sound Very Short Answer Type Questions Choose The Correct Answer

Question 1. When sound is produced by the mouth, vibration is observed in

1. Vocal chords
2. Pharynx
3. Trachea
4. Cochlea

Answer: 1. Vocal chords

Question 2. If sound is made at one end of a long pipe, how many times is it heard at the other end?

1. Once
2. Twice
3. Thrice
4. Four times

Answer: 2. Twice

Question 3. Sound waves can propagate through

1. Gaseous medium only
2. Solid and liquid mediums only
3. Solid, liquid, and gaseous mediums
4. Vacuum

Answer: 3. Solid, liquid, and gaseous mediums

Question 4. Velocity of a sound wave of wavelength 2 m is 350 m/s. What is its frequency?

1. 170 Hz
2. 175 Hz
3. 700 Hz
4. 350 Hz

Answer: 2. 175 Hz

Question 5. The frequency of a tuning fork is 300 Hz. How much distance does a sound wave traverse during 20 vibrations of the fork? Velocity of sound in air is 333 m/s.

1. 11.1m
2. 33.3 m
3. 44.4 m
4. 22.2 m

Answer: 4. 22.2 m

Question 6. Frequencies of two tuning forks are 100 Hz and 150 Hz respectively. The ratio of the wavelengths of the waves produced by them in air is

1. 2:3
2. 3:2
3. 4:9
4. 9:4

Answer: 2. 3:2

Question 7. Propagation of sound waves through a medium like air may be compared to

1. Compression and rarefaction of a spring
2. Vibration of a string
3. Waves generated in water
4. Propagation of light waves

Answer: 1. Compression and rarefaction of a spring

Question 8. If time period is 0.2 s, what is the corresponding frequency?

1. 4 Hz
2. 5 Hz
3. 10 Hz
4. 20 Hz

Answer: 2. 5 Hz

Question 9. Sound propagates fastest through which of the following mediums?

1. Solid medium
2. Liquid medium
3. Gaseous medium
4. Vacuum

Answer: 1. Solid medium

Question 10. Sound does not propagate at all through which of the following mediums?

1. Solid medium
2. Liquid medium
3. Gaseous medium
4. Vacuum

Answer: 4. Vacuum

Question 11. Sound is

1. Longitudinal elastic wave
2. Transverse elastic wave
3. Electromagnetic wave
4. None of the above

Answer: 1. Longitudinal elastic wave

Question 12. Frequency of sound is measured in the unit

1. watt
2. erg
3. Second
4. Hertz

Answer: 4. Hertz

Question 13. A simple pendulum takes 40 seconds to oscillate 20 times. Time period of oscillation of the pendulum is

1. 1s
2. 2s
3. 3s
4. 4s

Answer: 2. 2s

Question 14. A simple pendulum takes 40 seconds to oscillate 20 times. Frequency of oscillation of the pendulum is

1. 0.1 Hz
2. 0.2 Hz
3. 0.3 Hz
4. 0.5 Hz

Answer: 4. 0.5 Hz

Question 15. Production of sound requires

1. Vibration of the source of sound
2. Solid medium
3. The presence of audience
4. The presence of a reflector

Answer: 1. Vibration of the source of sound

Question 16. Propagation of sound requires

1. Vibration of the source of sound
2. Solid medium
3. The presence of audience
4. The presence of a reflector

Answer: 2. Solid medium

Question 17. It has been proved experimentally that the velocity of sound in air at 0°C is approximately

1. 332 m/s
2. 442 m/s
3. 1080 m/s
4. zero

Answer: 1. 332 m/s

Question 18. When velocity of sound in air at 0°C is 332 m/s, then which of the following cannot be the velocity of sound through a solid medium at the same temperature?

1. 300 m/s
2. 400 m/s
3. 500 m/s
4. 1000 m/s

Answer: 1. 300 m/s

Question 19. Air comes out of lungs and enters larynx through

1. Vocal chords
2. Pharynx
3. Trachea
4. Heart

Answer: 3. Heart

Question 20. Sound wave is a longitudinal wave. So vibration of the source of sound is

1. Longitudinal
2. Transverse
3. Longitudinal or transverse
4. None of the above

Answer: 3. Longitudinal or transverse

Question 21. What is the distance between two consecutive compressions in the air medium, if A is the wavelength of sound?

1. λ
2. 2λ
3. λ/2
4. 2λ/3

Answer: 1. λ

Question 22. If the distance between three consecutive compressions of longitudinal waves created by a spring is 30 cm, what is the wavelength of the wave?

1. 90 cm
2. 60 cm
3. 45 cm
4. 15 cm

Answer: 4. 15 cm

Question 23. When the direction of velocity of a wave produced in a medium is parallel to the vibration of the particles of the medium, then the wave is positively a

1. Transverse wave
2. Longitudinal elastic wave
3. Electromagnetic wave
4. Lightwave

Answer: 2. Longitudinal elastic wave

Question 24. Which is the essential property of a solid medium for the origin and propagation of mechanical waves?

1. Elasticity only
2. Inertia only
3. Cohesion only
4. All of the above

Answer: 4. All of the above

Question 25. What is the velocity of sound through copper’ if its density and Young’s modulus are 8.6 g/cm³ and 11.8 x 10¹¹ dyn/cm³ respectively?

1. 3704 m/s
2. 3604 m/s
3. 3504 m/s
4. 3404 m/s

Answer: 1. 3704 m/s

Chapter 7 Sources Of Sound And Propagation Of Sound Answer In Brief

Question 1. What type of wave is a sound wave?

Answer: Sound wave is a longitudinal elastic wave.

Question 2. Can sound wave propagate through vacuum?

Answer: No, sound wave cannot propagate through vacuum.

Question 3. What is a sonorous body?

Answer: A body which acts as a source of sound is called a sonorous body.

Question 4. How does the sound produced from the vocal chords of a human being change?

Answer: If there is change of tension of the two vocal chords or there is a change in air flow while breathing, then different types of sound are produced.

Question 5. How can one talk on the surface of the moon?

Answer: One has to take the help of walkie-talkie to talk on the surface of the moon.

Question 6. Write down the expression of velocity of sound through a solid medium.

Answer: Velocity of sound through a solid medium,

V = \(\sqrt{\frac{Y}{\rho}}\) where Y is Young’s modulus of the solid and ρ is the density of the solid medium.

Question 7. The velocity of sound is highest in which gaseous medium?

Answer: Velocity of sound is highest in hydrogen gas.

Question 8. How is sound produced in guitar, sitar and violin?

Answer: Sound is produced in these instruments due to vibration of strings.

Question 9. How is sound produced in a flute?

Answer: Sound is produced in a flute due to vibration of air particles inside the flute.

Question 10. How is sound produced in human larynx?

Answer: Sound is produced in human larynx due to vibration of the two elastic vocal chords present in the larynx.

Question 11. Which muscle controls the elasticity of two vocal chords of a human being?

Answer: A muscle named thyroarytenoid controls the elasticity of the vocal chords.

Question 12. Among iron, water, and air, velocity of sound is maximum in which medium?

Answer: Velocity of sound is maximum in iron.

Question 13. Among iron, water, and air, velocity of sound is minimum in which medium?

Answer: Velocity of sound is minimum in air.

Question 14. If a sound is made at one end of a long iron pipe, we hear sound twice at the other end (due to the two mediums, iron, and air). Which sound is heard at first?

Answer: The sound which is transmitted through iron is heard at first.

Question 15. Between the motion of the arm of a clock and the motion of a simple pendulum, which one is a periodic motion but not a linear simple harmonic motion?

Answer: The motion of the arm of a clock is a periodic motion but it is not a simple harmonic motion.

Question 16. A particle is executing a simple harmonic motion between two points A and B. If AB = 10 cm, what is the amplitude of the particle?

Answer: Amplitude of the particle = 10/5 = 5 cm

Question 17. If the time period of an oscillating particle is 0.01 s, then what is its frequency?

Answer: Time period, T = 0.01 s

∴ frequency, n = \(\frac{1}{T}\) = \(\frac{1}{0.01}\) = 100 Hz

Question 18. If the frequency of an oscillating particle is 5 Hz, what is its time period?

Answer: Frequency, n = 5 Hz

∴ time period, T = \(\frac{1}{n}\) = \(\frac{1}{5}\) = 0.2 s

Question 19. Which wave progresses parallel to the movement of particles oscillating in a material medium?

Answer: Longitudinal wave progresses in a direction parallel to the movement of particles oscillating in a material medium.

Question 20. Which wave progresses perpendicular to the movement of particles oscillating in a material medium?

Answer: Transverse wave progresses in a direction perpendicular to the movement of particles oscillating in a material medium.

Question 21. Can longitudinal waves propagate through solid, liquid, and gaseous mediums?

Answer: Yes, longitudinal waves can propagate through solid, liquid, and gaseous mediums.

Question 22. Can transverse waves propagate through solid, liquid, and gaseous mediums?

Answer: No, transverse waves can propagate only through solid medium.

Question 23. Which length is considered as a wavelength in case of a longitudinal wave?

Answer: Total length of one compression and one rarefaction is taken as a wavelength in case of a longitudinal wave.

Question 24. Which length is considered as a wavelength in case of a transverse wave?

Answer: The distance between two successive crests or two successive troughs is taken as a wavelength in case of a transverse wave.

Question 25. What is the relationship between propagation of a sound wave and medium?

Answer: Sound wave can propagate only through an elastic medium.

Question 26. How many types of waves are there? Name them.

Answer: There are two types of waves:

1. Longitudinal wave and
2. Transverse wave.

Chapter 7 Sources Of Sound And Propagation Of Sound Fill In the Blanks

Question 1. Sound is a  ______ elastic wave.

Answer: Longitudinal

Question 2. Any particle moving with linear simple harmonic motion periodically traverses the same path in _______

Answer: Opposite direction

Question 3. Transverse waves propagate only through solid medium because a solid medium can resist the change of its _______

Answer: Shape

Question 4. Light wave can propagate through vacuum because it is a type of ______ wave.

Answer: Electromagnetic

Question 5. The reciprocal of time period is ________

Answer: Frequency

Question 6. ______ is the number of complete oscillations made by an oscillating particle in one second.

Answer: Frequency

Question 7. Velocity of sound in air at 0°C is ______

Answer: 332 m/s

Question 8. During the propagation of ______ waves, the layers of the medium come very close to each other and the phenomenon is known as compression.

Answer: Longitudinal

Question 9. During the propagation of transverse waves, ______ and ______ are produced in a cyclic order.

Answer: Crests, troughs

Question 10. ______ waves propagate in a direction perpendicular to the direction of motion of the particles of the medium.

Answer: Transverse

Question 11. ______ waves propagate in a direction parallel to the direction of motion of the particles of the medium.

Answer: Longitudinal

Question 12. Velocity of sound waves is ______ compared to the velocity of light waves.

Answer: Less

Question 13. The unit of frequency is ______

Answer: Hertz(Hz)

Question 14. If the time period of a sound wave is 0.05s, its frequency is ______

Answer: 20Hz

Chapter 7 Sources Of Sound And Propagation Of Sound State Whether True Or False

Question 1. Sound is a type of transverse elastic wave which is produced due to the vibration of a body.

Answer: False

Question 2. Sound waves do not need any material medium for their propagation.

Answer: False

Question 3. Production and propagation of mechanical waves depend on the cohesive property of the material medium.

Answer: True

Question 4. Longitudinal waves propagate in a direction perpendicular to the motion of the vibrating particles of a material medium.

Answer: False

Question 5. Alternate compressions and rarefactions generate during the propagation of a longitudinal wave.

Answer: True

Question 6. Velocity of sound is highest in gaseous medium.

Answer: False

Question 7. Wavelength of sound wave is much larger than that of light wave.

Answer: True

Question 8. Sound is a kind of electromagnetic wave.

Answer: False

Question 9. Velocity of sound wave does not changes with the change of medium.

Answer: False

Question 10. Speed of light and speed of sound are same in a medium.

Answer: False

Question 11. Sound propagates in air medium in the form of longitudinal waves.

Answer: True

Chapter 7 Sources Of Sound And Propagation Of Sound Numerical Examples

Useful Information

1. A particle executes simple harmonic motion between A and B. If AB = d.
2. The amplitude of this oscillation a = \(\frac{d}{2}\)
3. If time period of oscillation of a particle = T and frequency = n, then, n = \(\frac{1}{T}\).
4. If the frequency and wavelength of a wave in a medium be n and λ respectively, then velocity of the wave V = nλ
5. Velocity of sound in a solid V = \(\sqrt{\frac{Y}{\rho}}\), Y = Young’s modulus, and ρ = density of the solid.

Question 1. The velocity of a sound wave of wavelength 1.7 m is 340 m/s. What is its frequency?

Answer:

Given

Velocity V of the sound wave is 340 m/s and wavelength λ = 1.7 m

Suppose, frequency of the sound wave = n

∴ V = nλ or,n = \(\frac{360}{1.7}\) or, n = 200 Hz

Question 2. The frequency of a tuning for k is 280 Hz. During the time the fork completes 70 full vibrations, a sound wave traverses 80 m through a medium. What is the velocity of sound in the medium?

Answer:

Given

The frequency of a tuning for k is 280 Hz. During the time the fork completes 70 full vibrations, a sound wave traverses 80 m through a medium.

Suppose, the wavelength of the sound produced by this tuning fork = λ, then

70λ = 80 or, λ = \(\frac{80}{70}\) or, λ = \(\frac{8}{7}\) m

Again, frequency, n = 280 Hz

So, velocity of sound, V = nλ

∴ V = 280 Hz x \(\frac{8}{7}\) m = 320 m/s

Question 3. The frequencies of two tuning forks are 200 Hz and 300 Hz respectively. What is the ratio of wavelengths of the waves produced by the forks in air?

Answer:

Given

The frequencies of two tuning forks are 200 Hz and 300 Hz respectively.

Frequency of the first tuning fork, n₁ = 200 Hz

In this case, if the wavelength of the sound produced = λ₁.

Then velocity of sound, V = n₁λ₁….(1)

Again, frequency of second tuning fork, n₂ = 300 Hz.

Here, if the wavelength of the sound produced = λ₂, then velocity of sound is given by, V = n₂λ₂…..(2)

Comparing equations (1) and (2), we get

\(n_1 \lambda_1=n_2 \lambda_2\)

 

or, \(\frac{\lambda_1}{\lambda_2}=\frac{n_2}{n_1}\)

or, \(\frac{\lambda_1}{\lambda_2}=\frac{300}{200}=\frac{3}{2}\)

∴ λ₁ :  λ₂= 3:2

Question 4. Young’s modulus of a metal, Y = 12.8 x 10¹¹ dyn/cm^2, and its density is 8 g/cm³. What is the velocity of sound in the metal?

Answer:

Given

Young’s modulus of a metal, Y = 12.8 x 10¹¹ dyn/cm^2, and its density is 8 g/cm³.

Velocity of sound in the metal, = 4 x 10⁵ cm/s

V = \(\sqrt{\frac{Y}{\rho}}=\sqrt{\frac{12.8 \times 10^{11}}{8}}=4 \times 10^5 \mathrm{~cm} / \mathrm{s}\)

= 4000 m/s = 4 km/s

Question 5. A and B are standing 1 km apart from each other. A fires a shot and B observes a flash of fire immediately but hears the sound of the shot after 2.5 s. What is the velocity of sound in air?

Answer:

Given

A and B are standing 1 km apart from each other. A fires a shot and B observes a flash of fire immediately but hears the sound of the shot after 2.5 s.

Distance between A and B, s = 1 km = 1000 m.

B sees the flash as soon as the shot is fired. Since the flash of fire and sound are produced at the same time as the firing of the shot, time taken by sound wave reaches B from A, t= 2.5 s.

∴ velocity of sound in air, v = \(\frac{s}{t}=\frac{1000}{2.5}=400 \mathrm{~m} / \mathrm{s}\)

Question 6. A man adjusts time in his watch by hearing the sound of siren in a factory. It is later found that the watch is running slow by 3 s. If the velocity of sound in air is 340 m/s, what is the distance of the factory from the man?

Answer:

Given

A man adjusts time in his watch by hearing the sound of siren in a factory. It is later found that the watch is running slow by 3 s. If the velocity of sound in air is 340 m/s,

If the watch is adjusted by hearing the sound of the siren and still it slows down by 3 s.

This means that sound takes 3 s to reacfi him from the factory.

∴ distance of the factory from the man = 340 X 3 = 1020 m

Question 7. The wavelength of a sound wave of frequency 2.5 kHz is 40 cm. How much time does this wave take to traverse a distance of 2 km?

Answer:

Given

The wavelength of a sound wave of frequency 2.5 kHz is 40 cm.

Frequency of sound wave, n = 2.5 kHz = 2500 Hz

Wavelength, λ = 40 cm = 0.4 m

Suppose, velocity of sound wave = V

∴ V = nλ = 2500 x 0.4 = 1000 m/s

So, required time = \(\frac{2000}{1000}\) = 2 s

Question 8. A body vibrating with a definite frequency produces waves of lengths 15 cm and 25 cm in mediums X and Y respectively. What is the ratio of the velocities of the waves in the two mediums?

Answer:

Given

A body vibrating with a definite frequency produces waves of lengths 15 cm and 25 cm in mediums X and Y respectively.

The wavelength of the wave in X medium = 15cm

The wavelength of the wave in Y medium = 25cm

As we know, V = nλ

∴ V_X = nλ_X

and V_Y = nλ_Y [frequency is constant]

So the ratio is given by

\(\frac{V_X}{V_Y}=\frac{n \lambda_X}{n \lambda_Y}=\frac{\lambda_X}{\lambda_Y}=\frac{15}{25}=\frac{3}{5}\)

∴ V_X : V_Y= 3:5

Question 9. The frequency of a tuning fork is 512 Hz. How many complete vibrations does the tuning fork make when sound produced by it traverses 550 m? Assume velocity of sound in air = 320 m/s.

Answer:

Given

The frequency of a tuning fork is 512 Hz.

Velocity of sound in air, V = 320 m/s

Frequency, n = 512 Hz

Wavelength of sound, λ = \(\frac{V}{n}\) = \(\frac{320}{512}\) = 0.625 m

∴ The tuning fork executes \(\frac{550}{0.625}\) = 880 complete

Vibrations when sound travels 550 m.

Chapter 7 Human Ear And Sound Pollution Synopsis

There are two major functions of the human ear:

1. Hearing and
2. Maintaining balance of the body.

Human ear can be divided into three parts:

1. Outer ear,
2. Middle ear and
3. Inner ear.

The middle ear consists of three bones:

1. Malleus,
2. Incus and
3. Stapes

1. The narrow passage from the pharynx to the cavity of the middle ear is known as the eustachian tube. It is a canal that connects the middle ear to the pharynx. Its main function is to maintain the equilibrium of the air pressure between the inside and outside of the middle ear.
2. The inner ear mainly consists of the vestibular apparatus and cochlea.
3. The last part of the inner ear that looks like a coiled tube similar to a snail is known as the cochlea. The organ which acts as the receptor of sound in the cochlea is called the organ of Corti.
4. The minimum intensity of, sound that produces a sensation of sound in the human ear is known as the threshold of hearing.
5. The threshold of hearing for a sound wave frequency 1000 Hz is I₀ = 10^-12 W/m².
6. This is known as zero-level intensity.
7. The maximum intensity for a sound of frequency 1000 Hz is 1 W/m², which a person can tolerate safely.
8. The relationship between the loudness L and intensity I is L = k • log I, k is a constant of proportionality.

If at a given frequency, l₁, l₀ be the intensities and L₁, L₀ be the loudness of two sound waves respectively, then,

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

⇒ \(L_1=k \log _{10} l_1 \text { and } L_0=k \log _{10} l_0\)

Hence the difference in the loudness of the two sounds is \(L=L_1-L_0=k \log _{10}\left(\frac{I_1}{I_0}\right)\)

If I₀ is taken as the reference intensities (10^-12  W • m^-2)

i.e., the minimum intensity of audible sound at frequency 1 kHz, then L is called the sound level,

Taking k = 1, the sound level is

L = \(1 \cdot \log _{10}\left(\frac{I_1}{I_0}\right)\)

= \(10 \cdot \log _{10}\left(\frac{l_1}{l_0}\right) \cdot \frac{1}{10}\) bel

L = \(10^{\circ} \cdot \log _{10}\left(\frac{I}{I_0}\right)\) decibel

For \(I=I_0 \quad L=10(1) \log _{10}=0 \mathrm{~dB} .\)

i.e., the sound of intensity 10^-12 W/m² at 1 kHz frequency has zero loudness. For increase in intensity at a point by 10 times the initial value, the increase in sound level will be 10log₁₀10 = 10 dB

The unit of loudness level is decibel. 1dB = 1/10 B

The harmful effect of undesirable sound on the environment is known as sound pollution. Undesirable sound refers to sound with high loudness and pitch and beyond the tolerance level of the ear.

World Health Organisation (WHO) has prescribed a safe intensity level of 45 dB and if the intensity of sound exceeds a level of 65 dB, it is detrimental to the health of human beings. Our hearing capacity may be permanently damaged if the intensity level rises to 150 dB-160 dB.

Chapter 7 Human Ear And Sound Pollution Short And Long Answer Type Questions

Question 1. What are the functions of a human ear?

Answer:

Functions Of A Human Ear:-

There are two major functions of a human ear:

1. Hearing and
2. Maintaining balance of the body.

Question 2. Draw a simple figure depicting different parts of a human ear.

Answer:

Different Parts Of A Human Ear Are Shown In Below:- 

The figure depicting different parts of a human ear is shown below:

Question 3. Describe the structure and function of the outer ear of a human being.

Answer:

Structure And Function Of The Outer Ear Of A Human Being:-

The outer portion of the ear is called outer ear. It consists of three parts

1. Pinna
2. Auditory canal and
3. Tympanic membrane or eardrum.

Pinna is made up of a thin plate of yellow elastic cartilage and soft tissue and is situated on two sides of the head. At the centre of the pinna, there is a hole which is connected with a 3 cm long zigzag canal named auditory canal which extends up to the eardrum (tympanic membrane).

The tympanic membrane is situated at the end of the auditory canal and is an elastic membrane mainly made up of muscles.

The pinna of ear collects external sound waves and reflects it to auditory canal. Sound waves, after passing through the auditory canal, create vibration on the tympanic membrane.

Question 4. What is pinna or auricle?

Answer:

Pinna:-

This is a portion of the outer ear protruding outwardly on two sides of the head. It is made up of a thin plate of yellow elastic cartilage, covered with an integument, and connected to the surrounding parts by ligaments and muscles.

Question 5. Describe the structure and function of the middle ear of a human being.

Answer:

Structure And Function Of The Middle Ear Of A Human Being:-

In between the outer ear and the inner ear, there lies the middle ear which consists of a small air-filled cavity and is made up of three adjacent bones — malleus, incus and stapes.

As the middle ear is connected with the pharynx through the eustachian tube, air pressure remains the same inside and outside the middle ear.

The vibrations that are created by an external sound wave in the eardrum enters the inner ear through the bones of the middle ear.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 6. Describe the structure and function of the inner ear of a human being.

Answer:

Structure And Function Of The Inner Ear Of A Human Being:-

The inner ear consists mainly of the vestibular apparatus and cochlea. There are three semicircular canals in the vestibular apparatus. These semi-circular canals are filled up with a liquid named endolymph.

The cochlea is the last part of the inner ear and looks like a coiled tube similar to a snail. The organ which acts as the receptor of sound in the cochlea is called the organ of Corti. It is a hairy instrument with three compartments filled with endolymph.

When a sound wave reaches the cochlea, the organ of the Corti is stimulated and this stimulation reaches the hearing centre of the brain through auditory nerves and creates the sensation of hearing.

Question 7. What is the role of the ear in sending sound waves?

Answer:

Role Of Ear In Sending Sound Waves:-

The pinna of the ear collects external sound waves and reflects it to the external auditory canal. It gathers sound energy and focuses it on the eardrum to vibrate it.

This vibration on the eardrum created by the external sound waves enters the inner ear through three tiny bones – malleus, incus and stapes of the middle ear.

When sound reaches the cochlea of the inner ear, the organ of Corti is stimulated and this stimulation reaches the hearing centre of the brain through auditory nerves and creates the sensation of hearing.

Question 8. What is the eustachian tube? What is its function?

Answer:

Eustachian Tube:-

It is a narrow passage from the pharynx to the cavity of the middle ear. It is a canal that connects the middle ear to the pharynx.

Its main function is to maintain the equilibrium of the air pressure between the inside and outside of the ear. Its secondary function is to clear any accumulated secretion, infection or debris from the space of the middle ear.

Question 9. What do you mean by sound pollution?

Answer:

Sound Pollution:-

The harmful effect of undesirable sound on the environment is known as sound pollution. Undesirable sound refers to sound with high loudness and pitch and beyond the tolerance level of the ear.

The sound produced by different factories, horns of traffic, loudspeakers used in various functions, fire-crackers, take-off and landing of aircraft, loud sounds from radio, television, sirens etc. create sound pollution in the environment.

Further, natural sources like the sound of lightning, and the rumbling of clouds also add to sound pollution.

Question 10. How does sound pollution happen in our daily lives?

Answer:

Sound Pollution Happen In Our Daily Lives As Follows:-

In daily life, our hearing gets so used to different high-pitched sounds that we get tremendous relief when these sounds stop.

Different aspects of our environment are responsible for the mental and physical health of every animal on the planet. Sound is one of the key features among them.

Different types of undesirable high-pitched sounds like noise, shouting, the sound of microphones, the sound of cars, the sound of air-horns, the sound of generators, and metallic and mechanical sounds from different factories and from different other sources get mixed together to disturb our normal life.

If the same type of disturbance continues in the same area regularly, it creates sound pollution. Mental and physical health is affected due to this sound pollution.

Question 11. Discuss in brief, the harmful effects of sound pollution on the animal world of the earth.

Answer:

Harmful Effects Of Sound Pollution On The Animal World Of The Earth:-

It has been observed in very recent research works that not only human beings but also other animals are badly harmed due to sound pollution created by humans.

Research has proved that a particular species of frog is used to hear calls of a specific low frequency from their friends.

However, due to sound pollution, the frogs are forced to communicate with other frogs in high-frequency sound which disrupts their normal livelihood. Birds also suffer due to the same reason.

The harmful effects of sound pollution extend to the deep sea also. Normal lives of whales, dolphins, fish etc. suffer enormously due to ultrasonic sounds from SONAR emitted on account of scientific research in deep seas Discuss the harmful effects of sound pollution in daily life.

Question 12. Discuss the harmful effects of sound pollution in daily life.

Answer:

Harmful Effects Of Sound Pollution On Daily Life:-

Sound pollution is harmful to human beings, both physically and mentally. It has been observed that the hearing power of a man is damaged if a high-pitched sound is heard continuously for a long time. In extreme cases, it may also lead to deafness.

The high-pitched sound may increase blood pressure, create heart problems, and affect the concentration of the mind and also memory. Work efficiency gradually decreases if a person is a victim of sound pollution due to professional reasons. Sound pollution is an important cause of insomnia.

Question 13. Discuss some ways to control sound pollution in our daily lives.

Answer:

Ways To Control The Sound Pollution In Our Daily Lives Are:

1. By increasing awareness among the citizens regarding sound pollution and its bad effects.
2. By taking preventive measures in factories regarding sound pollution.
3. By making laws regarding sound pollution and implementing those rigorously.
4. By prohibiting the use of loudspeakers and horns of car near schools, colleges, hospitals etc,
5. By constructing factories and airports away from the residential areas.

Question 14. What do you mean by the threshold of hearing? What is the maximum intensity of sound that a person can tolerate safely?

Answer:

Threshold Of Hearing And The Maximum Intensity Of Sound That A Person Can Tolerate Safely:-

The threshold of hearing is the minimum intensity of sound that produces a sensation of sound in the human ear. For a sound wave of frequency 1000 Hz, the threshold of hearing is given by, I₀ = 10^-12 W/m²

This is also known as zero-level intensity.

The maximum intensity for a sound of frequency 1000 Hz is 1 W/m², which a person can tolerate safely.

Chapter 7 Human Ear And Sound Pollution Very Short Answer Type Questions Choose The Correct Answer

Question 1. The ‘Decibel’ unit is used to measure

1. Loudness
2. Frequency
3. Wavelength
4. Time period

Answer: 1. Loudness

Question 2. What is the safe intensity level of sound indicated by the World Health Organisation (WHO)?

1. 45 dB
2. 65 dB
3. 80 dB
4. 90 dB

Answer: 1. 45 dB

Question 3. What is the sound intensity level of perception?

1. 100 dB
2. 110 dB
3. 120 dB
4. 140 dB

Answer: 3. 120 dB

Question 4. What is the threshold of hearing for sound with a frequency of 1000 Hz?

1. 10^-9 W/m²
2. 10^-10 W/m²
3. 10^-11 W/m²
4. 10^-12 w/m²

Answer: 4. 10^-12 w/m²

Question 5. What is the limit of perception for sound of frequency 1000 Hz?

1. 1 W/m²
2. 1.5 W/m²
3. 2W/m²
4. 10 W/m²

Answer: 1. 1 W/m²

Question 6. What is the unit of intensity of sound in SI?

1. W • m^-2 • K^-1
2. W • m^-2
3. W • m^-1
4. W • m^-1 • K^-1

Answer: 2. W • m^-2

Question 7. What is the dimensional expression of the intensity of sound?

1. MT^-3
2. MT^-2
3. MLT^-3
4. ML^-1T^-3

Answer: 1. MT^-3

Question 8. What is the persistence time of hearing?

1. 0.05 s
2. 0.1 s
3. 0.2 s
4. 1s

Answer: 2. 0.1 s

Question 9. It is the function of _____ to collect and centralise sound waves.

1. Pinna
2. Auditory canal
3. Tympanic membrane or eardrum
4. Tympanic cavity

Answer: 2. Auditory canal

Chapter 7 Human Ear And Sound Pollution Answer In Brief

Question 1. Which quantity has its unit as decibel?

Answer: Decibel (dB) is the unit of difference of intensity level of sound.

Question 2. Between loudness and intensity of sound, which one is a measurable quantity?

Answer: The intensity of sound is a measurable quantity.

Question 3. Where is the organ of Corti located?

Answer: The organ of Corti is located in the cochlea of the inner ear.

Question 4. What is endolymph?

Answer:

Endolymph

Endolymph is a liquid which is present in the scala media cavity of the cochlea in the inner ear in which the organ of Corti remains immersed.

Question 5. Where is the semicircular canal located in the ear?

Answer: The semicircular canal is located in the inner ear.

Question 6. What is a basilar membrane?

Answer:

Basilar membrane

The basilar membrane is a membrane in the cochlea on which the organ of Corti is located.

Question 7. What is the function of the auditory canal of the ear of a human being?

Answer: The auditory canal of the ear collects and focuses sound waves.

Question 8. What is the function of the tympanic cavity of the ear of a human being?

Answer: Waves created on the tympanic membrane vibrate the air of the tympanic cavity and sound waves move forward towards the next portion of the ear.

Question 9. What is the function of a eustachian tube of the ear of a human being?

Answer: The eustachian tube maintains the equilibrium of air pressure between the inside and the outside of the middle air.

Question 10. What are the different parts of the human ear?

Answer:

There are three main parts of the human ear, which are:

1. The outer ear
2. The middle ear
3. The inner ear.

Question 11. What are the three bones of the middle ear?

Answer: The three bones of the middle ear are malleus, incus and stapes.

Question 12. What is the function of the ear other than hearing?

Answer: In addition to hearing, the ear maintains the balance of the body.

Chapter 7 Human Ear And Sound Pollution Fill In The Blanks

Question 1. _______ maintain air pressure in the middle ear.

Answer: Eustachian tubes

Question 2. The unit of the difference of intensity levels of sound is ______

Answer: Bel

Question 3. The ear can mainly be divided into _______ parts.

Answer: Three

Question 4. Sound waves reach the auditory centre of the brain through ______ nerves.

Answer: Auditory

Question 5. The unit of intensity of sound is _______

Answer: W/m²

Question 6. The safe level of noise is ________ dB.

Answer: 65 db

Question 7. The part of the human ear that collects the sound and sends it to the auditory canal is _______

Answer: Pinna

Question 8. The vibrations produced by the eardrum are amplified by the _______, _________ and _______

Answer: Hammer, Anvil, The stirrup

Question 9. The _______ is a coiled and fluid-filled tube having the sense organ of hearing.

Answer: Cochlea

Chapter 7 Human Ear And Sound Pollution State Whether True Or False

Sound Exercise

Question 1. The ear only helps us to hear.

Answer: False

Question 2. The eustachian tube maintains pressure balance on the two sides of the eardrum.

Answer: True

Question 3. The three ossicles the hammer, anvil and the stirrup are located at the inner ear.

Answer: False

Question 4. The cochlea is a coiled and fluid-filled tube.

Answer: True

Question 5. The organ of Corti is located in the mammalian cochlea.

Answer: True

Question 6. The incus receives vibrations from sound pressure on the ear drum.

Answer: False

Question 7. Zero-level sound intensity is 10^-12W • m^-2.

Answer: True

Question 8. The safe level of noise as specified by the central pollution control board is 120 dB.

Answer: False

Chapter 7 Human Ear And Sound Pollution Miscellaneous Type Questions

Match the Columns

1.

Answer: 1. A, 2. C, 3. B, 4. D

2.

Answer: 1. B, 2. C, 3. D, 4. A

Chapter 4 Matter Concept Of Atom Synopsis

Subatomic Particles Of An Atom: Electron, proton, and neutron are the three main subatomic particles of an atom.

Cathode Rays: A strong potential difference (10000 V) is applied between two electrodes present inside an electric discharge tube containing a gas at a very low pressure (0.01 mm Hg).

As a result, the cathode emits certain invisible ray, known as cathode ray which produces a faint greenish fluorescence on the glass wall opposite to that of the cathode.

Discovery Of Electrons: Considering various properties of cathode rays, J. J. Thomson concluded (in 1897) that cathode rays are composed of negatively charged material particles and named them negatrons. Later these particles were named electrons.

Class 9 Physical Science Chapter 4 Atom

Anode Rays: When perforated cathode is used inside the discharge tube, a positively charged ray is emitted from the anode which moves towards the cathode. This is known as anode ray.

Discovery Of Protons: When hydrogen gas was taken in the discharge tube, a positively charged particle was obtained during the anode ray experiment. This particle is known as proton.

Mass Of Fundamental Particles Of An Atom:

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

The mass of an electron is 9.11 x 10^-13kg.

A proton is 1836 times heavier than an electron while a neutron is 1839 times heavier than an electron.

Charge of fundamental particles of an atom: The charge of an electron is -1.6 x 10^-19C.

The magnitude of charge of a proton is equal to that of an electron but is opposite in nature. Neutrons do not have any charge. Electrons are negatively charged whereas protons are positively charged particles.

Chapter 4 Matter Concept Of Atom Short And Long Answer Type Questions

Question 1. How is cathode ray produced in an electric discharge tube?

Answer:

The Cathode Ray Produced In An Electric Discharge Tube As Follows:-

When gas inside the discharge tube is kept at a very low pressure (about 0.01 mm Hg pressure) and a very high potential difference (10000 V) is applied between two metallic electrodes of the discharge tube, an invisible ray is produced at the cathode which then moves towards the anode. This ray is called the cathode ray.

It creates fluorescence on the glass wall opposite to that of the cathode in the discharge tube.

Question 2. Why is the charge of an electron considered to be the smallest unit of electricity?

Answer:

The Charge Of An Electron Considered To Be The Smallest Unit Of Electricity Because:-

Scientist Millikan was the first to determine the charge of an electron as -1.602 x 10^-19 coulomb or -4.8 x 10^-10 esu. However, no negatively charged particle with a lower charge value than an electron has been discovered till date. Thus, the charge of an electron is considered to be the smallest unit of electricity.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

The magnitude of charge of any positively or negatively charged particle is thus considered to be equal to or an integral multiple of the charge of an electron.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 3. How is anode ray produced in an electric discharge tube?

Answer:

An Anode Ray Produced In An Electric Discharge Tube:-

When gas inside the discharge tube is kept at a very low pressure (about 0.01 mm Hg pressure) and a very high potential difference (10000 V) is applied between two metallic electrodes and if during this process a perforated cathode is used, an invisible ray is produced at the anode which then moves towards the cathode.

This ray is called the anode ray. In this condition, if the anode rays are allowed to pass through the perforated cathode, they produce a reddish glow on the glass wall behind the cathode.

Question 4. What Is canal ray? Why is it named so?

Answer:

Canal Ray:-

Inside the discharge tube, if a high potential difference is applied between the two electrodes at a very low pressure (about 0.01 mm Hg pressure), an invisible ray consisting of positively charged particles is produced at the anode, moves towards the cathode and passes through the perforated cathode in a straight line. This ray is called the canal ray or anode ray.

The ray is so named because it passes through the perforated cathode.

Question 5. Which observation of Thomson’s discharge tube experiment led to the conclusion that cathode ray is not electromagnetic radiation?

Answer:

In Thomson’s experiment, it was observed that the cathode rays were deflected in the positive direction when an electric field was applied. This proved that cathode ray is not electromagnetic radiation but a stream of negatively charged particles.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

If a light paddle wheel is placed in the path of cathode rays, it is seen that the wheel starts rotating. This too proves that cathode ray is a stream of particles having fixed mass.

Question 6. Write four applications of electric discharge through a gas at low pressure.

Answer:

Applications Of Electric Discharge Through A Gas At Low Pressure:-

Electric discharge through a gas at low pressure has the following applications in

1. Neon-sign advertising boards,
2. Television tubes,
3. Fluorescent tube,
4. Sodium vapour lamp.

Question 7. State three important characteristics of cathode rays.

Answer:

Important Characteristics Of Cathode Rays:-

Three important characteristics of cathode rays are as follows

1. Cathode rays move in a straight line. If an opaque substance is placed in the path of cathode rays, its shadow is formed. The path of cathode rays remains unaffected by the position of the anode.
2. Cathode ray is a stream of particles, each having a definite but small mass. So, if a light paddle wheel is placed in the path of the cathode rays, it rotates due to collision with the particles.
3. Cathode rays consist of negatively charged particles. It gets deflected in presence of magnetic and electric field. In an electric field, cathode rays deflect towards the positive plate while in a magnetic field, cathode rays deflect towards the north pole.

Question 8. The e/m (charge/mass) ratio of cathode ray is the same for all gases taken in an electric discharge tube, but the e/m ratio of anode ray is different for different gases. Why?

Answer:

Given

The e/m (charge/mass) ratio of cathode ray is the same for all gases taken in an electric discharge tube, but the e/m ratio of anode ray is different for different gases.

Cathode ray is a stream of electrons. These electrons are emitted from the metallic cathode when high potential difference is applied between the electrodes in an electric discharge tube containing gases at a very low pressure.

Electron is a fundamental particle present in the atoms of all elements. Hence, the e/m ratio of cathode ray is constant irrespective of the gas being used in the discharge tube.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

On the other hand, anode rays are produced when high potential difference is applied in a discharge tube at a very low pressure. Under these circumstances, the gaseous atoms lose electrons and get converted into cations.

Anode ray is the stream of these cations. As the atomic mass is different for different gases, the mass of anode ray particles varies when different gases are used in the discharge tube. Hence, the e/m ratio is different for different gases.

Question 9. How was the existence of protons detected in anode rays?

Answer:

Existence Of Protons Detected In Anode Rays:-

The nature of deflection of anode rays in presence of electric and magnetic fields indicate that it is a collection of positively charged particles (it deviates towards the negatively charged plate).

Again, it is observed by using different gases in the electric discharge tube that e/m ratio of the anode ray is different for different gases. When hydrogen gas is used, the mass of the positively charged particle obtained in the electric discharge tube is found to be the minimum and the charge of each particle is equal to that of an electron.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

From this, it was concluded that the particles present in anode ray in the presence of hydrogen gas are similar to H⁺ ion. This led to the discovery of proton in anode rays by using naturally occurring hydrogen gas.

Question 10. Which incidents led to the following conclusions about anode rays

1. It moves in a straight line,
2. It is a stream of particles having definite mass,
3. It is a stream of positively charged particles.

Answer:

Conclusions About Anode Rays:-

1. If an opaque substance is placed in the path of anode rays, its shadow is formed. This proves that anode rays move in a straight line.
2. If a light paddle wheel is placed in the path of anode rays, it rotates due to the collision with the particles. It indicates that anode ray is a stream of particles with a definite mass.
3. Anode ray is deflected by electric or magnetic field. In an electric field it is deflected towards the negative plate. Thus, it can be said that, it is a stream of positively charged particles.

Question 11. Mention two differences between cathode ray and anode ray.

Answer:

The Differences Between Cathode Ray And Anode Ray Are As Follows:-

Question 12. Describe the plum pudding model or watermelon model of an atom.

Answer:

Watermelon Model Of An Atom:-

1. In 1898, Sir J. J. Thomson proposed the plumpudding model or watermelon model for describing the structure of an atom. According to this model—
2. An atom is a positively charged uniform sphere.
3. The electrons are evenly embedded into it to neutralise the positive charge. These are just like the seeds embedded in a watermelon or the plums placed inside a pudding.
4. The electrons are distributed throughout the sphere in such a manner that the total positive and negative charges are neutralised by each other properly. Hence, the structure is neutral.
5. The mass of the atom is uniformly distributed throughout the atom.

Question 13. Mention the similarities between a proton and an electron.

Answer:

Similarities Between A Proton And An Electron:-

The similarities between protons and electrons are as follows

1. Both are stable fundamental particles of an atom.
2. Both are charged particles and the magnitude of charge for both the particles is same, 1.602 x 10^-19 C or 4.8 x 10^-10 esu.
3. The number of protons and electrons are equal in an atom.
4. Both protons and electrons are deflected by electric and magnetic fields.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

Question 14. Between proton and electron, which particle has a higher e/m value and why?

Answer:

e/m Value Of An Electron Is Greater Than That Of A Proton:-

The e/m (charge/mass) value of a proton is 9.58 x 10⁴ C • g^-1 and that of an electron is 1.76 x 10⁸ C • g^-1. Thus, e/m value of an electron is greater than that of a proton.

Although the magnitude of charge of both electron and proton are equal but opposite in nature, mass of an electron (9.11 x 10^-28g) is 1/1836 times that of a proton. So, e/m value of an electron is greater than that of a proton.

Question 15. Write the differences between electrons and protons.

Answer:

The Differences Between Electrons And Protons Are As Follows:-

Question 16. Mass of a hydrogen atom is equal to the mass of a proton—explain.

Answer:

Mass Of A Hydrogen Atom Is Equal To The Mass Of A Proton:-

Ordinary hydrogen contains one proton at the nucleus and one electron in the extra-nuclear orbit. Mass of an electron is 1/1836 part to the mass of a proton. That’s why only the mass of proton is taken into account while expressing the mass of a hydrogen atom.

Class 9 Physical Science Chapter 4 Atom Short And Long Answer Type Questions

As the mass of an electron is not taken into account, an error takes place during this calculation but the extent of error is negligible. Hence the statement ‘Mass of a hydrogen atom is equal to the mass of a proton’ is justified.

Chapter 4 Matter Concept Of Atom Very Short Answer Multiple Choice Questions

Question 1. The constituent particle of cathode rays is

1. Electron
2. Proton
3. Neutron
4. Ion

Answer: 1. Electron

Question 2. Which of the following is not a fundamental particle of matter?

1. Electron
2. Proton
3. Positron
4. Neutron

Answer: 3. Positron

Class 9 Physical Science Chapter 4 Atom Very Short Answer Type Questions Choose The Correct Answer

Question 3. Negatron is also known as

1. Electron
2. Proton
3. Neutron
4. Positron

Answer: 1. Electron

Question 4. e/m ratio of cathode rays is

1. 2.276 x 10¹¹ C • kg^-1
2. 3.76 x 10¹¹ C • kg^-1
3. 1.76 x 10¹¹ C • kg^-1
4. 4.276 x 10¹¹ C • kg^-1

Answer: 3. 1.76 x 10¹¹ C • kg^-1

Question 5. The heaviest particle of an atom is

1. Neutron
2. Proton
3. Electron
4. None of these

Answer: 1. Neutron

Question 6. Speed of anode rays

1. Is higher than that of cathode rays
2. Is much less than that of cathode rays
3. Is equal to that of cathode rays
4. Cannot be determined

Answer: 2. Is much less than that of cathode rays

Question 7. Canal ray is commonly known as

1. Cathode ray
2. Anode ray
3. X-ray
4. UV-ray

Answer: 2. Anode ray

Question 8. Cathode ray consists of

1. Electron
2. Proton
3. Neutron
4. Ion

Answer: 1. Electron

Question 9. Which one is not electromagnetic radiation?

1. Cathode ray
2. X-ray
3. Radio wave
4. UV-ray

Answer: 1. Cathode ray

Class 9 Physical Science Chapter 4 Atom Very Short Answer Type Questions Choose The Correct Answer

Question 10. Thomson’s atomic model is known as

1. Plum pudding model
2. Proton-electron model
3. Plum cake model
4. Pomegranate model

Answer: 1. Plum pudding model

Question 11. Cathode ray is deflected by electric field as well as by magnetic field as it is

1. Electromagnetic radiation
2. Stream of positively charged particles
3. Stream of neutral particles
4. Stream of negatively charged particles

Answer: 4. Stream of negatively charged particles

Question 12. Nature of charge of an electron is

1. Neutral
2. Negative
3. Positive
4. None of the above

Answer: 2. Negative

Question 13. A proton is

1. 1836 times heavier than an electron
2. 1746 times heavier than an electron
3. Lighter than an electron
4. 1236 times heavier than an electron

Answer: 1. 1836 times heavier than an electron

Class 9 Physical Science Chapter 4 Atom Very Short Answer Type Questions Choose The Correct Answer

Question 14. The particle formed when an electron is removed from a hydrogen atom

1. Proton
2. Neutron
3. α – particle
4. β – particle

Answer: 1. Proton

Chapter 4 Matter Concept Of Atom Answer In Brief

Question 1. At what pressure are cathode rays produced in an electric discharge tube?

Answer: In an electric discharge tube, cathode rays are produced at a very low pressure of about 0.01 mm of Hg.

Question 2. In which direction will the cathode rays deflect in the presence of an electric field?

Answer: In presence of an electric field, the cathode rays will be deflected towards the positive plate.

Question 3. What Is the magnitude of e/m (charge/mass) of a neutron?

Answer: The magnitude of e/m of a neutron is zero.

Question 4. Why is cathode ray formed at very low pressure only?

Answer: At a very low pressure the resistance inside the discharge tube is less, hence the formation of cathode ray becomes feasible.

Question 5. What happens when cathode rays strike the surface of various hard metals?

Answer: Cathode rays produce X-rays when they strike the surface of various hard metals like tungsten, molybdenum etc.

Question 6. Write down the similarity between cathode rays and ordinary light.

Answer: Just like ordinary light, cathode rays also affect photographic plates.

Question 7. Who discovered electrons?

Answer: J. J. Thomson, in 1897, discovered electrons.

Question 8. Who named the negatively charged particles, present in cathode rays, electrons?

Answer: Scientist G.J. Stoney.

Question 9. Which scientist is considered as the ‘father of atomic physics’?

Answer: John Dalton.

Question 10. Who determined the charge of an electron by oil drop experiment?

Answer: Scientist Robert A. Millikan.

Question 11. Who performed the discharge tube experiment using a perforated cathode?

Answer: Scientist E. Goldstein.

Question 12. Who discovered protons?

Answer: Scientist Ernest Rutherford (in 1911).

Question 13. Who named the positively charged particles, found in anode rays, protons?

Answer: Scientist E. Rutherford .

Question 14. Who found a method to determine the number of protons in an atom?

Answer: Scientist Moseley (in 1913).

Question 15. Who proposed the watermelon model or plum pudding model for an atom?

Answer: Scientist J. J. Thomson.

Question 16. What is the mass of an electron in SI unit?

Answer: The mass of an electron is 9.1 x 10^-31 kg.

Question 17. What is the mass of a proton?

Answer: The mass of a proton is 1.6725 x 10^-27 kg.

Question 18. What is the charge of a proton?

Answer: In SI unit, the charge of a proton is + 1.602 x 10^-16 C and in CGS unit it is +4.8 x 10^-10 esu.

Question 19. What is the charge of an electron?

Answer: In SI unit, the charge of an electron is -1.602 x 10^-19 C and in CGS unit it is -4.8 x 10^-10 esu.

Question 20. For which ray e/m ratio is not constant?

Answer: e/m ratio is not constant for anode ray.

Chapter 4 Matter Concept Of Atom Fill In The Blanks

Question 1. Cathode ray is a stream of _____

Answer: Electrons

Question 2. Positive ray is commonly known as _______ ray.

Answer: Anode

Question 3. The nucleus of an atom does not consist of _______

Answer: Electrons

Question 4. Cathode rays are emitted _______ from the cathode surface and travel towards the _______

Answer: Perpendicularly, anode

Question 5. The watermelon model for the structure of an atom was proposed by _________

Answer: J.J Thomson

Question 6. ________ is produced when cathode rays hit the walls of a discharge tube or fall on the surface of substances like ______

Answer: Fluorescence, Zinc sulphide

Question 7. When cathode rays hit the surface of metals like tungsten or molybdenum, ______ rays are produced.

Answer: X

Question 8. During production of cathode rays, the pressure in the discharge tube is equal to the pressure of ________ mercury column.

Answer: 0.01

Question 9. Speed of anode rays is much ________ than the speed of cathode rays.

Answer: Less

Question 10. A proton is ________ times than an electron.

Answer: 1836, Heavier

Chapter 4 Matter Concept Of Atom State Whether True Or False

Question 1. Mass of a hydrogen atom is almost same to that of a proton.

Answer: True

Question 2. Cathode ray is streamed from anode to cathode.

Answer: False

Question 3. First discovered subatomic particle is proton.

Answer: False

Question 4. Mass of a hydrogen atom is part to the mass of an electron.

Answer: False

Question 5. Watermelon model of an atom is also known as the plum pudding model.

Answer: True

Question 6. An electron is 1836 times heavier than a proton.

Answer: False

Question 7. The e/m ratio of cathode ray depends upon the gas used in the discharge tube.

Answer: False

Question 8. The number of protons present influences the mass of an atom.

Answer: True

Question 9. Thomson named anode ray as positive ray.

Answer: True

Question 10. The plum pudding model of Thomson gives the concept of nucleus in an atom.

Answer: False

Question 11. Cathode ray is emitted perpendicularly from the cathode plane.

Answer: True

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Synopsis

Outcomes of Rutherford’s Alpha-Scattering Experiment:

Based on his famous alpha scattering experiment, in 1911, Ernest Rutherford first proposed the idea of nuclear atomic model. It was the first satisfactorily proposed atomic model, even though it failed to explain the stability of an atom.

Rutherford’s Atomic Model: 

1. All atoms contain a tiny but massive positively charged nucleus at their centre.
2. The size of the nucleus is very small compared to that of the atom as most of the space inside any atom is found to be empty.
3. The entire positive charge and almost whole of the mass of an atom is concentrated at its nucleus.
4. Negatively charged electrons revolve around the nucleus in circular paths present at the extranuclear part.
5. Total number of protons present in the nucleus is equal to the total number of electrons present in the extranuclear part of an atom.
6. The electrostatic force of attraction between the electrons and the nucleus is balanced by the centrifugal force of the revolving electrons. This prevents the electrons from falling into the nucleus.

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

Limitations Of Rutherford’s Atomic Model:

1. It is unable to explain the stability of an atom.
2. This model fails to explain the discontinuous nature of atomic spectra.

Discovery Of Neutron:

In 1932, scientist James Chadwick discovered neutrons, the neutral particle present in an atom, by bombarding beryllium (₄Be⁹) metal with fast-moving alpha (α) particles.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Short And Long Answer Type Questions

Question 1. On the basis of Rutherford’s α – scattering experiment, answer the given questions

1. What is an α – particle?
2. Why was gold foil used in α scattering experiment?
3. What arrangement was made to detect the path of α -particles?

Answer:

On the basis of Rutherford’s α – scattering experiment

α – particle

1. An α -particle is a particle having a mass of 4 units and 2 units of positive charge. It consists of 2 protons and 2 neutrons. As these are identical with helium nuclei, α – particles are often represented as ₂He²⁺. During the disintegration of the nuclei of many radioisotopes, emission of α – particles take place.
2. Thinner the metal foil used, higher will be the probability of α-particles to collide with the atoms of the metal foil. As gold is a soft and highly malleable metal, it can be easily converted into a very thin foil. Due to this Rutherford used gold foil in his experiment.
3. A circular fluorescent screen coated with zinc sulphide (ZnS) was set up around the metal foil. After penetrating through the metal foil, the α-particles hit the screen generating a flash of light.
4. This helped in detecting the path of scattered α-particles.

 

 

Question 2. State the observations made by Rutherford in his a -scattering experiment.

Answer:

Observations Made By Rutherford In His A -Scattering Experiment:-

Rutherford made the following observations in his α – scattering experiment

1. Most of the α – particles passed straight through the gold foil and hit the zinc sulphide (ZnS) screen Without suffering any deviation.
2. A few α-particles passed through the foil after suffering small deviations.
3. Very few α-particles (1 out of 20000) suffered deviation through large angles or even deflected by an angle of 180°.

 

 

Question 3. What were the conclusions drawn by Rutherford from his α-scattering experiment?

Answer:

The Conclusions Drawn By Rutherford From His Α-Scattering Experiment:-

From the observations of his α-scattering experiment, Rutherford drew the following conclusions

1. As metal foil is made up of metal atoms and most of the α-particles pass through the gold foil without any deviation, major part of an atom is empty.
2. α-particles are positively charged. So, they can be deflected only by another positive charge having greater mass than themselves. Hence, it was concluded that all the positive charge and mass of an atom is concentrated within a very small region in the atom. This is the nucleus of an atom.
3. Only those α-particles which hit the nucleus suffer deviation by large angles (90° or more) or even deflected back at an angle of 180°.
4. Negatively charged electrons remain outside the nucleus. As α-particles are much heavier than electrons, their paths are not affected by the presence of electrons in an atom.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 4. Describe Rutherford’s atomic model.

Answer:

Rutherford’s Atomic Model

Based on the conclusions of his α-scattering experiment, in 1911, Rutherford proposed an atomic model known as Rutherford’s atomic model. The postulates of this model are as follows

1. An atom has two parts—a positively charged nucleus and the extra nuclear part where the negatively charged electrons are present.
2. Entire mass and positive charge of an atom is concentrated within a very small region in the atom. This is known as the nucleus. Most of the space in an atom is empty.
3. The nucleus is very small in size with respect to an atom. The diameter of an atom is around 10^-8 cm while that of a nucleus is 10^-13 cm.
4. The electrons revolve around the nucleus in different circular orbits.
5. Total number of electrons revolving around the nucleus is equal to the number of positively .charged protons present within the nucleus.
6. Two opposite forces act on an electron moving along an orbit—the electrostatic force of attraction between negatively charged electrons and positively charged nucleus and the centrifugal force acting on the electrons due to its rotational motion which acts away from the nucleus.
7. These two forces being equal in magnitude but opposite in direction, maintain the stability of the electrons in the orbit.

Question 5. What is radioactivity? How did the proposing his atomic model?

Answer:

Radioactivity:-

Radioactivity is a nuclear phenomenon in which the nuclei of heavier elements, like uranium, thorium etc., in elemental state or in compound form, disintegrate spontaneously to form nuclei with greater stability [i.e., new elements are formed) and invisible rays are emitted (radioactive radiation) during this process.

From the results of electric discharge through gases at low pressure and the discovery of atom it was evident that atoms can be divided into smaller particles. After the discovery of radioactivity, Rutherford performed his famous q -scattering experiment on the basis of which he proposed his atomic model.

Question 6. Mention the merits of Rutherford’s atomic model.

Answer:

Merits Of Rutherford’s Atomic Model Are:-

1. The existence of a nucleus is first observed in this model. The concept that almost the entire mass of an atom is concentrated at its nucleus is accepted in future days.
2. The concept of revolving of electrons in several orbits around the nucleus first known from this atomic model.
3. The total negative (-ve) charge of revolving electrons is numerically equal to the total positive (+ve) charge of the nucleus—from this concept neutrality of an atom can be justified.

Question 7. Discuss the limitations of Rutherford’s atomic model.

Answer:

Limitations Of Rutherford’s Atomic Model:-

There were some limitations of Rutherford’s atomic model. These are discussed below

According to Maxwell’s theory of electromagnetic radiation, if a negatively charged electron revolves around the positively charged nucleus, then it should continuously emit radiation. Due to this, energy of the electron would decrease in every turp and it would move closer to the nucleus in a spiral pathway.

Ultimately the electron will fall into the nucleus, making the atom unstable. However, atoms are found to be quite stable. Thus, Rutherford’s model failed to explain the stability of an atom.

 

 

If electrons lose energy continuously, then the respective atomic spectra should be continuous in nature. However, atomic spectra are found to be discontinuous with well-defined lines (line spectrum or discontinuous spectrum). Rutherford’s theory failed to explain the formation of line spectra by atoms.

 

 

It gives no information about the velocity and energy of electrons as well as the electronic configuration of atoms.

Question 8. Why is an atom electrically neutral?

Answer:

An Atom Electrically Neutral Because:-

An atom contains equal number of protons and electrons. Now, the charge of a proton is equal in magnitude but opposite in nature with respect to the charge of an electron.

Hence, the total positive charge in the nucleus of an atom is equal to the total negative charge of electrons revolving in orbits. Hence, an atom is electrically neutral.

Question 9. The electrons outside the nucleus are negatively charged and the protons in the nucleus are positively charged. Then why do the electrons not fall into the nucleus due to its attraction?

Answer:

Given

The electrons outside the nucleus are negatively charged and the protons in the nucleus are positively charged.

According to Rutherford’s atomic model, the electrons revolve around the nucleus in different circular orbits with high velocity.

While rotating in an orbit, the centrifugal force acting on an electron is equal in magnitude to the electrostatic force of attraction acting between the negatively charged electron and the positively charged nucleus.

However, these two forces act in opposite directions. Hence, the resultant force acting on an electron is zero and the electron maintains its stability in the orbit and does not fall into the nucleus.

 

Question 10. How did Rutherford conclude that an atom contains equal number of protons and electrons?

Answer:

The presence of negatively charged electrons and positively charged protons in an atom had been proved experimentally. Also, a proton and an electron have equal but opposite charges. As an atom is electrically neutral, Rutherford concluded that an atom should contain equal number of protons and electrons.

Question 11. Which discovery became possible from the findings of α-particle scattering experiment?

Answer:

From α-particle scattering experiment it was proved that, all the positive charge and mass of an atom is concentrated in a very small region within the atom. This small region is known as nucleus.

Thus, discovery of nucleus became possible due to the α-particle scattering experiment.

Question 12. What would have happened if  Rutherford had used a foil of a tighter metal than gold foil in his α-scattering experiment?

Answer:

The relative mass of an α-particle is 4 units. In the α-scattering experiment, instead of gold foil, if a foil of any other lighter metal was used, then the α-particle might have passed through the foil without deflection. As A result, scattering of α-particles would not have taken place.

Question 13. Which observation led to the discovery of neutrons in an atom?

Answer:

Observation Led To The Discovery Of Neutrons In An Atom:-

Except ordinary hydrogen, the masses of all other atoms were found to be heavier than the total mass of electrons and protons in the atoms. This observation forced scientists to think about the presence of any other neutral particle having fixed mass in the atom. This ultimately led to the discovery of neutrons in an atom.

Question 14. Can an atom possess more number of protons than neutron?

Answer:

Except ordinary hydrogen, all other stable isotopes of elements have either equal or greater number of neutrons than protons in the nucleus. Neutrons present in the nucleus decrease the repulsion between protons.

If number of protons exceeds the number of neutrons in the nucleus, then repulsion between them will increase and the nucleus will become unstable. So, none of the stable isotopes have greater number of protons than neutrons. However there are certain exceptions.

For instance, in ordinary hydrogen (₁H¹), there is only 1 proton but no neutron. In an isotope of helium (₂He³), there are 2 protons but 1 neutron.

Question 15. Mention the similarities between a proton and a neutron.

Answer:

The similarities between a proton and a neutron are as follows

1. Both are stable fundamental particles of an atom.
2. Both are present in the nucleus of an atom.
3. The diameter of both the particles is about 2.4 x 10^-13 cm.
4. Both protons and neutrons are much heavier than electrons.
5. The mass of a nucleus depends on the number

Question 16. Write the differences between protons and neurons.

Answer:

The differences between protons and neutrons are as follows

Question 17. Write down the differences between electrons and neutrons.

Answer:

The differences between electrons and neutrons are as follows

Question 18. Mention the mass and charge of electron, proton and neutron.

Answer:

The mass and charge of the three fundamental particles of an atom, i.e., electron, proton and neutron is given below

Question 19. Why is the nucleus of an atom positively charged and the heaviest part of the atom?

Answer:

Nucleus contains positively charged protons and neutral particles neutrons. Due to this reason, nucleus of an atom is positively charged. On the other hand, as electron is the lightest particle of an atom, the total mass of protons and neutrons present in the nucleus is the main contributory factor to the entire mass of an atom.

Neutron is resides inside the nucleus. Presence of neutron inside the nucleus makes nucleus the heaviest part of the atom.

Question 20. Which problem regarding the structure Of an atom was solved by the discovery of neutrons?

Answer:

Rutherford observed that except ordinary hydrogen, the mass of all other atoms are greater than the combined mass of protons and electrons present in their atoms. On that basis, Rutherford, in 1920, suggested the presence of a neutral particle of unit mass within the nucleus of an atom.

He named the particle neutron. Later, in 1932, Chadwick with the help of a series of experiments proved the presence of neutrons in the nucleus. Thus, with the discovery of neutrons, the problem regarding the mass of an atom could be solved.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Multiple Choice Questions Answer

Question 1. Which metal plate was used in Rutherford’s α-scattering experiment?

1. Aluminium
2. Gold
3. Silver
4. Zinc

Answer: 2. Gold

Question 2. The existence of which of the following particles of an atom was proved by Rutherford’s α-scattering experiment?

1. Electron
2. Proton
3. Nucleus
4. Neutron

Answer: 3. Nucleus

Question 3. An alpha (α) particle is

1. Positively charged hydrogen ion
2. Unipositive helium ion
3. Dipositive helium ion
4. A chargeless particle

Answer: 3. Dipositive helium ion

Question 4. Which of the following elements is an α particle emitter?

1. Radium
2. Iron
3. Lead
4. Bismuth

Answer: 1. Radium

Question 5. In Rutherford’s experiment, most of the α – particles passed straight through the thin metallic foil without suffering any deflection. The reason is that

1. α-particles are very small in size with respect to electrons
2. α-particles are positively charged
3. Most of the part inside an atom is empty
4. α-particles are slow-moving particles

Answer: 3. Most of the part inside an atom is empty

Question 6. The nucleus of an atom is

1. Positively charged
2. Negatively charged
3. Neutral
4. Partly positive and partly negative

Answer: 1. Positively charged

Question 7. According to Rutherford’s atomic model, the entire mass of an atom,

1. Is uniformly distributed throughout the atom
2. Is concentrated within the nucleus
3. Is distributed outside the nucleus
4. Remains partly within the nucleus and partly outside the nucleus

Answer: 2. Is concentrated within the nucleus

Question 8. According to Rutherford’s atomic model, protons are

1. Present within the nucleus
2. Present outside the nucleus
3. Revolving around the nucleus
4. None of these

Answer: 1. Present within the nucleus

Question 9. Electrons present in an atom

1. Remain static within the nucleus
2. Remain static outside the nucleus
3. Revolve around the nucleus
4. Revolve within the nucleus

Answer: 3. Revolve around the nucleus

Question 10. The inference which cannot be drawn from Rutherford’s α-scattering experiment is that

1. The nucleus is small and heavy
2. The nucleus is always positively charged
3. The size of an atom is almost 10⁵ times that of the nucleus
4. The nucleus was hit by a large number of α-particles

Answer: 4. The nucleus was hit by a large number of α-particles

Question 11. An accelerated charged particle

1. Absorbs energy
2. Emits energy
3. Neither absorbs nor emits energy
4. Initially absorbs energy and then emits it

Answer: 2. Emits energy

Question 12. Which of the following atoms does not have any neutrons in it?

1. Protium
2. Deuterium
3. Tritium
4. Helium

Answer: 1. Protium

Question 13. Neutron is a

1. Positively charged particle
2. Negatively charged particle
3. Neutral particle
4. Either positively or negatively charged particle

Answer: 3. Neutral particle

Question 14. The mass of a neutron is almost equal to the mass of a/an

1. Electron
2. Proton
3. Nucleus
4. Helium atom

Answer: 2. Proton

Question 15. A neutron is heavier than an electron by

1. 1839 times
2. 1836 times
3. 1857 times
4. 1957 times

Answer: 1. 1839 times

Question 16. The heaviest subatomic particle is

1. Neutron
2. Proton
3. Electron
4. Positron

Answer: 1. Neutron

Question 17. \({ }_4^9 \mathrm{Be}+{ }_2^4 \mathrm{He} \rightarrow{ }_0^1 n+?\)

1. \({ }_6^{12} C\)
2. \({ }_6^{14} C\)
3. \({ }_7^{14} \mathrm{~N}\)
4. \({ }_8^{16} 0\)

Answer: 1. \({ }_6^{12} C\)

Question 18. Which one of the following is positively charged particle, emitted from nucleus?

1. α-ray
2. β-ray
3. Electron
4. γ-ray

Answer: 1. α-ray

Question 19. Neutrons are expressed as

1. \({ }_{-1}^0 n\)
2. \({ }0^1 n\)
3. \({ }_0^0 n\)
4. \({ }_1^1 n\)

Answer: 2. \({ }0^1 n\)

Question 20. Ratio of radius of an atom to the radius of the nucleus is

1. 10⁵
2. 10^-5
3. 10^-10
4. 10¹⁰

Answer: 1. 10⁵

Question 21. Which of the following can be detected by Rutherford’s α – scattering experiment?

1. Centre of the atom
2. Outer electronic part
3. Whole atom
4. Neutrons

Answer: 1. Centre of the atom

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Answer In Brief

Question 1. Which particles did Rutherford use for his scattering experiment?

Answer: Alpha (α) particles.

Question 2. Name the different rays that are emitted from radioactive elements.

Answer: Alpha (α) rays, beta (β) rays and gamma, (γ) rays

Question 3. What is the ratio of penetrating powers of α, β and γ-rays?

Answer: The ratio of penetrating powers of α, β and γ-rays is approximately 1:100:1000.

Question 4. What is the nature of γ-ray, obtained as a result of radioactive radiation?

Answer: γ-ray is electromagnetic radiation.

Question 5. Mention nature of the α-particle.

Answer:

Nature of the α-particle

α-particle is just like helium nucleus bearing 2 units of positive charge and 4 units of mass.

Question 6. What is the approximate measure of the diameter of nucleus of an atom?

Answer: 10^-12 to 10^-13 cm.

Question 7. How is the mass arranged in an atom according to Rutherford’s atomic model?

Answer: According to Rutherford’s atomic model almost all of the mass are centralised at the nucleus.

Question 8. What is the name of the heaviest particle present in an atom?

Answer: Neutron.

Question 9. What is the diameter of an atom?

Answer: The diameter of an atom is approximately 10^-8 cm.

Question 10. Which atom contains only two fundamental subatomic particles?

Answer: It is protium (₁H¹) which contains only two fundamental subatomic particles named proton and electron.

Question 11. Give an example of electromagnetic radiation.

Answer: Visible light.

Question 12. Who coined the term neutron?

Answer: Scientist Rutherford.

Question 13. What is the mass of a neutron particle?

Answer: Mass of a neutron particle is 1.675 x 10^-27 kg.

Question 14. Who discovered neutrons?

Answer:

Neutrons discovered by

Sir James Chadwick (In 1932).

Question 15. How many spectral lines are present in the visible region of hydrogen spectra?

Answer: The visible region of hydrogen spectra contains four spectral lines—red, bluish-green, blue and violet.

Question 16. What is the nature of atomic spectra?

Answer:

Nature of atomic spectra

Atomic spectra is discontinuous in nature and is basically a line spectra.

Question 17. Name the charge-less or neutral subatomic particle.

Answer: Neutral subatomic particle is neutron.

Question 18. What is the approximate diameter of the nucleus of an atom?

Answer: The diameter of the nucleus of an atom is in the range of 10^-12,-10^-13 cm.

Question 19. Do the electrons remain static within an atom?

Answer: According to Rutherford’s atomic model, electrons revolve around the nucleus in circular paths and thus, do not remain static within an atom.

Question 20. If a charged particle moves with accelerated motion, then what will be the change in its energy?

Answer: If a charged particle moves with accelerated motion, it will continuously emit radiation. Hence, the energy of the particle will gradually decrease.

Question 21. Which metal was bombarded by a- particles during the experiments conducted for the discovery of neutrons?

Answer: Beryllium (Be) metal.

Question 22. What is the approximate diameter of an atom?

Answer: Diameter of an atom is approximately 10⁸ cm.

Question 23. Which atom does not contain any neutron?

Answer: Protium (₁H¹) does not contain any neutron.

Question 24. Which atom contains two protons and two neutrons in the nucleus?

Answer: The helium atom (₂He⁴) contains two protons and two neutrons in the nucleus.

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron Fill In The Blanks

Question 1. Most of the part inside an atom is ________

Answer: Empty

Question 2. The positively charged heavy part of an atom is known as the ________

Answer: Nucles

Question 3. Scientist who carried out the a-particle scattering experiment was ________

Answer: Rutherford

Question 4. An electron is attracted towards the nucleus by ________ force of attraction.

Answer: Electrostatic

Question 5. The number of ________ in the nucleus is equal to the number of __________ revolving around the nucleus in different orbits.

Answer: Protons, electrons

Question 6. Rutherford’s atomic model fails to explain the __________ of an atom.

Answer: Stability

Question 7. The mass of a neutron is nearly equal to the mass of a __________

Answer: Proton

Question 8. The charge of a neutron is __________

Answer: Zero

Question 9. For a neutron, e/m value is __________

Answer: Zero

Chapter 4 Matter Rutherfords Atomic Model And Discovery Of Neutron State Whether True Or False

Question 1. Size of the nucleus is extremely small compared to the size of an atom.

Answer: True

Question 2. Radioactive elements emit alpha, beta and gamma rays.

Answer: True

Question 3. The chemical properties of an element depend on the number of neutrons present in the nucleus.

Answer: False

Question 4. Gold, being a soft and highly malleable metal, was used as a thin foil in Rutherford’s α-particle scattering experiment.

Answer: True

Question 5. Rutherford’s atomic model gives an idea about the velocity and energy of electrons.

Answer: False

Question 6. α-particle is bi positively charged helium ion.

Answer: True

Question 7. Neutron is absent in deuterium.

Answer: False

Question 8. The lightest fundamental particle in an atom is neutron.

Answer: False

Question 9. Atomic spectra is a discontinuous spectra.

Answer: True

Question 10. β-ray is the stream of positively charged particles.

Answer: False

Question 11. No idea about the radius of orbit of an electron can be obtained from Rutherford’s atomic model.

Answer: True

Chapter 4 Matter Bohr Rutherfords Atomic Model Synopsis

Rectification Of Rutherford’s Atomic Model:

In 1913, scientist Niels Bohr partly modified the drawbacks of Rutherford’s atomic model. This modified version is known as Bohr’s atomic model or Bohr-Rutherford’s atomic / model, which explains the idea of stationary orbits of electrons.

Basic Postulates of Bohr’s Atomic Model:

1. The electron in an atom revolves around the nucleus only in certain selected circular paths, called orbits, which are associated with definite energies.
2. When an electron revolves in such selected orbits, termed as stationary orbits, it neither emits nor absorbs energy.
3. When an electron jumps from a higher stationary energy level to a lower stationary energy level, it emits a fixed amount of energy in the form of radiations. If an electron absorbs a certain amount of energy, it moves to a higher energy level.

Limitations Of Bohr’s Atomic Model:

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

1. It fails to explain the spectra of atoms or ions having two or more electrons.
2. This is a two dimensional model, hence it is unable to provide any idea about the actual three dimensional electronic model of an atom.

Atomic Number: Total number of protons present in the nucleus of the atom of a particular element is called atomic number of that element. It is denoted by ‘T’.

Mass Number: Sum of the number of protons and the number of neutrons present in the nucleus of an atom is called the mass number of that element. It is denoted by ‘A’.

Relation Between Atomic Number And Mass Number Of An Atom: A = Z + N

where Z = atomic number, A = mass number, N = number of neutrons

Class 9 Physical Science Chapter 4 Matter

Nuclide: An atom or a nucleus characterised by a definite atomic number and a definite mass number is called a nuclide. For example, \({ }_{12}^{24} \mathrm{Mg},{ }_6^{12} \mathrm{C},{ }_8^{16} \mathrm{O}\) etc., are different nuclides.

Chapter 4 Matter Bohr Rutherfords Atomic Model Short And Long Answer Type Questions

Question 1. State the major postulates of Bohr- Rutherford atomic model.

Answer:

Postulates Of Bohr- Rutherford Atomic Model :-

The major postulates of the Bohr-Rutherford atomic model are as follows

1. Electrons do not revolve around the nucleus in any circular orbit. Instead, they revolve only in some selected orbits of definite radii.
2. While revolving along such an orbit, an electron neither emits nor absorbs any form of energy, i.e., the energy of the electron remains constant. So, these orbits are known as stationary orbits.
3. An electron absorbs or emits energy only when it jumps from one orbit to another. When an electron jumps from a higher energy orbit to a lower energy orbit, it emits energy. On the other hand, an electron jumps from a lower energy orbit to a higher energy orbit by absorbing energy.

Question 2. What are stationary orbits? Why are they calied so?

Answer:

Stationary Orbits:-

1. According to Bohr’s atomic model, the circular paths along which electrons revolve around the nucleus are called stationary orbits.
2. When an electron moves along such an orbit, it neither emits nor absorbs energy and hence the energy of the electron remains fixed. So these orbits are called stationary orbits.

Question 3. State the significance Rutherford’s atomic model.

Answer:

Significance Of Rutherford’s Atomic Model:-

According to Bohr-Rutherford’s atomic model, electrons moving in stationary orbits around the nucleus neither absorb nor emit energy continuously. Hence, the energy of electrons in those orbits remains constant. Thus, the theory successfully explains the stability of an atom which Rutherford’s theory failed to explain.

Question 4. What is meant by the ground state and excited state of an atom?

Answer:

Ground State And Excited State Of An Atom:-

Ground state: At normal condition, the revolving electrons in an atom occupy the lowest energy orbits. This state of an atom is called its ground state.

Excited state: When the electrons absorb energy from the surroundings in the form of heat or light, the electrons jump from lower energy orbits to higher energy orbits. This state of an atom is called its excited state.

Question 5. What is a nuclide? Give example.

Answer:

Nuclide:-

A nuclide is an atom or a nucleus, specified by its atomic number and mass number.

Example: \({ }_{12}^{24} \mathrm{Mg}\) or, \({ }_{12} \mathrm{Mg}^{24}\) is a nuclide of magnesium, \({ }_{17}^{35} \mathrm{Cl}\) is a nuclide of chlorine.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 6. Write the differences between mass number and atomic mass of an element.

Answer:

Class 9 Physical Science Chapter 4 Matter 

Differences Between Mass Number And Atomic Mass Of An Element:-

Mass number and atomic mass of an element have very close values. However, atomic mass differs from mass number in the following aspects

Question 7. What is the relation between atomic number and mass number of an atom?

Answer:

Relation Between Atomic Number And Mass Number Of An Atom:-

Mass number (A) of an atom = number of protons (p) + number of neutrons (n).

So, it can be said that, A = p + n

Now, the number of protons in the nucleus of an atom is the atomic number (Z) of the element.

Therefore, Z = p. Hence, A = Z+ n.

Thus, mass number = atomic number + number of neutrons.

or, atomic number = mass number – number of neutrons.

Question 8. State the similarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model.

Answer:

Similarities Between Rutherford’s Atomic Model And Bohr-Rutherford’s Atomic Model:-

Eminent scientist Niels Bohr in 1913 rectified the drawbacks of Rutherford’s atomic model. This corrected model was known as Bohr-Rutherford’s model. The similarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model are as follows

1. According to both models, the total positive charge and the entire mass of the atom is concentrated in the nucleus present at the center of the atom.
2. As per both the atomic models, negatively charged electrons revolve around the nucleus in different circular orbits.
3. Both the models suggested that the major part of an atom is vacant.
4. Both the atomic models concluded that the electrostatic force of attraction between the negatively charged electrons and positively charged nucleus and the centrifugal force acting upon the electrons due to their rotational motion are equal in magnitude.

Question 9. State the dissimilarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model.

Answer:

Dissimilarities Between Rutherford’s Atomic Model And Bohr-Rutherford’s Atomic Model:-

The dissimilarities between Rutherford’s atomic model and Bohr-Rutherford’s atomic model are as follows

Class 9 Physical Science Chapter 4 Matter Short And Long Answer Type Questions

Question 10. Write down three differences between mass number and atomic number.

Answer:

Differences Between Mass Number And Atomic Number:-

Three differences between mass number and atomic number are

Question 11. Mention the limitations of Bohr- Rutherford’s atomic model.

Answer:

Limitations Of Bohr-Rutherford’s Atomic Model:-

1. Spectra of atoms or ions having two or more electrons cannot be explained with the help of Bohr’s theory. This atomic model is applicable only for single electron system like H, He⁺, Li²⁺, Be^3+, etc.
2. Bohr-Rutherford’s atomic model fails to give any idea about the actual three-dimensional electronic model of an atom.
3. Bohr-Rutherford’s atomic model ignored the dual character (i.e., particle and wave nature) of electron while according to de-Broglie (1924) electrons have dual character (wave as well as particle nature).
4. Bohr-Rutherford’s theory cannot give any explanation for the splitting of spectral lines under the influence of magnetic field or electric field.

Question 12. Why is atomic number of an element considered as its intrinsic property?

Answer:

Atomic number of an element is considered as its intrinsic property for the following reasons

1. Two elements can never have the same atomic number.
2. Atomic number of an element = Number of protons; as proton is a fundamental particle, the fundamental characteristics of an element depend on its atomic number.
3. With the change in atomic number of an element, the nature of the element changes i.e., a new element is formed.
4. Existence of isotopes has proved that atoms of different mass numbers but same atomic number belong to the same element while existence of isobars has proved that atoms of same mass number but different atomic numbers do not belong to the same element.
5. Moseley’s experiments on X-rays have also proved that atomic number of an element is its intrinsic property.

Question 13. Complete the following table.

Class 9 Physical Science Chapter 4 Matter Short And Long Answer Type Questions

Answer:

Question 14. X^2- ion contains 10 electrons and 8 neutrons. Find the atomic number and mass number of the element. What is the name of the element?

Answer:

Given

X^2- ion contains 10 electrons and 8 neutrons.

The number of electrons in the X^2- ion is 10.

Here the atom X forms a negative ion. So, the number of electrons accepted by the atom is 2.

Hence, the number of electrons in X-atom = 10 – 2 = 8.

Now in an atom, number of protons = number of electrons.

Therefore, number of protons in X-atom = 8

So, the atomic number of the element X = 8

The number of neutrons in X-atom = 8

Therefore, mass number of the atom = number of protons + number of neutrons = 8 + 8 = 16.

As the atomic number of the element is 8, the element is oxygen.

Question 15. The number of protons and neutrons in the nuclei of two atoms A and B are given below

1. A: number of protons = 6, number of neutrons = 6
   B: number of protons s 6, number of neutrons = 8.

Find the mass number of A and B. Is there any relation between 4 and S?

Answer:

Mass number of A = number of protons + number of neutrons = 6 + 6 = 12

Mass number of B = number of protons + number of neutrons = 6 + 8 = 14

Number of protons in both the atoms is the same. Hence, the atoms A and B are isotopes of an element.

Question 16. The atomic number of chlorine is 17. What will be the atomic number of Cl^– ion?

Answer:

The Atomic Number Of Both Cl -atom And CI^– ion Will Be The Same, i.e., 17

The atomic number of an element is the number of protons present in the nucleus of the atom of that element. When Cl -atom converts to Cl^– ion, it accepts an electron but the number of protons in the nucleus remains the same. Hence, the atomic number of both Cl -atom and CI^– ion will be the same, i.e., 17.

Question 17. Can an electron absorb or emit unlimited energy? Explain.

Answer:

No, an electron cannot absorb or emit unlimited energy. It only absorbs that amount of energy which is required to jump to a higher orbit. Similarly, when an electron jumps from a higher orbit to a lower orbit, it releases energy equal to the difference in energies of the two orbits.

Question 18. How does the energy vary when an electron moves from one orbit to another?

Answer:

The Energy Vary When An Electron Moves From One Orbit To Another Is As Follows:-

According to Bohr-Rutherford’s atomic model, when an electron jumps from a higher stationary energy level to a lower one, it emits fixed amount of energy in the form of radiation. On the contrary, when an electron moves to a higher energy level from a lower one, it absorbs certain amount of energy.

Question 19. Mass number and atomic number of a nuclide of an element are 35 and 17 respectively. Calculate the number of charged and neutral particles in its nucleus.

Answer:

Given

Mass number and atomic number of a nuclide of an element are 35 and 17 respectively.

Mass number of the nuclide = 35

Atomic number of the nuclide = 17

∴ Number of charged particles (protons) in the nucleus= 17

∴ Number of neutral particles (neutrons) in the nucleus = 35 – 17 = 18

Question 20. Calculate the number of electrons and protons in H⁺ and H”.

Answer:

Atomic number of H = 1

Number of electrons in H^⊕ = 1 – 1 = 0

And number of protons in H^⊕ = 1

Class 9 Physical Science Chapter 4 Matter Short And Long Answer Type Questions

Again number of electrons in H^Θ = 1 + 1 = 2

And number of protons in H^Θ = 1

Chapter 4 Matter Bohr Rutherfords Atomic Model Very Short Answer Type Questions Choose The Correct Answer

Question 1. In an atom, there are

1. Equal number of neutrons and protons
2. Equal number of protons and electrons
3. Equal number of neutrons and electrons
4. Equal number of protons and positrons

Answer: 2. Equal number of protons and electrons

Question 2. Mass number of an atom is usually expressed by the alphabet

1. X
2. M
3. Z
4. A

Answer: 4. A

Question 3. Atomic number of an atom is usually expressed by the alphabet

1. X
2. M
3. Z
4. A

Answer: 3. Z

Question 4. The circular paths along which electrons revolve around the nucleus are known as

1. Ground state
2. Stationary orbits
3. Excited state
4. Stationary state

Answer: 2. Stationary orbits

Question 5. The lowest energy state of an atom is known as its

1. Ground state
2. Equilibrium state
3. Excited state
4. Stationary state

Answer: 1. Ground state

Question 6. By absorbing energy, an electron

1. Jumps to an outer orbit
2. Jumps to an inner orbit
3. Does not change its orbit
4. Revolves faster

Answer: 1. Jumps to an outer orbit

Question 7. Mass number of an atom indicates the total number of

1. Protons and electrons
2. Neutrons
3. Protons and neutrons
4. Protons, neutrons and electrons

Answer: 3. Protons and neutrons

Question 8. Number of neutrons present in deuterium

1. 0
2. 1
3. 2
4. 3

Answer: 2. 1

Question 9. According to Bohr’s atomic model number of stationary orbits is

1. 9
2. 5
3. 7
4. 11

Answer: 3. 7

Question 10. Mass number and number of neutrons of an atom X are 27 and 14 respectively. What is the total number of electrons in X³⁺ ion?

1. 13
2. 10
3. 14
4. 27

Answer: 2. 10

Question 11. Total number of charged particles present in 6C

1. 6
2. 8
3. 12
4. 14

Answer: 3. 12

Question 12. If the total number of electrons present in a trivalent cation is 10, its atomic number is

1. 10
2. 7
3. 13
4. 14

Answer: 3. 13

Chapter 4 Matter Bohr Rutherfords Atomic Model Answer In Brief

Question 1. Bohr’s theory is not applicable for which of the given species— H, H⁺, He⁺, Li²⁺?

Answer:

Application Of Bohr’s Theory:-

Bohr’s atomic model is applicable only for single electron system. As H⁺ does not have any electron, Bohr’s theory is not applicable for this ion.

Question 2. What is an orbit?

Answer:

Orbit:-

According to Bohr-Rutherford’s atomic model, an orbit is the circular path around the nucleus along which the electrons revolve around the nucleus in an atom.

Question 3. What is atomic number?

Answer:

Atomic Number:-

Atomic number of an atom is defined as the number of protons present in the nucleus of an atom.

Question 4. What is mass number?

Answer:

Mass Number:-

Mass number of an atom is the sum of the number of protons and neutrons present in the nucleus of an atom.

Question 5. What are the names of the Bohr orbits in increasing order of their distance from the nucleus?

Answer:

Bohr Orbits In Increasing Order:-

According to increasing order of their distance from the nucleus, the seven Bohr orbits are named as K, L, M, N, O, P, and Q orbits.

Question 6. How does the energy of the orbit change on moving away from the nucleus?

Answer: On moving away from the nucleus, the energy of the orbits gradually increases.

Question 7. How does an atom absorb or emit energy?

Answer:

An Atom Absorb Or Emit Energy As Follows:-

The absorption or emission of energy by an atom always takes place discontinuously. (Absorption or emission of energy always takes place in an integral multiple of the smallest unit of energy, called quantum)

Question 8. What happens when an electron absorbs excess energy?

Answer: When an electron absorbs excess energy, it jumps from a lower energy orbit to a higher energy orbit. (If sufficient energy is supplied, then the electron may be removed from the atom.)

Question 9. Which theory forms the basis of Bohr- Rutherford’s atomic model?

Answer: Quantum theory.

Question 10. What is the minimum number of orbits that an atom can have?

Answer: An atom may have a minimum 1 orbit.

Question 11. Write down the relation between atomic number and mass number.

Answer: Mass number = Atomic Number + Number of neutrons.

Question 12. There are 11 protons in an atom. Find the number of electrons in the uni-positive ion of that atom.

Answer: The atom contains 11 protons. So the number of electrons in the uni-positive ion of that atom will be 10.

Question 13. If an atom contains 8 protons, then find the number of electrons in the di-negative ion of that atom.

Answer: There are 8 electrons in the said atom. Hence, the di-negative ion of that atom will contain 10 electrons.

Question 14. What do 13 and 27 signify in the symbol \({ }_{13}^{27} \mathrm{Al}\)?

Answer: Here, 13 stands for the atomic number and 27 standsforthe mass number of aluminium.

Question 15. State whether the outermost electron in \({ }_3 \mathrm{Li}\) atom will absorb or release energy when it jumps to the third orbit.

Answer: The third electron in \({ }_3 \mathrm{Li}\) atom is present in the second orbit. So, when it jumps to the third orbit, it will absorb energy.

Question 16. What is expressed by the symbol, \({ }_8^{16} 0\)?

Answer: \({ }_8^{16} 0\) represents a nuclide of oxygen whose atomic number is 8 and mass number is 16.

Question 17. Number of electrons and neutrons in a bivalent anion are 18 and 20 respectively. What is the mass number of the corresponding atom?

Answer: Number of electrons in the corresponding atom = 18 – 2 = 16

∴ Number of protons in the atom = 16

∴ Mass no. of the atom = 16 + 20 = 36

Question 18. How many charged particles are present in an \({ }_9^{19} F\)-atom?

Answer: 18 (9 protons and 9 electrons).

Question 19. \({ }_6^{12} C\) and \({ }_6^{14} C\) differ in number of which sub atomic particle?

Answer: Neutrons.

Number of neutrons in \({ }_6^{12} C\) and \({ }_6^{14} C\) are 6 and 8 respectively.

Question 20. Number of which particle is same in \({ }_{15} \mathbf{p}^{31}\) and \({ }_{16} S^{32}\)?

Answer: It is neutron whose number is same (16) in \({ }_{15} \mathbf{p}^{31}\) and \({ }_{16} S^{32}\).

Question 21. What is the total number of charged particles present in Ca²⁺ ion?

Answer: 38 (number of protons and electrons are 20 and 18 respectively).

Question 22. Write down the nuclide consists of 8 neutrons and 6 electrons.

Answer: Atomic no. = no. of protons = no. of electrons = 6,

Mass no. = (n + p) = (8 + 6) = 14

∴ The corresponding nuclide is \({ }_6^{14} C\).

Question 23. Write down the number of neutrons and electrons present in \({ }_{92}^{235} \mathrm{U}^{3+}\).

Answer: Number of neutrons =235 – 92 = 143

Number of electrons = 92 – 3 = 89

Class 9 Physical Science Chapter 4 Matter Short And Long Answer Type Questions

Question 24. What is meant by \({ }_{11} \mathrm{Na}^{23}\)?

Answer:

\({ }_{11} \mathrm{Na}^{23}\)

It stands for the nuclide of sodium atom.

Question 25. Calculate the number of electrons in a binegative ion of an atom containing 8 protons.

Answer: Number of electrons in the atom = Number of protons in it = 8. [As the atom is neutral]

∴ Number of electrons in the bi-negative ion = 8 + 2 = 10

Question 26. Mention the charge of the sample containing 6 protons, 8 electrons, and 6 neutrons.

Answer: Charge of the sample will be (6-8) = -2.

Question 27. Give example of an atom whose mass number and atomic number are same.

Answer: It is protium (\({ }_1 H^1\)) where atomic number = mass number.

Chapter 4 Matter Bohr Rutherfords Atomic Model Fill In The Blanks

Question 1. The number of neutrons in ₁₁Na²³ is _______

Answer: 12

Question 2. Maximum number of electrons that can be present in a principal energy level is ________

Answer: 2n²

Question 3. Bohr-Rutherford atomic model is ________ in nature.

Answer: Two dimensional

Question 4. Atomic number is always a _______ number.

Answer: Whole

Question 5. The circular path along which an electron revolves around the nucleus is known as _______

Answer: Orbit

Question 6. The orbits whose energy remain fixed are called ________ orbits.

Answer: Stationary

Question 7. When an electron jumps to a higher energy level by absorbing energy, it is said that the atom is in its _________ state.

Answer: Excited

Question 8. If an electron revolves in a selected orbit, it neither ________ nor _______ energy.

Answer: Emits, absorbs

Question 9. Atomic number indicates the number of _________ in an atom.

Answer: Protons

Question 10. Mass number = Number of ________ + Number of ________

Answer: Protons, neutrons

Chapter 4 Matter Bohr Rutherfords Atomic Model State Whether True Or False

Question 1. The atomic number of the element M is 13. Thus, a number of electrons in M^3- ion will be 10.

Answer: False

Question 2. Bohr’s atomic model is applicable for multi-electron systems.

Answer: False

Question 3. Bohr’s atomic model is unable to give an idea about the actual three-dimensional electronic model of an atom.

Answer: True

Question 4. Mass number = Number of protons + Number of electrons.

Answer: False

Question 5. An electron neither emits nor absorbs energy while moving along a stationary orbit.

Answer: True

Question 6. In \({ }_{15} p^{31}\) the atomic number of the respective element is 31.

Answer: False

Question 7. Atomic number of an element is the fundamental property of that element.

Answer: True

Question 8. \({ }_8^{16} 0\) is an example of nuclide.

Answer: True

Question 9. According to Bohr’s theory electrons can revolve in any circular orbit around the nucleus.

Answer: False

Question 10. Electrons do not emit energy during revolving in stationary orbit.

Answer: True

Question 11. Atomic number and mass number of protium is equal.

Answer: True

Question 12. Niels Bohr rectified the limitations of Rutherford’s model.

Answer: True

∴ Total mechanical energy at point B, \(E^{\prime \prime}=E_k^{\prime \prime}+E_p^{\prime \prime}\)

and potential energy, E’_p =mg(h – h₁)

Chapter 5 Power Synopsis

Power is the rate of work done with respect to time, i.e., power is the work done in unit time. Power is a scalar quantity.

If W amount of work is done by a body in time t, then the power, P = \(\frac{W}{t}\).

Dimensional formula of power is ML²T^-3

In CGS system and SI, the absolute units of power are erg/s and watt (W or J/s), respectively.

1 horse power is given by, 1hp = 550 ft • lb • s^-1 = 746 W

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

Chapter 5 Power Short And Long Answer Type Questions

Question 1. Define power. What is the relationship between power and work done?

Answer:

Power:-

Power is defined as the rate of doing work with respect to time.

If the amount of work W is done in time t, then power, P = \(\frac{W}{t}\).

Question 2. What is the dimensional formula of power? What are the dimensions of power?

Answer:

Dimensional Formula Of Power:-

= \(\frac{\text { dimension expression of work }}{\text { dimension expression of time }}=\frac{\mathrm{ML}^2 \mathrm{~T}^{-2}}{\mathrm{~T}}\)

= ML²T^-3

Dimensions of power are 1 in length, 2 in mass, and -3 in time.

Question 3. What is the relationship between power and velocity?

Answer:

Relationship Between Power And Velocity:-

We know, power = \(\frac{\text { work }(W)}{{time}(t)}\)

Suppose a small displacement s of a body takes place in the direction of a constant force F while being acted on the body for a time t.

∴ work done, W = Fs

So, power, P = \(\frac{W}{t}=\frac{F s}{t}\) = Fv, where v is the instantaneous velocity and time interval t is very small.

Question 4. Define absolute units of power in CGS system and SI.

Answer:

Absolute Units Of Power In CGS System And SI:-

Absolute unit of power in CGS system is erg/s.

1erg/s: 1 erg/s is the power of doing lerg of work in 1 second.

Absolute unit of power in SI is J/s or watt.

1 watt: 1 watt is the power of doing 1 joule of work in 1 second.

Question 5. Suppose a man walks a distance of 3 m in a horizontal path in 5 s with a bucket of water in hand. Then he climbs up 3 m in a staircase in 5 s with the same bucket. Explain in which case, the man has more power.

Answer:

Given

Suppose a man walks a distance of 3 m in a horizontal path in 5 s with a bucket of water in hand. Then he climbs up 3 m in a staircase in 5 s with the same bucket.

While walking in a horizontal plane with a bucket in hand, the force of graving acts in a direction opposite to the weight of the bucket. But the displacement of the bucket is horizontal, i.e., in a direction perpendicular to the force applied on the bucket.

Hence, this is a no-work force. As the amount of work is zero, the value of power is also zero. But while going up the staircase with the bucket, work is done against gravity. As a result, value of power is not zero. Hence, power is more in the second case.

Question 6. What do you mean by kilowatt and horsepower?

Answer:

Kilowatt And Horsepower:-

Kilowatt: 1 kW (kilowatt) is defined as the power of doing 1000 J of work in Is. So, lkW = 1000W.

Horsepower: 1 horsepower is defined as the power of doing 550ft. ib of work by any machine or by a system in 1s.

∴ 1 horsepower = 550 ft • lb • s^-1 = 746 W

Question 7. The power of an engine is 5 horsepower. What do you mean by the above statement?

Answer:

The power of an engine is 5 horsepower.

As 1 hp = 746 W, 5 hp = 5 x 746 = 3730 W.

That is, the power of the engine is 3730 W. This means that the engine can do a work of 3730 J per second.

Question 8. Sand is falling from the lower portion of a sand-filled moving truck. How does the kinetic energy of this truck change if the power spent by the engine remains unchanged?

Answer:

Given

Sand is falling from the lower portion of a sand-filled moving truck.

Power of the engine of the moving truck

= \(\frac{\text { work done by the engine }(W)}{\text { time of work done }(t)}\)

= \(\frac{\text { force } \times \text { displacement }}{\text { time }}\)

= \(\frac{\text { weight of the truck } \times \text { displacement }}{\text { time }}\)

= \(\text { weight of the truck } \times \text { average velocity }\)

= mg x v

In this case, due to the fall of sand from the truck, its mass (m) decreases gradually. But according to the question, in order to keep the power spent by the engine constant, the velocity (v) of the truck has to be increased in the same proportion.

Thus, the value of the quantity mg x v remains constant.

∴ mg x v = constant

or, mv = constant [g = constant]

Again, kinetic energy of the truck

= 1/2 mv² = 1/2(mv) x v

Since v increases in spite of mv remaining constant, hence the kinetic energy of the truck increases in this case.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Chapter 5 Power Very Short Answer Type Questions Choose The Correct Answer

Question 1. The product of applied force on a body with its speed gives a measure of its

1. Power
2. Energy
3. Work
4. Momentum

Answer: 1. Power

Question 2. Horsepower is the unit of which of the following physical quantities?

1. Work
2. Power
3. Kinetic energy
4. Potential energy

Answer: 2. Power

Question 3. Amount of work done by a machine of 1000W power in 1 minute is

1. 60000 J
2. 1000 J
3. 600 J
4. 100 J

Answer: 1. 60000 J

Question 4. A body is lifted from the ground slowly to a certain height in the first case. Again in the second case, the same body is lifted more rapidly to the same height from the ground. Which of the following statements is true?

1. Though the amount of work done is same in both the cases, amount of power is higher in the first case
2. Though the amount of work done is same in both the cases, amount of power is higher in the second case
3. In both the cases, work and power are different
4. In both the cases, work and power are same

Answer: 2. Though the amount of work done is same in both the cases, amount of power is higher in the second case

Question 5. 100W is written on an electric bulb. W is the unit for which quantity?

1. Electrical energy
2. Electrical power
3. Electrical work
4. Mechanical energy

Answer: 2. Electrical power

Question 6. Dimensional formula of power is

1. MLT^-3
2. ML²T³
3. ML²T^-2
4. ML²T^-3

Answer: 4. ML²T^-3

Question 7. Relationship between work (W) and power (P) is

1. P = Wt
2. P = W/t
3. P = \(\frac{W}{t^2}\)
4. P = \(\sqrt{W t}\)

Answer: 2. P = W/t

Question 8. The power of a pump is 490 W. How much time is required to raise 400 L of water to a height of 15 m using this pump?

1. 60s
2. 90s
3. 120s
4. 30s

Answer: 3. 120s

Question 9. Unit of power in CGS system is

1. J/s
2. erg/s
3. W
4. erg.s

Answer: 2. erg/s

Question 10. Power of the engine of a motorbike is generally expressed in which unit?

1. Horsepower
2. Watt
3. Joule/second
4. erg/second

Answer: 1. Horsepower

Question 11. Which of the following physical quantities has joule/hour as its unit?

1. Work
2. Kinetic energy
3. Force
4. Power

Answer: 4. Power

Question 12. A boy of mass 40 kg climbs up 40 steps of a staircase in 20 s. Each step is 20 cm high. What is the power applied by the boy? (g = 9.8 m/s²)

1. 156.7 erg/s
2. 156.8 J/s
3. 165.84 W
4. 165.84 erg/s

Answer: 2. 156.8 J/s

Question 13. Rate of doing work by a body with respect to time is called

1. Force
2. Power
3. Energy
4. Linear momentum

Answer: 2. Power

Question 14. If Y = \(\frac{\text { work }}{\text { power }}\), then the dimensional formula of Y is

1. M⁰L⁰T⁰
2. M⁰L²T⁰
3. M⁰L²T¹
4. T

Answer: 4. T

Question 15. Compared to one kilowatt (kW), one horsepower (hp) is

1. More
2. Less
3. Same
4. Cannot be said

Answer: 2. Less

Question 16. The relationship between horsepower and watt is

1. 1hp = 476 W
2. 1hp = 764 W
3. lhp = 746 W
4. lhp = 674 W

Answer: 3. lhp = 746 W

Question 17. If Z = \(\frac{\text { work }}{\text { power }}\) work , then which quantity is represented by Z?

1. Kinetic energy
2. Potential energy
3. Time
4. Linear momentum

Answer: 3. Time

Question 18. A train moves against a frictional force of 5000 N at a velocity of 25 m/s. What is the power of the engine of the train?

1. 1250 W
2. 125000 J/s
3. 1.25 X 106W
4. 12.5W

Answer: 2. 125000 J/s

Question 19. A boy does a work of 400 erg in 5 s and a girl takes 10 s to do the same work. Which of the following statements is correct?

1. The boy has more power
2. The girl has more power
3. Both have the same power
4. None of the above

Answer: 1. The boy has more power

Question 20. Which of the following physical quantity have the unit J/h?

1. Work
2. Kinetic energy
3. Force
4. Power

Answer: 4. Power

Question 21. Work done = power x

1. Velocity
2. Speed
3. Time
4. Displacement

Answer: 3. Time

Question 22. Which one of the following is not the unit of power?

1. erg/s
2. W
3. W.h
4. J/s

Answer: 3. W.h

Chapter 5 Power Answer In Brief

Question 1. What is the unit of power in SI?

Answer: Watt is the unit of power in SI.

Question 2. What is the relationship between work and power?

Answer: Relationship between work and power is expressed by power = \(\frac{work}{time}\)

Question 3. What is the relationship between power and velocity?

Answer: The relationship between power and velocity is expressed by power = applied force x velocity of the body

[In this case, velocity of the body means instantaneous velocity.]

Question 4. Horsepower is the unit of which quantity?

Answer: Horsepower is the practical unit of power in FPS system.

Question 5. 1 horsepower = how many W?

Answer: 1 horsepower = 746 W.

Question 6. 1 kW = how much horsepower?

Answer: 1 kW = 1.34 horsepower

Question 7. What do you mean by 1 kilowatt of power?

Answer:

1 kilowatt of power

One kW (kilowatt) is defined as the power required to do a work of 1000J in one second, i.e., 1 kW = 1000W.

Question 8. The power of a pump is 1.2 kW. What do you mean by the above statement?

Answer: The power of a pump is 1.2 kW means that the pump can do a work of 1.2 kJ or 1200 J in one second.

Question 9. What is average power?

Answer:

Average power

Average power of is defined as the ratio of total work done to the total time taken.

i.e., average power = \(\frac{\text { total work done }}{\text { total time }}\)

Chapter 5 Power Fill In The Blanks

Question 1. Division of the unit of work by the unit of power gives the unit of _______

Answer: Time

Question 2. Multiplication of the unit of force by the unit of ________ gives the unit of power.

Answer: Velocity

Question 3. Power x _________ = work.

Answer: Time

Question 4. Power is a ______ physical quantity.

Answer: Scalar

Question 5. Power of a motor is 373 W. In hp unit the power of the motor is ________

Answer: 0.5

Question 6. Power of an agency depends upon how fast ______ is done by it.

Answer: Work

Chapter 5 Power State Whether True Or False

Question 1. Power is a scalar quantity.

Answer: True

Question 2. Power is the capacity of a body to do work.

Answer: False

Question 3. 1 horsepower = 746 kW.

Answer: False

Question 4. Rate of work done is power.

Answer: True

Question 5. Horsepower is a practical unit of power.

Answer: True

Question 6. The power of a pump is 1 kW means that the pump can perform 1000 J work in 1 s.

Answer: True

Chapter 5 Power Numerical Examples

Useful Information

1. If W amount of work in done in time t, then W power P = \(\frac{W}{t}\).
2. If the force applied on a body moving with velocity v be F then, power P = F x v
3. If m mass is lifted to a height h against gravity in time t then power P = \(\frac{mgh}{t}\)

Question 1. How much energy is required to lift 200 L of water every minute to a height of 15 m ? [Mass of 1L of water is 1 kg]

Answer:

Mass of 200 L of water, m = 200 kg

Work W to be done to raise 200 kg of water to h = 15 m against gravity, W = mgh

Time, t = 1 min (=60 s) is required to do this work.

∴ power, P = \(\frac{W}{t}\) = \(\frac{mgh}{t}\)

= \(\frac{200 \times 9.8 \times 15}{60}\) J/s = 490 w

Question 2. The power of a man is 6W. How much work does he perform in 10 minute?

Answer:

Given

The power of a man is 6W.

Power of the person P = 6W, time to do work t= 10 x 60 = 600s

∴ Work done by the person W = P x t =6 X 600 = 3600 J

Question 3. A boy of mass 30 kg can go up 20 steps, each of height 10 cm, in 25 s. Calculate the power of the boy.

Answer:

Given

A boy of mass 30 kg can go up 20 steps, each of height 10 cm, in 25 s.

Mass of the boy, m = 30 kg

So, his weight = mg = 30 x 9.8 N

Height of 20 steps,

h = 10 x 20 cm = 200 cm = 2m

Work done by the boy,

W = mgh =30 x 9.8 x 2J = 588 J

This amount of work is done in t = 25 s

∴ power of the boy,

P = \(\frac{W}{t}\) = \(\frac{588}{25}\)J/s = 23.52W

Question 4. The power of a dump is 2kW. How much time does it take to fill up a tank of 800 L capacity kept at a height of 12 m with its help? [Mass of 1 L of water is 1 kg]

Answer:

Given

Power of the pump, P = 2 kW = 2000 W

Mass of 800 L of water = 800 kg

Work done to raise a mass of m = 800 kg to a height of h = 12 m against gravity,

W = mgh = 800 x 9.8 x 12J

∴ if t time is required to fill up the tank, then

t = \(\frac{W}{p}=\frac{800 \times 9.8 \times 12}{2000}=47.04 \mathrm{~s}\)

Question 5. A pump is lifting 600 kg of water per minute to a height of 20 m. If the efficiency of the pump is 80%, what is its power?

Answer:

Given

A pump is lifting 600 kg of water per minute to a height of 20 m. If the efficiency of the pump is 80%,

Suppose, power of the pump = P

Efficiency of the pump =80%

∴ effective power of the pump,

\(P_1=\frac{80}{100} P=0.8 P\)

With the help of the pump, a mass (m) of 600 kg of water is raised to a height of h = 20 m in t = 1 min = 60s.

So, \(P_1=\frac{m g h}{t} \text { or, } 0.8 P=\frac{m g h}{t} \text { or, } P=\frac{m g h}{0.8 t}\)

∴ P = \(\frac{600 \times 9.8 \times 20}{0.8 \times 60}=2450 \mathrm{~W}=2.45 \mathrm{~kW}\)

Question 6. A car is moving on a rough horizontal road with a uniform velocity. Friction of the road is 200 N and the power of the engine is 2 kW. Calculate the velocity of the car in km/h units.

Answer:

Given

A car is moving on a rough horizontal road with a uniform velocity. Friction of the road is 200 N and the power of the engine is 2 kW.

Friction of the road, F = 200 N

Power of the engine, P = 2 kW = 2000 W

Suppose, velocity of the car = v

So, P = f x v

∴ v = \(\frac{P}{F}=\frac{2000}{200}=10 \mathrm{~m} / \mathrm{s}\)

= \(10 \times \frac{18}{5} \mathrm{~km} / \mathrm{h}\)

[1 m/s = 18/5 km/h]

= \(36 \mathrm{~km} / \mathrm{h}\)

Question 7. A motor car moves with velocity 36 km/h by applying an average force of 20N. Find power of the car.

Answer:

Given

A motor car moves with velocity 36 km/h by applying an average force of 20N.

Velocity of the car,

v = 36 km/h = \(\frac{36000 \mathrm{~m}}{3600 \mathrm{~s}}\) = 10 m/s

Force applied by the car F = 20 N

∴ the power of the car P = F x v = 20 x 10 = 200 W

Question 8. The work done by a human heart is 11 per beat. Calculate power of the heart if it beats 72 times in a minute.

Answer:

Given

The work done by a human heart is 11 per beat.

Work done by the heart in 72 beats

W = 1 x 72 = 72 J

∴ power of the heart

P = \(\frac{W}{t}=\frac{72 \mathrm{~J}}{60 \mathrm{~s}}=1.2 \mathrm{~W}\)

Chapter 5 Work Synopsis

1. When on object is displaced from its initial position under the action of a force, then work is said to be done. Work is a scalar quantity.
2. If d is the displacement of the object and F is the force applied, then work done, W=Fd.
3. During application of force on a body, if the body gets displaced in the direction of the applied force, then work is said to be done by the force. This is also called positive work.
4. During application of force on a body, if the body gets displaced in the direction opposite to that of the applied force, then work is said to be done against the force. This is also called negative work.
5. When the displacement of an object is at an angle of 90° with the direction of force, then no work is done by the applied force. This type of force is called no-work force.
6. The dimensional formula of work is ML²T^-2.
7. In CGS system and SI, the absolute units of work are erg and joule or 1 respectively.
8. 1J = 1N x1m= 10⁵ dyn . 100 = 10⁷ erg.

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

Chapter 5 Work Short And Long Answer Type Questions

Question 1. Define work. How is the quantity of work measured?

Answer:

Work And Its Quantity Measurements :-

1. When an object is displaced from its initial position under the action of a force, then work is said to be done.
2. If applying a force F on a body displaces it by a distance d in the direction of the force, then work done, W = Fd.

Question 2. Define absolute units of work in CGS system and SI. Establish a relationship between the two units.

Answer:

Absolute Units Of Work In CGS System And SI And Its Relationship:-

The absolute unit of work in CGS system and SI are erg and joule, respectively.

1 erg: 1 erg is defined as the work done when a force of one dyne (dyn) applied to a body displaces it by 1cm in the direction of the force.

1 erg = 1 dyn x 1 cm = 1 dyn • cm

1 Joule: 1 joule is defined as the work done when a force of 1 N (newton) applied to a body displaces it by 1 metre (m) in the direction of the force.

1J = 1N x lm = 1N • m

Relationship Between Joule (J) And Erg:

1J = 1N x 1 m = 10⁵ dyn x 10² cm = 10⁷ erg

Question 3. What do you mean by work done by a force or positive work? Explain with examples.

Answer:

Work Done By A Force Or Positive Work Means:-

During the application of force on a body, if the body gets displaced in the direction of the applied force, then work is said to be done by the force. Since force and displacement are in the same direction, the value of this work is positive. Hence, it is called positive work.

For example, when a book is raised upward from the ground, work is said to be done by the force and is positive as the displacement of the book takes place in the same direction as that of the applied force.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 4. What do you mean by work done against a force or negative work? Explain With examples.

Answer:

Work Done Against A Force Or Negative Work Means:-

During application of force on a body, if the body gets displaced in the direction opposite to that of the applied force, then work is said to be done against the force. Since applied force and displacement are in the opposite directions, the value of this work is negative. Hence, it is called negative work.

For example, when a book is lowered slowly with constant velocity, an upward force is applied on the book at every moment. In this case, work is said to be done against the force and it is negative work as the displacement of the book takes place in the opposite direction as that of the applied force.

Question 5. What is a no-work force? Give two examples of no-work force.

Answer:

No-Work Force And Its Example:-

When the displacement of an object takes place in a direction perpendicular to the direction of the force, then no work is done by the applied force. This type of force is called no-work force.

Examples:

A man walks along a horizontal path with a suitcase in his hands. Force of gravity acts downward on the suitcase, but in a direction perpendicular to the displacement of the suitcase. Here, force of gravity is a no-work force.

Let us assume that due to the gravitational force of the sun, the earth is moving in circular orbits around the sun. At any position of the earth, the gravitational force acts along the radius towards the centre of the orbit and the direction of displacement is along the tangent at that point of the circular path.

This force of the sun on the earth is a no-work force since the direction of displacement of the earth at every point of its orbit is perpendicular to the force.

Question 6. A stone attached to a thread, is rotated around a finger in a horizontal plane. Which force is working in which direction due to this rotation? What is the direction of the displacement of the stone at any moment? Give reasons in favour of your answer. Explain how much work is done by you due to this rotation.

Answer:

A strong pull is felt by the finger due to rotation of the stone. The faster this stone is rotated, the more this pull is experienced. The stone rotates due to the pull toward the centre along the radius of the circle (along the thread in this case). This I force is called centripetal force.

It can be said from experience that if the thread breaks during rotation by any means, then no force is applied on the stone through the thread. The stone is dislodged tangentially from the circular path. This means that during revolution, the direction of displacement of the stone is always tangential to the circle.

The pull of the thread is along the radius of the circle and the displacement of the stone is tangential to the circle. This means that applied force and displacement are perpendicular to each other. Hence, this force on the stone is a no-work force.

Question 7. A man is walking up an inclined plane with a box in his hand. Is the gravitational force doing any work on the box?

Answer:

Given

A man is walking up an inclined plane with a box in his hand.

It is shown how gravity attracts the box in a straight downward direction.

As a result, a component of gravity, W₂ works downward along the inclined plane. But the displacement of the box takes place in upward direction along the inclined plane. So, gravity is doing positive work in this case.

Question 8. In a tug-of-war game, both the sides pull the rope with equal strength in opposite directions. What is the amount of work done by each side?

Answer:

In a tug of war game, both the sides pull the rope with equal strength but there is no displacement of the rope towards either side. As there is no displacement of the point of application of force, no work is done by the applied force and hence, the work done by each side is zero.

Question 9. In a tug-of-war game between teams A and B, team A which applies a force of 10 N is defeated by team B which applied a force of 12 N. In this case, which team has done positive work and which team has done negative work?

Answer:

In a tug-of-war game, team B has won. This means that team B has applied comparatively more force than team A. So, displacement of team A has taken place towards team B.

In this case, point of application of force i.e., displacement of team A has taken place towards direction of force applied by team B . Hence, team B has done positive work.

On the other hand, team A has also applied a force of 10 N on team B. In spite of that, displacement has taken place towards team B. Actually, team B has slide backwards in the game.

Hence, it may be said that displacement of team B was brought about by team A against the direction of force. So, team A has done negative work.

Question 10. A motor car is running on a horizontal road with uniform velocity. Is the engine of the car doing work in this condition?

Answer:

Given

A motor car is running on a horizontal road with uniform velocity.

Friction of the ground works against the motion of the car when it moves on a horizontal plane. To maintain the uniform velocity of the car, its engine applies a force equal to the frictional force in the direction of movement and for the displacement of the car against friction, the engine of the car works.

If the velocity of car=v; is frictional force = F, then work done by the engine per second = Fv.

Question 11. A man walks to the top of a hill along a steep path. Another man having the same mass as the first one goes to the top via a complex route. Which individual does greater amount of work?

Answer:

Given

A man walks to the top of a hill along a steep path. Another man having the same mass as the first one goes to the top via a complex route.

Work done by an object or an individual is the product of the force applied on it and the displacement of the object or individual.

As the masses of the two men are the same in this case, so their weights are also same. This means that each man has applied the same force against his weight to climb the hill.

Again, displacement is defined as the linear distance between the initial and final positions of the moving object or individual. In this case, displacements for both the men are the same as they have climbed the top of the hill from the ground level. As a result, both have done the same work.

[Note: Frictional force has not been taken into account in the given case. In reality, different amounts of frictional forces act on them while climbing the hill. Hence, the amount of work done by the two men is different in that case.]

Question 12. A man is swimming against the current in such a way that he is stationary with respect to the shore. Is he doing any work?

Answer:

Given

A man is swimming against the current in such a way that he is stationary with respect to the shore.

As there is no displacement with respect to the shore, no work is done with respect to the shore. But there is displacement with respect to the river water, so work done by the man is not zero with respect to water.

Chapter 5 Work Very Short Answer Type Questions Choose The Correct Answer

Question 1. Which of the following vehicles does more work to traverse a fixed distance in the same path carrying the same weight?

1. Bullock cart
2. Bicycle
3. Horse cart
4. Same work is done by all the vehicles

Answer: 4. Same work is done by all the vehicles

Question 2. Unit of work in SI is

1. Watt
2. Erg
3. Joule
4. Dyne

Answer: 3. Joule

Question 3. In a tug of war,

1. The defeated team does positive work
2. The defeated team does negative work
3. The victorious team does negative work
4. No work is done by either of the teams

Answer: 2. The defeated team does negative work

Question 4. A body makes a complete round in a circular path and during its journey, average applied force on it was F, r being the radius of the circular path. What is the total amount of work done by the body?

1. Zero
2. 2πrF
3. 2πF
4. πrF

Answer: 1. Zero

Question 5. Gravitational force is a no-workforce. This is because

1. Displacement of the body does not occur in the direction of gravitational force
2. Displacement of the body occurs in a direction perpendicular to the direction of gravitational force
3. Displacement of the body occurs in the direction opposite to that of gravitational force
4. There is no displacement of the body

Answer: 2. Displacement of the body occurs in a direction perpendicular to the direction of gravitational force

Question 6. A body starts its journey from point A, traverses a path ACB and reaches point B. Average applied force, F acts on the body during the entire course of its journey. If the path ACB is semicircular with a radius r, the amount of work done by the body is

1. Zero
2. πrF
3. 2πrF
4. 2rF

Answer: 4. 2rF

Question 7. A body held in hand is brought downward. Here, work is done against the force because

1. The direction of applied force is downward
2. The direction of gravitational force on the body is downward
3. The direction of displacement of body is downward
4. The direction of force applied on the body is upward but displacement of the body is downward

Answer: 4. The direction of force applied on the body is upward but displacement of the body is downward

Question 8. Dimensional formula of work is

1. MLT^-2
2. ML²T^-3
3. ML²T^-2
4. MLT^-3

Answer: 3. ML²T^-2

Question 9. Application of 200 dyn force displaces a body 4 m along the direction of force. Work done is

1. 0.004 J
2. 0.008 J
3. 0.08 J
4. 800 J

Answer: 2. 0.008 J

Question 10. Work done to raise a mass of 4 kg to a height of 2 m against gravity (g = 9.8 m/s²) is

1. 79.4J
2. 39.2J
3. 72.4J
4. 78.4J

Answer: 4. 78.4J

Question 11. Example of no-work force is

1. Frictional force
2. Centripetal force
3. Surface tension
4. All of the above

Answer: 2. Centripetal force

Question 12. Work done by winding the spring of a manual-winding watch is stored in the watch as

1. Sound energy
2. Kinetic energy
3. Potential energy
4. Light energy

Answer: 3. Potential energy

Question 13. The relationship between joule and erg is

1. 1J = 10⁵erg
2. 1J = 10⁷erg
3. 1J = 10¹⁰erg
4. 1J = 10 erg

Answer: 2. 1J = 10⁷erg

Question 14. Suppose you start racing from a fixed position in a field and come back to the same place. What is the total work done?

1. Zero
2. Positive quantity
3. Negative quantity
4. None of these

Answer: 1. Zero

Question 15. Two teams P and Q participate in a tug of war contest and pull the rope with the same force. Which team does more work?

1. TeamP
2. TeamQ
3. No work is done by either team
4. None of the above

Answer: 3. No work is done by either team

Question 16. A man drags a block 4 m on the floor. Friction force being 100 N, work done against friction is

1. 40 J
2. 0.4 J
3. 400 J
4. 4000 J

Answer: 3. 400 J

Question 17. A man weighing 40 kg, carrying a box of mass 10 kg, climbs up 10 steps of a staircase. If the height of each step is 20 cm, what is the amount of work done? [g = 10m/s²]

1. 1000J
2. 980J
3. 900J
4. 1100J

Answer: 1. 1000J

Question 18. Suppose you start racing from a fixed position in a field and come back to the some place. What is the total work done?

1. Zero
2. Positive quantity
3. Negative quantity
4. Unity

Answer: 1. Zero

Question 19. The angle between the applied force and displacement in the case of no work force is

1. 0°
2. 45°
3. 90°
4. 180°

Answer: 3. 90°

Question 20. The work done on an object does not depend upon the

1. Force applied
2. Initial velocity
3. Displacement
4. Angle between force and displacement

Answer: 2. Initial velocity

Chapter 5 Work Answer In Brief

Question 1. Product of two vector quantities is a scalar quantity. Give an example.

Answer: Force and displacement are vector quantities but their product, work is a scalar quantity.

Question 2. Give an example of no-work force.

Answer:

Example of no-work force

Centripetal force is a no-work force.

Question 3. What is the dimensional formula of work?

Answer: Dimensional formula of work is ML²T^-2.

Question 4. What is the absolute unit of work in SI?

Answer: The absolute unit of work in SI is joule (J).

Question 5. What is the absolute unit of work in CGS system?

Answer: The absolute unit of work in CGS system is erg.

Question 6. 1J = how many erg?

Answer: 1J = 10⁷erg

Question 7. What is the angle between force and displacement in case of no-work force?

Answer: In case of no-work force, angle between force and displacement is 90°.

Question 8. How much work is done by a weight-lifter when he stands with a weight above his head?

Answer: As the weight-lifter stands still with the weight above his head without any displacement, no work is done by him.

Question 9. The earth moves around the sun due to the force exerted by the sun on the earth. Is any work being done by the sun due to this motion of the earth?

Answer: No, force of attraction or gravitational force of the sun on earth is a centripetal force which is a no-work force.

Question 10. Does work depend on the speed with which a body is raised upward?

Answer: No, work done depends on displacement of the body and not on the speed with which the body is raised upward.

Question 11. Apart from no-work force, is it possible for the value of work done to be zero in any other case?

Answer: Yes, if the displacement of a body is zero even after the application of force, amount of work done is zero.

Question 12. What is the value of work related to potential energy in a place where gravity is zero?

Answer: Value of work related to potential energy in a place of zero gravity, is zero.

Question 13. In a tug of war game, which side does work when the rope remains motionless?

Answer: As there is no displacement of the rope, none of the sides is doing any work.

Question 14. A book is raised upward with uniform velocity. In this case, is work being done against the force or by the force?

Answer: As the displacement of the book is in the direction of the applied upward force, work is being done in favour of the force.

Question 15. A book is lowered downward with uniform velocity. In this case, is work being done against the force or by the force?

Answer: As the displacement of the book is downward but the book is held using an upward force, work is done against the force.

Question 16. A book held in hand, is displaced horizontally with uniform velocity. What is the relationship between force and work in this case?

Answer: In this case, the applied force is a no-work force.

Question 17. Can we recover the work done under a force?

Answer: Work done can be recovered if it is done under a force where mechanical energy is conserved.

Question 18. A stone is thrown upward. Is work being done by or against the force of gravity?

Answer: In this case, work is done against the force of gravity since displacement of the stone is upward but gravitational force acts downward.

Question 19. When is work done by a force positive?

Answer: Work done by a force is positive when the displacement of the body is in the direction of the force.

Question 20. When is work done by a force negative?

Answer: Work done by a force is negative, when the displacement of the body is in the direction opposite to the direction of the force.

Question 21. How much work is done by a weight-lifter when he stands with a weight above his head?

Answer: If the weighter-lifter stands still with the weight above his head without any displacement, no work is done by him.

Question 22. Does work depend on the speed with which a body is displaced?

Answer: No, work done depends on displacement of the body, not on the speed with which the body is displaced.

Question 23. What is the work done by the force of gravity on a satellite moving round the earth?

Answer: The force of gravity acts at right angle to the displacement of the satellite, so work done is zero.

Chapter 5 Work Fill In The Blanks

Question 1. Work is a ______ quantity.

Answer: Scalar

Question 2. The applied force does not do any work if the angle between force and displacement is ________

Answer: 900

Question 3. Centripetal force is _______ force.

Answer: No Work

Question 4. Any car traversing the same path carrying the same weight does the same amount of ______

Answer: Work

Question 5. Negative work denotes that work is done ________ the force.

Answer: Against

Question 6. A boy fails to lift a water-filled bucket in spite of his attempts. Amount of work done by the boy is ________

Answer: Zero

Question 7. Work done by the string of a simple pendulum during oscillation is _______

Answer: Zero

Question 8. _________ work is done when a mass is taken up with constant velocity by holding it in hand.

Answer: Positive

Chapter 5 Work State Whether True Or False

Question 1. It is said that work has been done when there is displacement of an object due to the application of an external force on a body.

Answer: True

Question 2. Negative work is done when the point of application of force moves in a direction opposite to that of the applied force.

Answer: True

Question 3. It is said that work has been done when there is displacement of an object due to the application of an external force on a body.

Answer: True

Question 4. Positive work is done when the point of application of force moves in a direction of the applied force.

Answer: True

Question 5. When an airplane takes off the work done by its weight is positive.

Answer: False

Question 6. Work is a vector quantity.

Answer: False

Question 7. In a tug-of-war the stronger team does positive work.

Answer: True

Chapter 5 Topic A Work Numerical Examples

If applying a force F on a body displaces it by a distance d in the direction of the force, then work done, W = Fd

The absolute units of work in CGS system and SI are erg and joule (J) respectively.

Question 1. A body is displaced 2 m in the direction of an applied force of 400 dyn. Calculate the work done.

Answer:

Given

A body is displaced 2 m in the direction of an applied force of 400 dyn.

Applied force, F= 400 dyn

Displacement of the body in the direction of force, d = 2 m = 200 cm

∴ work done,W = Fd = 400 dyn x 200 cm = 8 x 10⁴ erg

Question 2. A body of mass 10kg is raised upward by 5 m. Calculate the work done.

Answer:

Given

A body of mass 10kg is raised upward by 5 m.

Mass of the body, m = 10 kg

∴ applied force,

F = mg – 10 kg x 9.8 m/s² = 98 N

Displacement of the body, d = 5 m

∴ W = Fd = 98N x 5m = 490J

Question 3. A man drags a .box 10 m on the floor. What is the work done against friction, if frictional force of the floor is 200 N?

Answer:

Given

A man drags a .box 10 m on the floor.

Frictional force of the floor = 200 N Displacement of the box against friction, s = 10 m

∴ work done against friction, W = Fs = 200 N x 10 m = 2000 J

Question 4. The mass and the length of a uniform chain are M and L respectively. This chain is kept on a smooth table, 1/4th length of the chain is hanging from the side of the table. What is the work done to raise this hanging , chain on the table, if acceleration due to gravity is g?

Answer:

Given

The mass and the length of a uniform chain are M and L respectively. This chain is kept on a smooth table, 1/4th length of the chain is hanging from the side of the table.

Mass of the hanging portion = \(\frac{M}{4}\) and its weight = \(\frac{Mg}{4}\)

As the chain is uniform, we may assume that the mass of the hanging chain is centred around its mid-point.

That point is situated at \(\frac{L}{4}\) x \(\frac{1}{2}\) = \(\frac{L}{8}\) below the surface of the table.

∴ work done to raise this hanging portion on the table,

W = \(\frac{Mg}{4}\) X \(\frac{L}{8}\) = \(\frac{MgL}{32}\)

Question 13. What do you mean by the statement ‘Young’s modulus 1.26 x 10¹² dyn/cm²?

Question 15. Calculate the dimensional formula of the force constant of a spring and write down the dimension of force constant.

∴ Total change in length of the combined rod \(l =l_1+l_2\)

Chapter 3 Topic C Viscosity And Bernoulli’s Theorem Synopsis

1. Viscosity is the property of a fluid by virtue of which it tries to reduce the relative motion between its two adjacent layers.
2. Viscosity is a general property of the fluid. An ideal fluid has no viscosity. It is also called the internal friction of a fluid.
3. Terminal velocity is defined as the maximum uniform velocity with which a falling body falls through a viscous medium.
4. If a liquid is flowing through a pipe, then the volume of liquid flowing per second through any cross-section of the pipe is called the rate of flow of that liquid.

Bernoulli’s Theorem:

In case of the streamline flow of an ideal fluid, the summation of its kinetic energy, potential energy, and pressure energy (energy due to pressure) per unit volume at every point of a streamline is constant.

The equation of Bernoulli’s theorem is given by

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

\(\frac{1}{2} \rho v^2+\rho g h+P\) = constant

or, \(\frac{v^2}{2 g}+h+\frac{p}{\rho g}\) = constant

or, velocity head + elevation head + pressure head = constant

Chapter 3 Topic C Viscosity And Bernoulli’s Theorem Short And Long Answer Type Questions

Question 1. What is viscosity?

Answer:

Viscosity

Viscosity is the property of a fluid by which it tries to reduce the relative velocity between two adjacent layers.

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

Question 2. What is the relationship between the viscosity and mobility of a liquid?

Answer:

The relationship between the viscosity and mobility of a liquid

Viscosity is a general property of matter. It is different for different liquids. When viscosity of a fluid increases, mobility decreases.

Example: Honey has greater viscosity than water.

Question 3. Why is viscosity also called internal friction?

Answer:

Viscosity also called internal friction

When a body moves or is about to move on another body or on a surface, then the opposing force that works against this motion or tendency of motion is called friction.

On the other hand, the property by virtue of which a liquid tries to reduce the relative velocity between two adjacent layers is called viscosity of the liquid. Because of this type of similarity between viscosity and friction, viscosity is also called internal friction of a liquid in many cases.

Question 4. What are the differences between viscosity and friction?

Answer:

The differences between viscosity and friction are:

Question 5. The viscous force depends on which factors?

Answer:

Viscous force depends on

1. Nature of the liquid,
2. Area of the layers in contact,
3. Velocity gradient between the layers.

Question 6. What is velocity gradient? What are the unit and dimensional formula of velocity gradient?

Answer:

Velocity gradient

1. Velocity gradient is the change in velocity between adjacent layers with distance perpendicular to the flow of fluids.

The unit and dimensional formula of velocity gradient

1. Dimensional formula of velocity gradient is T^-1and its unit in SI is s^-1.

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

Question 7. What is meant by an ideal fluid?

Answer:

Ideal fluid

While flowing, a fluid tries to reduce the relative motion between its two adjacent layers due to viscous force. But for an ideal fluid, no such force acts between layers. The ideal fluid is hence non-viscous and streamlined.

Question 8. What do you mean by streamline flow or laminar flow?

Answer:

Streamline flow or laminar flow

If the magnitude and direction of the flow always remain unchanged at any point of the flow line during flow of a fluid, then that flow is called streamline flow or laminar flow. In this condition, there is not any collision among the particles of the liquid.

Question 9. What do you mean by a streamline? What is the type of streamline in case of streamline motion of a liquid through a right circular cylindrical tube?

Answer:

Streamline

In case of a streamline motion, the path in which a particle of the fluid flows is called a streamline. In case of a streamline motion, any particle of the fluid always has the velocity of the preceding particle. A tangent drawn at any point of a streamline expresses the direction of the velocity of the fluid at that point.

In case of streamline motion of a liquid through a right circular cylindrical tube, streamlines are parallel to the axis of the tube.

NEET Biology Class 9 Question And Answers	WBBSE Class 9 History Notes	WBBSE Solutions for Class 9 Life Science and Environment
WBBSE Class 9 Geography And Environment Notes	WBBSE Class 9 History Multiple Choice Questions	WBBSE Class 9 Life Science Long Answer Questions
WBBSE Solutions for Class 9 Geography And Environment	WBBSE Class 9 History Long Answer Questions	WBBSE Class 9 Life Science Multiple Choice Questions
WBBSE Class 9 Geography And Environment Multiple Choice Questions	WBBSE Class 9 History Short Answer Questions	WBBSE Solutions For Class 9 Maths
WBBSE Solutions for Class 9 History	WBBSE Class 9 History Very Short Answer Questions

 

Question 10. What are the characteristics of a streamline?

Answer:

Characteristics of a streamline are:

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

1. A streamline may be a straight line or a curved line.
2. A tangent drawn at any point of a streamline indicates the direction of the velocity of the fluid at that point.
3. Two streamlines never intersect with each other.
4. Velocity of the fluid increases at that place in the tube where the streamlines are very close to each other and velocity decreases at that place where the streamlines maintain greater distances between them.

Question 11. What do you mean by turbulent flow?

Answer:

Turbulent flow

At any point of a flow line during the flow of a fluid, if the magnitude and direction of the flow change in a haphazard way, then the flow is called turbulent flow.

Suppose, a liquid flows through a tube. During this condition, if the liquid particles collide with each other continuously and also move simultaneously, then this type of flow is called turbulent flow.

During this time, whirls are created at some places inside the liquid.

Question 12. Why do two streamlines never intersect each other?

Answer:

Two streamlines never intersect each other

If a tangent is drawn at any point of a streamline, the tangent indicates the direction of the velocity of the fluid at that point. Now, suppose two streamlines intersect each other at the point A.

Two tangents can be drawn at the point of intersection. As a result, two directions of the velocity of the fluid are obtained at the point A, which is not possible. Therefore, two streamlines can never intersect each other.

Question 13. What do you mean by terminal velocity?

Answer:

Terminal velocity

The velocity of a small body that falls through a viscous medium due to gravity increases steadily at the beginning. But as its velocity increases, value of the viscous resistance inside the perpendicular layers of the adjacent fluid of the body also increases.

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

Along with this, the buoyancy of the medium acts on the body in an upward direction. As a result, downward acceleration gradually decreases. At a particular point of time, when value of resistance force due to viscosity and buoyancy are equal to the gravitational force, the resultant force on the body becomes zero.

Then the body falls through the medium with a steady or uniform velocity. This uniform velocity is called terminal velocity.

Thus, terminal velocity is defined as the highest uniform velocity with which a body finally falls through an infinitely spread viscous medium.

Question 14. Draw the velocity-time graph of a body falling through an infinitely spread viscous medium.

Answer:

The velocity-time graph of a body falling through an infinitely spread viscous medium is shown here. Here the acceleration of the body decreases gradually. After a certain time the body falls with a constant velocity. This is the terminal velocity, shown in the figure with v₀.

Question 15. write down Bernoulli’s theorem.

Answer:

Bernoulli’s theorem

In case of the streamline flow of an ideal fluid, summation of its kinetic energy, potential energy, and pressure energy (energy due to pressure) per unit volume at every point of a streamline is constant.

Question 16. Write down Bernoulli’s theorem on the basis of conservation of energy.

Answer:

Bernoulli’s theorem on the basis of conservation of energy

In case of streamline motion of an ideal fluid, the net mechanical energy (i.e., summation of its kinetic energy, potential energy and energy due to pressure per unit volume) at any point of a streamline is always constant.

Question 17. State the mathematical form of Bernoulli’s theorem.

Answer:

Mathematical form of Bernoulli’s theorem

Suppose, an ideal liquid is flowing in streamline flow through a tube. Let us assume that streamline flow of a liquid takes place through a tube of non-uniform cross section.

If v is the velocity of the liquid at any point on the streamline, kinetic energy of unit volume = 1/2ρv² (where p is the density of liquid).

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

If h is the height of that point from a particular reference level, potential energy in unit volume = ρgh and if P is the pressure at that point, then according to Bernoulli’s theorem,

1/2ρv² + ρgh + P = constant……..(1)

If equation (1) is divided by ρg, we get

\(\frac{v^2}{2 g}+h+\frac{P}{\rho g}=\text { constant }\)……(2)

In equation (2), \(\frac{v^2}{2 g}\) is called the velocity head, h is called the elevation head and \(\frac{P}{\rho g}\) is called the pressure head.

In the velocities of flow of the liquid at points A and B are v₁ and v₂, respectively. h₁ and h₂ are the heights of the points A and B from a particular reference level (CD) and pressures at those two points are P₁ and P₂, respectively.

Then, according to Bernoulli’s theorem,

\(\frac{1}{2} \rho v_1^2+\rho g h_1+P_1=\frac{1}{2} \rho v_2^2+\rho g h_2+P_2\)

Question 18. Write down Bernoulli’s theorem with respect to the horizontal flow of a fluid.

Answer:

Bernoulli’s theorem with respect to the horizontal flow of a fluid

Bernoulli’s equation is given by

\(\frac{1}{2} \rho v^2+\rho g h+P\) = constant;

where ρ is the density of the liquid, v is its velocity and P is its pressure at depth h.

Here, the kinetic energy, potential energy, and pressure energy of unit volume of the fluid are 1/2ρv², ρhg, and P, respectively.

For horizontal flow of the fluid, Bernoulli’s theorem can be written as \(\frac{1}{2} \rho v^2+P\) = constant

So at the place where kinetic energy of the fluid is more, pressure is less, and vice versa.

Question 19. On the basis of velocity and pressure of the fluid, how do you describe Bernoulli’s theorem?

Answer:

In respect of a fluid flowing in a horizontal way, pressure of the fluid is less where its velocity is more and vice versa—this is the essence of Bernoulli’s theorem.

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

Question 20. With the help of Bernoulli’s theorem, calculate the pressure of water at a depth of h for a stationary liquid.

Answer:

The pressure of water at a depth of h for a stationary liquid

Let us assume that a liquid of density ρ is in a vessel at a steady condition. Now at a depth h from the free surface, a point A is taken.

Pressure of liquid at the point A has to be calculated. If the bottom surface of the liquid is taken as the reference surface, height of point A is h₁. Now as the liquid is still, so the velocities of the liquid at points A and B are zero i.e., v_A = v_B = 0.

If the atmospheric pressure is P_a, then pressure at point B, P_B = P_a

If P is the pressure due to the liquid at point A, then total pressure at point A, P_A = P_a + P

By applying Bernoulli’s theorem, we get

\(\frac{1}{2} \rho v_A ^2+\rho g h_1+P_a=\frac{1}{2} \rho v_B^2+\rho g\left(h+h_1\right)+P_B\)

or, \(\rho g h_1+P_a+P=\rho g h+\rho g h_1+P_a\)

or, ρ = hρg

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

Question 21. Why is it dangerous to stand near a fast-moving train?

Answer:

One should not stand near a fast-moving train. The air near the train starts flowing at a very high speed due to the high speed of the train. Consequently, pressure in that region decreases compared to the air pressure of the surrounding region.

This excess surrounding pressure behind the person tends to push the person towards the train and may cause a serious accident.

Question 22. Why is the tin roof shade of a house blown away during stormy wind?

Answer:

When there is a stormy wind, velocity of air and hence, its kinetic energy increases and thus pressure of air decreases. As the air inside the room remains more or less still, atmospheric pressure becomes greater than the outside.

As a result, there is an upward thrust on the roof shade. When this force is more than a specific value, the roof shade is blown away.

Applications Of Bernoulli’s Theorem

Question 23. Why does the velocity of water through a pipe increase if the nozzle of the pipe is slightly closed by a finger?

Answer:

If the nozzle of the pipe is slightly closed by a finger, a stream of waterfalls at a greater distance. This means velocity of water stream has increased. According to the equation of continuity, av = constant, where a is the area of cross-section and v is the velocity of the fluid.

Class 9 Physical Science Chapter 3Short And Long Answer Type Questions

When the nozzle of the pipe is closed by a finger, area of the cross-section of the pipe decreases, causing the velocity of the water stream to increase.

Question 24. If you blow between two pages of a book, the pages stick together instead of spreading apart – explain with reason.

Answer:

If I blow between two pages of a book, the pages stick together instead of spreading apart

If air is blown between two pages of a book, speed of air increases between them. Hence, according to Bernoulli’s theorem air pressure between the pages becomes less than outside. Due to this difference in pressure, the pages come closer and stick together.

Question 25. If two boats in a river move side by side, they tend to come closer – explain with reason.

Answer:

If two boats in a river move side by side, they tend to come closer

If two boats in a river move side by side, the speed of water between the boats becomes more than that of the other sides. According to Bernoulli’s theorem, the pressure at the other sides of the boats becomes more than that between them. Therefore the boats tend to come closer.

Chapter 3 Topic C Viscosity And Bernoulli’s Theorem Very Short Answer Type Questions Choose The Correct Answer

Question 1. On a plane horizontal surface, some amount of water, shampoo, and tar are poured slowly. They move with different speeds and the tar stops at first. This is because, among these three, tar has the minimum

1. Viscosity
2. Surface tension
3. Fluidity
4. Elasticity

Answer: 1. Viscosity

Question 2. Which of the following quantities is similar to friction?

1. Viscosity
2. Surface tension
3. Buoyancy
4. All of these

Answer: 1. Viscosity

Question 3. Which of the following properties is applicable only for a flowing liquid material?

1. Elasticity
2. Surface tension
3. Malleability
4. Viscosity

Answer: 4. Viscosity

Class 9 Physical Science Chapter 3Very Short  Answer Type Questions

Question 4. Which of the following statements is incorrect?

1. Like friction, viscosity is a force against the motion
2. More the viscosity of a liquid, lesser is its mobility
3. After reaching its terminal velocity, a body starts falling with uniform velocity
4. If the area of a liquid surface decreases, value of viscous force also increases

Answer: 4. If the area of a liquid surface decreases, value of viscous force also increases

Question 5. If there is a laminar flow of a liquid through a pipe similar to the shape of a right circular cylinder, the streamlines look like

Answer: 1.

Question 6. Laminar flow of a liquid through a pipe of a non-uniform shape is shown. If velocities of the flowing liquid at points A and B are given by V_A and V_B respectively, then

 

1. V_A > V_B
2. V_A < V_B
3. V_A = V_B
4. Cannot be determined

Answer: 1. V_A > V_B

Question 7. There is laminar flow of a liquid through a horizontal tube of non-uniform shape. Velocity of liquid is V at the point where area of the cross section is A. What is the velocity at the point where the cross-section is A/2?

1. V/2
2. V
3. 2V
4. 4V

Answer: 3. 2V

Question 8. lf the velocity of water through a pipe is 1 m/s, velocity head is [g = 10 m/s²]

1. 1 cm
2. 2 cm
3. 4 cm
4. 5 cm

Answer: 4. 5 cm

Question 9. Stir a liquid kept in a vessel and then leave it to itself. After some time the motion subsides because of

1. Viscosity
2. Surface tension
3. Elasticity
4. Buoyancy

Answer: 1. Viscosity

Class 9 Physical Science Chapter 3Very Short  Answer Type Questions

Question 10. Bernoulli’s theorem is based on the law of

1. Conservation of momentum
2. Conservation of mass
3. Conservation of angular momentum
4. Conservation of energy

Answer: 4. Conservation of energy

Question 11. Action of a sprayer depends on

1. Bernoulli’s theorem
2. Jurin’s law
3. Avogadro’s theorem
4. Stoke’s law

Answer: 1. Bernoulli’s theorem

Question 12. A body falling through a viscous liquid attains the terminal velocity. Afterwards it falls with an acceleration equal to

1. g
2. 0
3. -g
4. g/2

Answer: 2. 0

Question 13. Bernoulli’s theorem is applicable for

1. Viscous fluid
2. Nonviscous fluid
3. Incompressible and nonviscous fluid
4. Compressible fluid

Answer: 3. Incompressible and nonviscous fluid

Chapter 3 Topic C Viscosity And Bernoullis Theorem Answer In Brief

Question 1. Bernoulli’s theorem is established on which conservation law?

Answer: Bernoulli’s theorem is established on the law of conservation of energy.

Question 2. If temperature is increased, does the viscosity of a liquid increase or decrease?

Answer: Viscosity of a liquid decreases, if temperature is increased.

Question 3. If temperature is increased, does the viscosity of a gas increase or decrease?

Answer: Viscosity of a gas increases, if temperature is increased.

Question 4. Do two streamlines ever intersect with each other?

Answer: No, two streamlines never intersect with each other.

Question 5. What type of a fluid gives rise to a whirlwind?

Answer: Turbulent flow of a fluid gives rise to a whirlwind.

Question 6. What is the velocity of a liquid layer in contact with the bottom surface when laminar flow of liquid takes place over a firmly fixed horizontal plane?

Answer: Velocity of the liquid layer in contact with the bottom surface is zero.

Question 7. What is that velocity called when a body falls through a viscous medium with maximum uniform velocity?

Answer: That velocity is called terminal velocity.

Question 8. If a body is moving with terminal velocity through a viscous medium and density of the body is greater than the density of the medium, then what is the direction of terminal velocity?

Answer: As the density of the body is greater than the density of the medium, direction of terminal velocity is perpendicularly downward.

Question 9. lf a body is moving with terminal velocity through a long viscous medium and density of the body is less than the density of the medium, then what the direction of terminal velocity?

Answer: As the density of the body is less than the density of the medium, direction of terminal velocity is perpendicularly upward.

Question 10. What is the relationship between viscosity of a liquid and its mobility?

Answer: When viscosity of a liquid increases, its mobility decreases.

Question 11. Bernoulli’s theorem is fully applicable for what type of fluid?

Answer: Bernoulli’s theorem is fully applicable for an ideal fluid.

Question 12. What are the characteristics of an ideal fluid?

Answer: An ideal fluid is incompressible and non-viscous.

Question 13. When a car is running very fast, it is found that light polythene packets, etc. keep flying behind the running car. This phenomenon takes place due to which principle?

Answer: This happens due to Bernoulli’s theorem.

Chapter 3 Topic C Viscosity And Bernoullis Theorem Fill In the Blanks

Question 1. Water is ______ viscous than kerosene.

Answer: More

Question 2. ________ of a fluid is called its internal friction.

Answer: Viscosity

Question 3. When viscosity of a liquid increases, its _______ decreases.

Answer: Mobility

Question 4. When a small ball of iron falls through water, three forces act on it, namely gravitational force, buoyant force and ________ force.

Answer: Viscous

Question 5. Water is _______ viscous than kerosene.

Answer: More

Question 6. Ideal fluid has no _______

Answer: Viscosity

Question 7. The maximum velocity of a fluid, up to which the flow of the fluid is ________ and beyond which the flow becomes _________ is regarded as the critical velocity for that fluid.

Answer: Streamline, turbulent

Question 8. Two _________ never intersect each other.

Answer: Streamlines

Question 9. Raindrops fall to the ground with _______ velocity.

Answer: Terminal

Chapter 3 Topic C Viscosity And Bernoullis Theorem State Whether True Or False

Question 1. Bernoulli’s theorem follows the law of conservation of energy.

Answer: True

Question 2. A smooth, uninterrupted flow in ordered layers, without any energy transfer between the layers is called laminar flow.

Answer: True

Question 3. Mobility of water is greater than that of honey.

Answer: True

Question 4. Two streamlines can intersect each other.

Answer: False

Question 5. Viscosity is called internal friction of a liquid.

Answer: True

Question 6. Viscosity of a fluid decreases with the rise in temperature.

Answer: True

Question 7. For a streamline flow of an fluid the sum of the velocity head, elevation head, and pressure head always remain constant at any point in the fluid.

Answer: True

Question 8. If the relative motion between the layers in contact in a flowing liquid decreases, viscosity decreases.

Answer: True

Chapter 3 Topic C Viscosity And Bernoullis Theorem Numerical Examples

Useful information

If the cross-sectional area at any place of a tube is a and the velocity of the fluid at that place is v, then rate of flow of liquid at that place is av.

According to the equation of continuity av = constant.

If v = velocity of a liquid, h = height from any standard level, P = pressure, ρ = density of the liquid, g = acceleration due to gravity,

1. according to Bernoulli’s theorem, \(\frac{v^2}{2 g}+h+\frac{p}{\rho g}=\) constant
2. for horizontal flow of liquid, \(\frac{1}{2} \rho v^2+P\) = constant
3.  velocity head = \(\frac{v^2}{2 g}\), elevation head = h and pressure head = \(\frac{P}{\rho g}\)

Question 1. Velocities of air below and above the surface of wings of a model plane are v and 3v, respectively. If the area of a wing is A and density of air is ρ, what is the dynamic lift?

Answer:

Given

Velocity of air below the surface of the wing of a model plane, v₁ = v, and velocity of air above the surface of wing of a model plane, v₂ = 3v.

Suppose, pressure of air below and above the surface of wing are P₁ and P₂, respectively.

If we take the wing as horizontal and thickness of wing as negligible, then by Bernoulli’s theorem,

\(\frac{1}{2} \rho v_1^2+P_1=\frac{1}{2} \rho v_2^2+P_2\)

or, \(\frac{1}{2} \rho v^2+P_1=\frac{1}{2} \rho \cdot 9 v^2+P_2\)

Hence the dynamic lift,

F = (P₁ – P₂) . A = 4ρv²A

Question 2. Water flows through a horizontal pipe. At one point of the pipe, velocity of water is v and its pressure is P. If velocity of water at another point having same height as the first is 2v, then what is the pressure at that point?

Answer:

Given

Water flows through a horizontal pipe. At one point of the pipe, velocity of water is v and its pressure is P. If velocity of water at another point having same height as the first is 2v

At the first point, velocity of water, v₁ = v and pressure of water, P₁ = P.

At the second point, velocity of water, v₂ = 2v.

Suppose, pressure of water at the second point be P₂.

Since both the points are situated at the same height, so according to Bernoulli’s theorem,

\(\frac{1}{2} \rho v_1^2+P_1=\frac{1}{2} \rho v_2^2+P_2\)

or, \(P_2=P_1+\frac{1}{2} \rho\left(v_1^2-v_2^2\right)=P+\frac{1}{2} \rho\left(v^2-4 v^2\right)\)

= \(P-\frac{3}{2} \rho v^2\)

Question 3.  Water is flowing through a horizontal pipe with non-uniform cross section. At two points A and B inside the pipe which are at the same height, velocity of water are 20 cm/s and 50 cm/s, respectively. What is the difference of pressure between the points A and B?

Answer:

Given

Water is flowing through a horizontal pipe with non-uniform cross section. At two points A and B inside the pipe which are at the same height, velocity of water are 20 cm/s and 50 cm/s, respectively.

Velocity of water at point A, v₁ = 20 cm/s

The velocity of water at point B, v₂ = 50 cm/s

Density of water, ρ = 1 g/cm³

Suppose, pressure at points A and B are P₁ and P₂, respectively.

Since points A and B are at the same height, then according to Bernoulli’s theorem,

\(\frac{1}{2} \rho v_1^2+P_1=\frac{1}{2} \rho v_2^2+P_2\)

or, \(P_1-P_2=\frac{1}{2} \rho\left(v_2^2-v_1^2\right)=\frac{1}{2} \times 1 \times\left(50^2-20^2\right)\)

or, \(P_1-P_2=1050 \mathrm{dyn} / \mathrm{cm}^2\)

Question 4. Find the velocity of flow of water at a point where the velocity head is 0.4 m.

Answer:

Velocity head =\(\frac{v^2}{2 g}\), where, v = velocity of water and g = acceleration due to gravity.

Here, \(0.4=\frac{v^2}{2 \times 9.8}\)

or, v = \(\sqrt{0.4 \times 2 \times 9.8}=2.8 \mathrm{~m} / \mathrm{s}\)

∴ The velocity of flow of water is 2.8 m/s.