Chapter 1 Physical Environment Heat
Measurement of Heat and Its Unit
Heat is a form of energy. Heat energy flows from a hot body to a cold body. Temperature is a measure of heat energy in a body.
It is the degree of hotness or coldness of a body. Absorption of heat increases the temperature of a body, while extraction of heat from a body decreases its temperature.
- The amount of heat gained (or lost) by a body to change its temperature of the body depends on
- The mass of the body
- Nature of the material of the body, and
- The amount of change in the temperature. This can be understood by the following experiments.
Change of State of substance
Substances generally exist in either of the three forms:
- Solid – for example, wood, iron, gold, aluminum, ice, etc.
- Liquid – for example, water, alcohol, milk, etc.
- Gas or vapor – for example, steam, oxygen, nitrogen, etc.
Read And Learn More WBBSE Notes For Class 8 School Science
1. Melting and Freezing
These three states are inter-convertible with Melting (or fusion) meaning the transformation of exchange of heat.
For example, the supply of heat to a solid to its liquid state at some fixed solid ice produces liquid water and supplies temperature by absorption of heat energy.
The more heat to water produces steam the fixed temperature at which melting occurs is Again, on cooling (i.e. by extracting heat) liquid called the melting point of the substance, and water is formed from steam and extraction of different for different substances, further heat from the water will produce solid ice.
Freezing (or solidification) is the process of Thus the exchange of heat and plays the most transformation of liquid to its solid state at some important role in the interconversion of the three fixed temperatures by extraction of heat energy, states of matter.
Physical Environment Heat summary
The fixed temperature at which the freezing (or solidification) occurs is called the freezing point of the substance.
The melting point and freezing point of metals and crystalline solids are the same under the same pressure. For example, the freezing point of water and melting point of ice at normal pressure (1 atmospheric pressure) is 0°C.
But non-crystalline substances like wax, glass, butter, pitch, etc. melt and freeze at two different temperatures. For example, the butter melts at a temperature between 28°C to 37°C and freezes between 20°C to 25°C.
2. Change of volume during melting and solidification
Normally, a solid expands in volume on melting, and liquid contracts on freezing. So, in general, the density of the liquid is less than that of its solid state.
An example is wax. But water is an exception. In the case of water, the density of ice (solid state) is less than that of water (liquid state). So, during freezing, water expands in volume as it becomes solid.
3. Factors Affecting The Melting Point Of A Substance
The melting point of a solid substance depends on two factors:
- The pressure applied to the substance
- Presence of impurities in the substance
- Effect of pressure on the melting point of a substance
For those substances whose volume increases due to melting, their melting point increases with increasing pressure.
For example, copper, gold, etc. Increased pressure resists the volume increase, thus increasing the melting point.
Melting point of wax increases by approximately 0.04°C due to an increase of pressure by one atmosphere.
For those substances, whose volume decreases due to melting, their melting point decreases with increasing pressure. For example, ice, brass, cast iron, etc.
Increasing pressure helps in melting by decreasing their volume. The melting point of ice decreases by approximately 0.0007°C due to an increase of pressure by one atmosphere.
When two pieces of ice are pressed together for some time and then released, they form a single lump. This is because when pressure is applied, then at the contact area between two pieces of ice, the melting point decreases.
So some ice melt at the contact point. When pressure is released, the melting point again returns to the original value.
So the water formed at the contact area again freezes, forming a single lump of ice. This is called regelation.
Effect Of Presence Of Impurity In The Substance On Melting Point
The presence of impurities or presence of any other substance decreases the melting point of the substance. For example,
1. Melting point of ice is 0°C at normal pressure. But if some salt is added, its melting point becomes much lower than 0°C.
Again, a mixture of metals (called alloys) has a melting point lower than the melting point of any of its constituents.
[Example: The melting point of a Fuse wire used in safety fuse, is made up of lead and tin. The melting point of their mixture is lower than the melting point of either of them.
When excess current flows through this wire, heat is produced and the wire melts easily thereby preventing further passage of electricity through it and thus preventing fire hazards.
A freezing mixture is prepared by mixing two substances. At the existing temperature, one of them should melt requiring heat for this.
This heat is taken from the mixture and as a result, the temperature of the mixture falls. When salt is mixed with ice, its temperature decreases. This mixture is called a freezing mixture.
When ice and common salt are mixed in a 3:1 weight ratio, part of the ice undergoes melting and the heat required for this is taken from the salt mixture as a result of which the temperature becomes approximately – 23°C.
This freezing mixture is frequently used for the preservation of fish, meat, etc. and to carry medicine at low temperatures from one place to another place.
WBBSE Class 8 Heat notes
Latent Heat
When a substance undergoes a change of state without changing temperature it either absorbs or releases a certain quantity of heat.
For example, when ice changes into water, it absorbs a definite quantity of heat without showing any rise in temperature.
Similarly, when water changes into steam, it also absorbs some definite quantity of heat without showing any rise in temperature.
In both cases, this hidden heat which simply changes the matter from one state to another without affecting its temperature is known as latent heat.
The same latent heat is given out by a substance when the change of state takes place in the reverse direction, ie when the substance changes from its liquid state to a solid state or from its gaseous state to a liquid state.
It has been found experimentally that the quantity of heat ‘O’ given out or absorbed by a substance during the change of its state without a change of its temperature is directly proportional to the mass ‘m’ of the substance.
a mixture of 87% (by weight lead in antimony is 246°C, while the melting point of pure lead is 327°C and that of pure antimony is 631°C.]
∴Q = L.m where L = latent heat of the substance.
If m = 1, then Q = L.
Definition: Latent heat is the quantity of heat absorbed or liberated by a substance of unit mass when it undergoes a change of state at a particular temperature.
Latent heat of fusion :
The latent heat of fusion of a substance is the quantity of heat that is required to convert the unit mass of the substance from its solid state to its liquid state without changing its temperature.
The CGS unit of latent heat is calorie per gram (or cal per g) and the SI unit is Joule per kilogram (J/kg).
The latent heat of the fusion of ice is 80 calories per gram at 0°C and normal atmospheric pressure.
This means that under normal pressure, 80 calories of heat has to be supplied to 1 gram of ice at 0°C to convert it into 1 gram of water at 0°C.
If an ice block at 0°C is taken inside a room, having room temperature greater than 0°C, and if some water is taken inside a hole made in the ice block, the water does not freeze.
This is because 80 calorie of heat has to be extracted per gram of water to freeze it. But only those substances maintained at a temperature less than 0°C can absorb this much heat from the water.
Heat concepts for Class 8
Since the temperature of the ice block is 0°C and the room temperature is greater than 0°C, they cannot absorb heat from water placed Freezing is the reverse process of melting or fusion.
The same amount of heat, ie. equal to the latent heat of fusion, is given out during within the hole made in an ice block.
The initial temperature of the water was equal to room temperature, which was greater than 0°C. So ice at 0°C will absorb some heat from the water.
the process of freezing (or solidification) and is called the latent heat of freezing or solidification.
Heat effects and applications for Class 8
Some ice will melt in this process and the temperature of the water will be lowered. After some amount of ice melts, the temperature of both the ice block and water will be at 0°C, and then an exchange of heat between the water and the ice block will stop.
Evaporation
It is our common experience that wet clothes get dry when left in the sunlight for some time. This is because of the evaporation of water from wet clothes.
During this process, the water on the surface of the cloth takes up heat from the surrounding and is slowly converted into vapor.
All liquids undergo evaporation but the rate of evaporation of volatile liquids such as spirits and alcohols is very fast.
Definition: Evaporation is a process in which liquid is slowly and gradually converted into vapor. It takes place from the surface of the liquid and it can occur at any temperature and pressure.
Characteristic properties of evaporation
1. More the exposed area of the liquid, more will be the rate of evaporation. For example, a folded sari left in the sun will take more time to dry compared to a sari left for drying in the sun without any fold.
2. Evaporation can occur at any temperature. No particular temperature is needed. But the higher the temperature the faster will be the rate of evaporation.
For example, a wet cloth becomes dry even in the winter season, but the time taken for that will be more than the time required during the summer.
3. If air flows over the liquid, it facilitates the process of evaporation and the rate of evaporation becomes faster.
Wet clothes become dry quickly if air flows over them. They dry up faster in the winter season than in the rainy season (when the moisture content in the air is very high).
4. The rate of evaporation depends on the nature of the liquids. Volatile liquids like spirits, alcohol, etc. evaporate more quickly than water.
Rate of evaporation increases as air pressure decreases.
Boiling
If we take some water in a container, dip a thermometer in the water and heat it, then the temperature of the water gradually rises and after some time, it starts boiling.
Turbulence occurs throughout the water. But if we notice the reading of the thermometer it shows 100°C. The temperature remains constant during boiling.
Definition: Boiling is the process due to which a liquid changes rapidly into a gaseous state at some fixed temperature by absorbing heat energy.
The fixed temperature at which a liquid is rapidly transformed into the gaseous state is called the boiling point of that liquid.
At normal pressure boiling point of pure water is 100°C. Factors affecting the boiling point of a liquid are following
1. Nature of the liquid: Volatile liquids like spirits, alcohol, etc. have lower boiling points than non-volatile liquids.
liquids generally increase. For example, the normal boiling point of water is 100°C, but in presence of some dissolved common salt, its boiling point increases.
2. Presence of soluble impurities in the liquid: In presence of soluble impurities, the boiling point of the liquids generally increases.
For example, the normal boiling point of water is 100°C, but in presence of some dissolved common salt, its boiling point increases.
3. Pressure over the liquid: The boiling point of the liquid increases or decreases with an increase or decrease in the pressure over it. When a liquid boils to form vapor, its volume increases.
If pressure over the liquid is increased, it requires more temperature to boil, and the boiling point increases. During cooking, the cooking utensil is generally covered with a lid.
hen covered with a lid, the vapor produced in the utensil cannot escape easily and increases the pressure over the liquid surface. As a result, water boils at a higher temperature (greater than 100°C).
So the food materials get properly boiled and cooked faster. The pressure cooker is used to cook faster because here the lid covers it very tightly and high pressure is generated within.
Thus boiling point increases more and cooking becomes faster.
Latent heat of vapourization:
The quantity of heat required for the unit mass of a liquid to convert it into vapor at a constant temperature is called the latent heat of vaporization.
The latent heat of the vaporization of water at normal pressure is 537 calories per gram. This indicates that under normal pressure (i.e., 1 atmospheric pressure) 537 calories of heat energy is required to convert 1 gram of water into 1 gram of vapor at 100°C.
Condensation
This is the reverse process of vaporization. This is the phenomenon by which vapor is transformed
In summer, when we sweat heavily, it feels comfortable if we stand beneath a fan.
This is because the airflow produced by the fan facilitates liquid. The latent heat of condensation is equal to the latent heat of vapourization. Condensation of water vapor in the air produces clouds, fog, etc. in nature.
1. Cloud: The water that evaporates from sea, ocean, river, lakes, etc. mixes with hot air. Air containing water vapor is lighter than “dry” air which contains much less water vapor.
So, lighter air goes upwards towards the higher altitudes. But with increasing altitude, the air pressure reduces. The air cools down and the water vapor of the air condenses as water droplets on the tiny dust particles floating in the air, forming clouds.
2. Fog: It is often seen during dawn in the winter season. During day time, water evaporates and water vapor thus formed mixes with the air.
At night, when the temperature falls, the air becomes saturated with water vapor. It then condenses as water droplets on suspended dust and smoke particles, coal dust, etc.
And floats in the air, thus forming fog. In big cities and industrial belts, where the pollution level is significantly high, the air contains more dust and smoke particles, thus increasing the chance of fog formation.
At noon, the fog disappears, because, at higher temperatures, the small water droplets are again converted to water vapor.
3. Dew: In the winter season, dew is found on leaves and grasses in the morning. During the daytime, the surface of the earth and objects near it becomes hot.
But after sunset, the earth gradually cools down by radiating heat. The air in contact with the earth’s surface also cools down and when it becomes sufficiently cold, it becomes saturated with water vapor.
The water vapor then condenses into water droplets on the cold surface of leaves and grasses, which is called dew.
Dew is not formed immediately after sunset but is formed in the later part of the night and at the dawn. This is because it takes time to cool down the earth’s surface temperature so that water vapor can condense.
The flow of Heat: Conduction, Convection, and Radiation
To carry heat from one part of an object to its other part or from one object to another object is called transfer of heat.
Transfer of heat energy from one place to another with or without the help of a medium can take place in three ways
- Conduction
- Convection
- Radiation.
1. Conduction
It is our common experience that when one end of a long metal rod is heated, the other end of it gradually warms up.
Obviously, heat propagates from the hotter region of the rod to its cooler end, without any actual movement of its constituent particles. This process of transfer of heat is called conduction.
Definition: The process by which heat energy is transmitted through a substance from a region of higher temperature to a region of lower temperature without any movement of its constituent particles from one place to another is called conduction.
By the process of conduction, heat is not only transferred within the same object but heat is also transferred from one object to another provided:
The two objects are in contact with each other, and They are at different temperatures
So long as there is a temperature difference between the two objects (or between two regions at the same object), the transfer of heat continues.
Once the temperature of the two objects (or two regions of the same object) becomes the same, heat transfer ceases.
WBBSE Chapter 1 Heat exercises
The substance through which heat can flow easily is called a good conductor of heat. The substances through which heat cannot flow easily are called the bad conductors of heat.
Generally, metals are good conductors of heat. For example, copper, steel, brass, etc. are good conductors. So cooking utensils (sometimes the base of the cooking utensils) are made up of these materials.
So, heat quickly reaches the foodstuffs that are being” cooked. Substances like wool, stone, glass, wood, water, air; diamond, paper, etc. are bad conductors of heat. Generally, most liquids and gases are also, bad conductors.
1. Demonstrative Experiments On Conduction Of Heat Describe a demonstrative experiment to show that water is a bad conductor of heat.
Apparatus needed: A glass test tube, some water, a small piece of ice, some iron wire, a test tube holder, bunsen burner.
Procedure: Let us take a small piece of ice and wrap it with iron wire such that when dropped inside a test tube filled with water, it sinks to the bottom of the test tube.
Now, by holding the test tube in a tilted position with the help of a test tube holder, the upper layer of water in the test tube is strongly heated till it starts boiling.
Observation: Even when the upper layer of water starts boiling, it is found that the piece of ice wrapped with iron wire does not melt.
Inference: This shows that water is a bad conductor of heat. Because when the heat energy was supplied to the upper layer of water, the water in the upper part of the test tube started boiling.
But the heat energy was not transferred to the lower part of the test tube. So the ice did not melt.
2. Boiling Of Water In A Paper Vessel
Let us take a thin piece of paper and let us fold it in quadrants and then by taking three folds of it together let us make a small packet or vessel.
Some little amount of water is taken in it and the water is now boiled by holding the thin paper packet into the flame of the fire. Surprisingly the paper does not burn.
It needs a higher temperature to burn the paper than to boil water. Heat actually flows easily through the thin piece of paper.
But if the same experiment is performed with a thick sheet of paper, the paper burns. Thick paper is a bad conductor of heat, so heat does not flow through it easily and the temperature of the paper rises quickly and it ultimately burns.
2. Uses of good conductors of heat
1. Boilers, cooking utensils, etc. are usually made of copper or aluminum, which are good conductors of heat.
So the heat is rapidly transferred from the outside to the inside of the vessel and it heats up the contents inside and serves the necessary purpose.
2. Davy’s safety lamp: it is an oil lamp surrounded by a wire gauge of cylindrical shape. In mines, it is used mainly for lighting purposes.
If there is any inflammable gas, it may enter the wire gauze of the lamp and burns there. The wire gauge is made up of materials that are good conductors of heat.
When the inflammable gas (if any) burns, the major part of the heat produced during its burning, is evenly distributed throughout the wire gauge.
So the temperature cannot reach a higher value where the inflammable gas can catch fire and may eventually explode. So the lamp serves the purpose of lighting with safety.
3. Uses Of Bad Conductors Of Heat
1. Handles of cooking utensils are generally made up of or covered with bad conductors such as wood, cane, and bakelite so that the utensil can be held by the handle with our hands even when the rest of the utensil is very hot.
2. In winter, it feels warmer and more comfortable, if we wear two garments made up of thin threads rather than one made up of thick threads, the total thickness of the garments being the same in both cases.
This is because when we wear two garments, the air is trapped between the two. Air being a bad conductor of heat prevents the outflow of heat from our body to the colder surroundings.
So two shirts, one above the other, give more warmth than a single shirt of thickness equal to that of the two together.
2. Woollen clothes are more comfortable in winter. This is because wool itself is a bad conductor of heat. It has innumerable tiny pockets of air enclosed within its fibers.
Air is also a bad conductor of heat. So wool and air pockets together prevent the outflow of heat from our body to the colder surroundings.
4. Ice blocks are covered with sawdust. Saw dust is a bad conductor of heat. It prevents the easy flow of heat from the hotter surroundings to the ice blocks. Thus melting of ice is partly avoided.
5. Ice is a bad conductor of heat. So igloos are built with ice blocks. When the temperature of the surroundings is much below zero degrees centigrade, it is warmer inside the igloos.
6. Building materials should be bad conductors of heat. Hay and mud are bad conductors of heat. So mud houses or houses made up of hay are warmer in winter while colder in the summer.
7. ln winter, birds fluff their feathers. This is because when birds bulge out feathers, layers of air are trapped within the feathers. These layers of air prevent the outflow of heat from their body to the colder surrounding.
8. Table mats are made of poor conducting materials like rubber, spun jute, etc. This is because when hot utensils are placed on it, heat from the hot utensils cannot pass on easily through the mat, and the surface of the wooden table is not damaged.
9. In winter, a new cotton quilt is more comfortable than an old one. In a new quilt, cotton fibers remain in a loose state with plenty of air pockets in between.
Air being a poor conductor of heat prevents the flow of heat through it. But in an old quilt, fibers are in a compressed state, and the quantity of trapped air within the fibers is small. So, it is less effective in preventing heat flow across it.
10. Water is a bad conductor of heat. So the upper surface of the water of a pond is warmer in summer than its lower surface.
In winter, the situation is reversed. The upper layer of water in the pond is colder compared to its bottom layers.
11. A thick glass tumbler is cracked when hot water is poured on it. This is because glass is a poor conductor of heat.
So, when hot water is poured into it, the part of the glass in direct contact with the hot water is heated and it expands.
But glass is a poor conductor, the heat is not evenly distributed throughout the tumbler. So the heated portion (in direct contact with hot water) expands more than the portion which is not in direct contact with hot water. Due to uneven expansion of a different portion of the same tumbler, it cracks.
2. Convection
Heat is transmitted through liquids and gases by another mechanism, known as convection. Heat transfer by the convection process is limited to liquids and gases.
The mechanism may be demonstrated with an experiment. Some water and a small quantity of saw dust (they are insoluble in water) are taken in a beaker.
The beaker is slowly heated at the bottom. After some time, it will be seen that some sawdust is moving down and some are moving up.
The reason is that water particles at the bottom of the beaker are first heated and become lighter. They move up and carry some wet and heavy sawdust along with them.
Water particles in the upper part of the beaker are colder and heavier, so they move down and they also carry some sawdust along with them.
From the movement of sawdust within the water, we can realize that the water particles in the hotter region of the liquid move up while the water particles in the colder region of the beaker move down.
Thus a circulatory motion of water particles is created within the liquid. In this way ultimately the whole of the water is heated. The circulatory motion of the water particles during heating is called a convection current.
Definition: The process of transmission of heat within a liquid or gas by the actual movement of heated particles from a hotter region to a colder region is called convection. When liquids and gases are heated, they expand.
As a result, their densities decrease and they become lighter and move upwards. But the liquid and gas away from heat remains unchanged and is comparatively denser, which is heavier.
So, as the lighter liquid or gas moves upwards, the heavier liquid or gas comes downwards, thus creating a convection current.
[The downward or upward motions of the fluid (i;e. liquid and gas) particles are controlled by gravity. Particles of larger mass experience greater gravitational pull than lighter particles.
At a place, where the force of gravity is absent as in artificial satellites, the transmission of heat by convection will not take place.]
Demonstrative experiment to show convection current in water Let us fill a glass beaker with water and a small, pink-colored crystal of potassium permanganate is carefully dipped in it.
The beaker is heated gently over a flame. It is seen that violet-colored water is initially flowing upwards. From the movement of the colored water in the beaker, the convection current can be observed.
This is the convection current due to which our hands feel hotter when placed over the oven than when placed beside the oven.
1. Ventilation
The air which is breathed out by us is warm, damp, and less dense compared to ordinary air. So it rises upwards in a room and can escape through openings called ventilators, which are situated near the ceilings of the room.
The gap is replaced by cooler air entering the room through the doors and windows. So ventilation is the free circulation of air.
In some rooms, exhaust fans are also used to facilitate the escape of warm, damp, and relatively heavier air.
It may be noted that during winter, if a kerosene lamp or oven is placed inside a confined room, then it may prove fatal to sleep inside that room.
Kerosene lamps or fire burns with oxygen available in the room leaving behind carbon dioxide and poisonous carbon monoxide.
Since doors and windows are tightly shut on a winter night, it prevents fresh air from entering the room.
Gradually the concentration of oxygen decreases and the concentration of poisonous carbon monoxide gas within the room increases and it may eventually kill a human being if exposed to this poisonous gas for a longer time.
2. Sea Breeze And Land Breeze
Water has high specific heat than that land masses. So, during day time, absorption of heat by water will produce less temperature increase compared to the land masses.
As a result, the air above the warmer land mass becomes more warm compared to air over the sea. So, the air over the land moves up and creates a partial vacuum.
Relatively colder air from the sea moves toward the land to fill up the vacuum. This flow of air from the sea towards land is called sea breeze.
After sunset, both land and sea will radiate heat and become cooler. But due to the low specific heat of land mass, the rate of decrease of temperature of the land mass will be much more than that of seawater (having higher specific heat).
So at night, the land mass becomes colder than seawater. Now, the air over the sea will become warmer and moves upwards, creating a partial vacuum.
This is filled up by the movement of colder air from the land. This flow of air from land toward the sea is called the land breeze.
Understanding Heat in Physical Environment
3. In a refrigerator the cooling unit is placed at the top, Why?
The reason is that air in contact with the cooling unit is cooled and becomes heavier. So it moves to a lower region.
To fill the vacuum formed near the said unit, warmer air from other parts of the refrigerator moves up. This air also cools and moves downwards. Thus the whole refrigerator is cooled.
Had the cooling unit been kept at the bottom, a small quantity of air would have been cool. This air being heavier would have remained at the bottom.
The warmer and lighter air would never come down near the cooling unit. Thus there would have been little and partial cooling inside the refrigerator,
4. Trade winds
The equatorial regions which receive sun rays directly are very hot compared to the polar regions, where the sun rays fall obliquely.
The hot air of the equatorial zones being lighter rises up and the colder air from the polar regions fills this gap. This flow of air from the polar region to the equatorial region is called trade winds.
Eagles can fly at a high altitude without flapping their wings This is possible because they fly over the rising convection currents of air, moving upwards and this lends support to float.
3. Radiation
If someone sits nearby a hot oven or a fireplace, he/she feels warm. Here the only medium between the oven and the person is air.
Air is a very poor conductor of heat, so heat cannot pass from the oven by the process of conduction.
Again, in the convection process, heated particles of air move upwards, so heat cannot also pass to the person by the convection process. Hence, heat must have reached the person without the help of any material medium.
In this case, the transfer of heat energy has occurred through a process called radiation. Every hot object emits invisible heat rays in all directions.
These heat rays carry heat energy. When these heat rays fall on a cold object, the cold object receives heat energy and gets heated.
This is the process of radiation. The best example of radiation is the transfer of heat energy of the sun to the earth.
When we come out in the sunshine, heat from the sun is transferred to us by radiation and we feel hot.
Definition: Radiation is the mode of transfer of heat that takes place from a hotter body to a colder space without the aid of any medium and in the process, the intervening medium, if any, is not heated up.
Properties of radiant heat The properties of the radiant heat (i.e. the heat transmitted by the radiation process) are as follows:
Radiant heat can travel even without the help of any medium.
Heat radiation travels as electromagnetic waves like light and has the properties of light such as rectilinear propagation, reflection, refraction, etc.
The radiant heat is called infrared radiation which is invisible. The frequency of infrared radiation is small in comparison to visible light. The velocity of propagation of radiant heat in a vacuum is equal to that of light in a vacuum.
It does not warm up effectively the intervening medium if any. It reflects better from a white surface than from a dark or colored surface.
The amount of radiant heat falling normally on a unit area at a point per second is inversely proportional to the square of the distance of the point from the source of radiant heat.
The amount of radiant heat generated from a black and rough surface is greater than the heat radiated from a white and polished surface, both surfaces being at the same temperature.
Utilization of radiation phenomena of heat The amount of heat that an object can absorb by radiation depends on color of the object.
Objects having dark colors absorb more heat radiations than objects having light colors. A white object is a poor absorber of heat radiation.
It also means that a white object is a good reflector of heat radiation. In summer we prefer to wear white clothes.
Since the white surface is a good reflector and poor absorber of heat, the white clothes we wear reflect most of the radiant heat of the sun and so we are relieved of intense heat,
WBBSE Class 8 Environmental Science Heat
2. Cloudy nights are hotter than cloudless nights. The reason is, after sunset, the heated surface of the earth begins to cool down by radiating heat.
If there is a cloud, the heat radiated by the earth is reflected by the clouds back to the earth. So the earth remains warm.
On a cloudless night, the earth is cooled by radiating the absorbed heat (absorbed during day-time) during the night without any chance of being reflected back to the earth.
3. Outer surface at bottom of the cooking utensils is coated black. The reason Is that a blackened surface absorbs heat very well and so cooking is done more quickly in such utensils than in a new one with a polished outer surface.
4. The box of the solar cooker (and solar water heater) is painted black from the inside. This is because a black surface is a very good absorber of heat and it will absorb maximum heat radiations coming from the sun.
Thermos-Flask
Thermos-flask is also called Dewar’s flask, named after its designer British scientist Sir James Dewar. It is normally used to keep hot things hot and cold things cold for a longer time.
Construction
It mainly consists of a double-walled glass bottle. The space between the two walls is made vacuum. The outer surface of the inner wall and the inner surface of the outer wall is silvered.
The bottle is placed inside a non-conducting cylindrical jacket. The space between the double-walled bottle and the jacket is filled with felt pads and springs
so that the bottle is not damaged due to any jerk. The mouth of the bottle is closed with a cork.
The efficiency of a thermos-flask in maintaining the temperature of a substance placed within it depends on the minimization of the heat transfer between the substances kept within it and the surroundings.
How does a thermos-flask function
Let some hot liquid be kept within a thermos- flask. Loss of heat from the liquid is kept in check in the following ways:
The space between the walls of the double-walled bottle is without any medium i.e.; vacuum; so heat transfer from the hot liquid by conduction and convection is significantly reduced.
WBBSE Class 8 science practice on Heat
The outer surface of the inner wall may, if at all, radiate a very small quantity of heat, but since it is silvered, the traces of radiated heat is reflected back from the silvered inner surface of the outer wall.
The mouth of the double-walled bottle is corked, so the loss of heat by evaporation is minimized. Thus, a thermos-flask minimizes the loss of heat from its content (placed within it).
