## Unit 7 Properties Of Bulk Matter Chapter 9 Transmission Of Heat Multiple Choice Questions And Answers

**Question 1. Through a solid medium heat**

- Cannot be conducted but convection occurs
- Cannot be transmitted by convection
- Cannot be transmitted by conduction
- Can be transmitted by both conduction and convection

**Answer:** 2. Cannot be transmitted by convection

**Question 2. Through a solid or a liquid medium heat**

- Cannot be conducted but convection occurs
- Cannot be transmitted by convection
- Cannot be transmitted by conduction
- Can be transmitted by both conduction and convection

**Answer:** 4. Can be transmitted by both conduction and convection

**Question 3. During the conduction of heat**

- The molecules of the substance remain static
- The molecules of the substance move in the direction of heat conduction
- The molecules of the substance move randomly in different directions
- The molecules vibrate around their equilibrium positions

**Answer:** 4. The molecules vibrate around their equilibrium positions

**Question 4. An example of a liquid which is a good conductor of heat is**

- Water
- Alcohol
- Mercury
- Aqueous solution of an acid or a base or a salt

**Answer:** 3. Aqueous solution of an acid or a base or a salt

**Question 5. If the thickness of a plate is d, the cross-sectional area is A and the difference in temperature of its two ends is T then the amount of heat conducted through it in unit time is**

- \(Q \propto d A T\)
- \(Q \propto \frac{d T}{A}\)
- \(Q \propto \frac{A T}{d}\)
- \(Q \propto \frac{T}{d A}\)

**Answer:** 3. \(Q \propto \frac{T}{d A}\)

**Question 6. The SI unit of the coefficient of thermal conductivity is**

- J · m
^{-1 }K^{-1} - W · m
^{-1}· K^{-1} - J · m
^{-2}· K^{-1} - J · m
^{-2}· K^{-2}

**Answer:** 2. W · m^{-1} · K^{-1}

**Question 7. The coefficient of thermal conductivity of copper is nine times that of steel in the composite cylindrical bar shown in the figure. What will be the temperature at the 18cm 6cm junction of copper and steel?**

- 33°C
- 90°C
- 75°C
- 65°C

**Answer:** 3. 75°C

**Question 8. The area of the cross-section of a conductor of thickness d is A, the coefficient of thermal conductivity of its material is k and the difference in temperature of its two ends is T. If Q amount of heat is conducted through it in time t then the thermal resistance of the conductor is**

- \(\frac{Q}{t}\)
- \(\frac{A T}{d}\)
- \(\frac{k A}{d}\)
- \(\frac{d}{k A}\)

**Answer:** 4. \(\frac{d}{k A}\)

**Question 9. The ratio of the lengths, cross-sectional areas and the differences in temperature of the two ends of two conducting rods are 2:3 each. If the rate of heat conduction through them be equal, then the ratio of the coefficients of thermal conductivity of their materials is**

- 2:3
- 3:2
- 4:9
- 9:4

**Answer:** 2. 3:2

**Question 10. Thermal resistance of a substance denotes**

- The coefficient of thermal conductivity
- The rate of heat conduction
- The reciprocal of the coefficient of thermal conductivity
- The reciprocal of the rate of heat conduction

**Answer:** 3. The reciprocal of the coefficient of thermal conductivity

**Question 11. The rate of heat conduction through the window of a room is 273 J • s ^{-1} when the difference in temperatures of air inside and outside the room is 20°C. If this difference in temperature be 20 K, then the rate of heat conduction will be**

- 293 J · s
^{-1} - 273 J · s
^{-1} - 253 J · s
^{-1} - 263 J · s
^{-1}

**Answer:** 2. 273 J · s^{-1}

**Question 12. The thicknesses of two metallic plates of a combined strip are equal. The coefficients of their thermal conductivities are k _{1} and k_{2} and the temperatures of their hot and cold ends are T_{1 }and T_{2} respectively. The temperature of the junction of the combined strip will be**

- \(\frac{k_1 T_1+k_2 T_2}{k_1+k_2}\)
- \(\frac{k_1 T_1-k_2 T_2}{k_1-k_2}\)
- \(\frac{k_2 T_1+k_1 T_2}{k_1+k_2}\)
- \(\frac{k_2 T_1-k_1 T_2}{k_2-k_1}\)

**Answer:** 1. \(\frac{k_1 T_1+k_2 T_2}{k_1+k_2}\)

**Question 13. The areas of a cross-section of two metallic rods of equal lengths, having coefficients of thermal conductivity k _{1} and k_{2}, are A_{1} and A_{2} respectively. The rate of heat conduction in them will be the same if**

- \(k_1 A_1^2=k_2 A_2^2\)
- \(k_1 A_2=k_2 A_1\)
- \(k_1 A_1=k_2 A_2\)
- \(k_1^2 A_1=k_2^2 A_2\)

**Answer:** 3. \(k_1 A_2=k_2 A_1\)

**Question 14. The temperatures of the two ends A and B of a rod AB of length 20 cm are 100°C and 0°C respectively. In thermal equilibrium the temperature of the rod at a distance 6 cm from the end A will be**

- 50°C
- 80°C
- 70°C
- 60°C

**Answer:** 3. 70°C

**Question 15. Three rods made of the same material and having the same cross-section have been joined as shown. Each rod is of the same length. The left and right ends are kept at 0°C and 90°C respectively. The temperature of the junction of the three rods will be**

- 45°C
- 60°C
- 30°C
- 20°C

**Answer:** 2. 60°C

**Question 16. Two metallic rods of lengths x _{1} and x_{2} have equal cross-sectional area. Their thermal conductivities are k_{1} and k_{2} respectively. If they are connected in series, then the equivalent thermal conductivity of the combination will be**

- \(\frac{\frac{x_1}{k_1}+\frac{x_2}{k_2}}{k_1+k_2}\)
- \(\frac{x_1+x_2}{\frac{x_1}{k_1}+\frac{x_2}{k_2}}\)
- \(\frac{k_1+k_2}{\frac{x_1}{k_1}+\frac{x_2}{k_2}}\)
- \(\frac{\frac{x_1}{k_1}+\frac{x_2}{k_2}}{x_1+x_2}\)

**Answer:** 2. \(\frac{x_1+x_2}{\frac{x_1}{k_1}+\frac{x_2}{k_2}}\)

**Question 17. The dimension of thermal resistance is**

- \(\mathrm{ML}^2 \mathrm{~T}^{-2} \Theta\)
- \(\mathrm{M}^{-2} \mathrm{~L}^{-2} \mathrm{~T}^2 \Theta^3\)
- \(\mathrm{ML}^2 \mathrm{~T}^{-3} \Theta\)
- \(\mathrm{M}^{-1} \mathrm{~L}^{-2} \mathrm{~T}^3 \Theta\)

**Answer:** 4. \(\mathrm{M}^{-1} \mathrm{~L}^{-2} \mathrm{~T}^3 \Theta\)

**Question 18. The process by virtue of which water in a pond gets heated at noon in summer up to a certain depth is**

- Conduction
- Convection
- Conduction as well as convection
- Convection as well as radiation

**Answer:** 1. Conduction

**Question 19. If the velocity of light in a vacuum be c then the velocity of radiant heat in any other medium is**

- Always equal to c
- Always greater than c
- Always less than c
- Equal to or less than c

**Answer:** 3. Always less than c

**Question 20. We can protect our body from the heat of the sun by means of an umbrella. From this, it can be said that**

- Radiant heat travels in a straight line
- Velocity of radiant heat is equal to that of light
- Radiant heat does not warm the medium
- Radiant heat spreads in the form of waves

**Answer:** 1. Radiant heat travels in a straight line

**Question 21. With the increase in temperature of a radiator**

- Intensity of radiant heat increases
- The wavelength of radiant heat increases
- The absorptive power of the radiator increases
- The emissive power of the radiator increases.

**Answer:** 1. Intensity of radiant heat increases

**Question 22. The specific design, for which heat lost by radiation from a thermos flask is the least, is**

- Glass vessel with stopper made of cork
- Shiny wall of the glass vessel
- Vacuous space in between two walls of the glass vessel
- Both the characteristics 1 and 3

**Answer:** 2. Shiny wall of the glass vessel

**Question 23. Felt is widely used as an insulator of heat instead of air, because**

- The coefficient of thermal conductivity of air is more than that of felt
- Convection of heat occurs in air but not in felt
- No air remains trapped inside the fibres of felt
- Felt is a better insulator of heat than air

**Answer:** 2. Convection of heat occurs in air but not in felt

**Question 24. If the temperature of the sun gets doubled, the rate of energy received on earth will be**

- Doubled
- Quadrupled
- 16 times
- 8 times

**Answer:** 3. 16 times

**Question 25. The coefficients of radiation of two bodies of equal shapes and volumes are 0.2 and 0.8 respectively. If they radiate heat at the same rate then the ratio of their temperatures will be**

- √3:1
- √2:1
- 1:√5
- 1:√5

**Answer:** 2. √2:1

**Question 26. If the temperature of a black body rises from T to 2 T, how many times will its rate of radiation be?**

- 4
- 2
- 16
- 8

**Answer:** 3. 16

**Question 27. The radii of two spheres made of the same material are 1 m and 4 m respectively. If their temperatures are 4000 K and 2000 K respectively, then the ratio of the amount of heat radiated per second is**

- 1:1
- 16:1
- 4:1
- 1:9

**Answer:** 1. 16:1

**Question 28. Which law is used to determine the temperature of stars?**

- Stefan’s law
- Wien’s law
- Kirchhoff’s law
- Planck’s law

**Answer:** 1. Stefan’s law

**Question 29. When the body has the same temperature as that of its surroundings, it**

- Does not radiate heat
- Radiates same quantity of heat as it absorbs
- Radiates less quantity of heat as it receives from surroundings
- Radiates more quantity of heat as it receives from surroundings

**Answer**: 2. Radiates same quantity of heat as it absorbs

**Question 30. A piece of iron appears colder in winter than a piece of wood when touched because**

- The temperature of iron is less than that of wood
- The temperature of iron is less than our body temperature but that of wood is higher on
- Wood being an insulator of heat, no heat from it can be transmitted to our body
- Iron being a good conductor, heat can be transmitted from our body to the piece of iron

**Answer:** 4. The temperature of iron is less than that of wood

**Question 31. The specific property that a cooking utensil must possess, is**

- High absorptive power
- High thermal conductivity
- Low specific heat
- All of the above three characteristics

**Answer:** 4. All of the above three characteristics

**Question 32. Woolen clothes are used in winter because wool is**

- An insulator of heat
- A material having higher specific heat.
- A material haring lower specific heat
- A good conductor of heat

**Answer:** 1. An insulator of heat

**Question 33. The mode in which a cup of hot tea placed on a metallic table loses heat is**

- Conduction
- Convection
- Radiation
- All of them

**Answer:** 4. All of them

**Question 34. Which mode of transmission of heat depends on the force of gravity?**

- Conduction
- Convection
- Radiation
- None of 1, 2 and 3

**Answer:** 2. Convection

**Question 35. The depletion in the ozone layer is mainly caused by**

- Nitrous oxide
- Methane
- Sulphur dioxide
- Clilorofluro carbon

**Answer:** 4. Clilorofluro carbon

**In this type of question, more than one options are correct.**

**Question 36. A hollow and a solid sphere of same material and identical outer surface are heated to the same temperature.**

- In the beginning, both will emit equal amount of radiation per unit of time
- In the beginning, both will absorb an equal amount of radiation per unit of time
- Both spheres will have the same rate of fall of temperature \(\left(\frac{d t}{d t}\right)\)
- Both spheres will have equal temperatures at any moment

**Answer:**

- In the beginning, both will emit equal amount of radiation per unit of time
- In the beginning, both will absorb equal amount of radiation per unit time
- Both spheres will have same rate of fall of temperature \(\left(\frac{d t}{d t}\right)\)

**Question 37. A composite block is made of slabs A, B, C, D, and E of different thermal conductivities (given in terms of a constant k) and size (given in terms of length, L) as shown. All slabs are of same width. Heat Q flows only from left to right through the blocks. Then in a steady state**

- Heat flow through slabs A and E are same
- Heat flow through slab E is maximum
- The temperature difference across slab E is the smallest
- Heat flow through C = heat flow through B + heat flowthrough D

**Answer:**

1. Heat flow through slabs A and E are the same

4. Heat flow through C = heat flow through B + heat flowthrough D

**Question 38. The two ends of a uniform rod of thermal conductivity k are maintained at different but constant temperatures. The temperature gradient at any point on the rod is \(\frac{d \theta}{d l}\) (equal to the difference in temperature per unit length). The heat flow per unit time per unit cross-section of the rod is l.**

- \(\frac{d \theta}{d l}\) is the same for all points on rod
- l will decrease as we move from higher to lower temperature
- q = \(k \cdot \frac{d \theta}{d l}\)
- None of these

**Answer:**

1. \(\frac{d \theta}{d l}\) is the same for all points on rod

3. q = \(k \cdot \frac{d \theta}{d l}\)

**Question 39. A heated body emits radiation which has maximum intensity at frequency v _{m}. If the temperature of the body is doubled**

- The maximum intensity of radiation will be at frequency 2v
_{m} - The maximum intensity of radiation will be at frequency
- The total emitted energy will increase by a factor 16
- The total emitted energy will increase by a factor 2

**Answer:**

1. The maximum intensity of radiation will be at frequency 2v_{m}

3. The total emitted energy will increase by a factor 16

**Question 40. The energy radiated by a body depends on**

- Area of body
- Nature of surface
- Mass of body
- Temperature of body

**Answer:**

1. Area of body

2. Nature of surface

4. Temperature of body