Thermal energy from a hot body flows to a cold body in the form of heat. This is called as transfer of heat. There are three ways by which the transfer of heat takes place. These are:

• conduction
• convection
• radiation

QUICK QUIZ

Think of objects around us getting heat or giving out heat.

Ans:    (i)     Melting of ice cubes. Ice cube gets heat from the surroundings.

           (ii)    A hot piece of iron radiates heat.

           (iii)   A black surface absorbs heat.

           (iv)   The glass of the greenhouse get heat from the sun.

Conduction:

The mode of transfer of heat by vibrating atoms and free electrons in solids from hot to cold parts of a body is called conduction of heat.

Explanation:

The handle of metal spoon held in hot water soon gets warm. But in case of a wooden spoon, the handle does not get warm. Both the materials behave differently regarding the transfer of heat. Both metals and non-metals conduct heat. Metal are generally better conductors than non-metals.

Conduction in solids:

Transfer of heat takes place from hot to cold parts in solids:

In solids, atoms and molecules are packed close together. They continue to vibrate about their mean position. When one of its ends is heated then the atoms or molecules present at that end begin to vibrate more rapidly. They also collide with their neighboring atoms or molecules. In doing so, they pass some of their energy to neighboring atoms or molecules during collisions with them with the increase in their vibrations. These atoms or molecules in turn pass on a part of the energy to their neighboring particles. In this way some heat reaches the other parts of the solids. This is a slow process and very small transfer of heat takes place from hot to cold parts in solids.

Metals have free electrons. These free electrons move with very high velocities within the metal objects. They carry energy at a very fast rate from hot to cold parts of the object as they move. Thus, heat reaches the cold parts of the metal objects from its hot part much more quickly than non-metals.

                   All metals are good conductors of heat.

Bad conductors or insulators:

             The substances through which heat does not conduct easily are called bad conductors or insulators. Wood, cork, cotton, wool, glass, rubber, etc. are bad conductors or insulators

DO YOU KNOW?

             Why Styrofoam boxes are used to keep food hot or ice cream cold for a long time? Styrofoam is a bad conductor of heat. It does not allow heat to leave or enter the box easily.

Thermal conductivity:
Let two opposite faces each of cross-sectional area A be heated to a temperature $T_{1}$. Heat Q flows along its length L to opposite face at temperature $T_{2}$ in t seconds. 

The amount of heat that flows in unit time is called the heat flow rate. 

Thus, Rate of flow of heat = $\frac{Q}{t} $                          (i) 

It is observed that the rate at which heat flows through a solid object depends upon various factors. 

CROSS-SECTIONAL AREA OF THE SOLID:
Larger cross-sectional area A of a solid contains a larger number of molecules and free electrons on each layer parallel to its cross-sectional area; hence, greater will be the flow of heat through the solid. 

Thus, Rate of flow of heat $\frac{Q}{i}\propto A$                            (ii)   

LENGTH OF THE SOLID:
The larger the length between the hot and cold ends of the solid, the more time it will take to conduct heat to the colder end, and the smaller will be the heat flow rate.

Thus, Rate of flow of heat $\frac{Q}{t}\propto \frac{1}{L}$                       (iii)

TEMPERATURE DIFFERENCE BETWEEN ENDS:

Greater is the temperature difference $T_{1} – T_{2}$ between hot and cold faces of the solid, greater will be the rate of flow of heat. 

Thus, Rate of flow of heat $\frac{Q}{t} \propto ( T_{1} – T_{2} )$              (iv)

Combining the above factors I, ii, iii, and iv, we get

$\frac{Q}{t}\propto \frac{(〖A(T〗_{1}  – T_{2}))}{L}$ 
Rate of flow of heat = $\frac{Q}{t}$ = $\frac{(〖kA(T〗_{1}  – T_{2}))}{L}$

 
Here k is the proportionality constant called thermal conductivity of the solid.
k = $\frac{Q}{t}\times \frac{L}{(〖A(T〗_{1}  – T_{2}))}$ 

Coefficient of thermal conductivity: 
Thus, the thermal conductivity of a substance can be defined as:
The rate of flow of heat across the opposite faces of a meter cube of a substance maintained at a temperature difference of one kelvin is called the thermal conductivity of that substance. 

Unit of thermal conductivity:
W $m^{-1} K^{-1}$ Or J $m ^{-1} K ^{-1} s^{-1}$

Use of conductors and non-conductors:

In houses, good thermal insulation means lower consumption of fuel. For this, the following measures may be taken to save energy.

• Hot water tanks are insulated by plastic or foam lagging.
• Wall cavities are filled with plastic foam or wool.
• Ceiling of rooms is covered by insulating materials (false ceiling).
• Double glazed window panes are used. These window panes have air between glass sheets that provides good insulation.

Uses of a good conductor:

         Good conductors are used when the quick transfer of heat is required through a body. Thus cookers, cooking plates, boilers, radiators, condensers of refrigerators, etc. are made of metals such as aluminum or copper. Similarly, metal boxes are used for making ice, ice cream, etc.

Uses of Insulators or bad conductor:

                Insulators or bad conductors are used in home utensils such as handles of sauce pans, hot plates, spoons, etc. They are made up of wood or plastic. Air is one of the bad conductors or best insulators. That is why cavity walls i.e. two walls separated by an air space and double-glazed windows keep the houses warm in winter and cool in summer. Materials that trap air i.e. wool, felt, fur, feathers, polystyrenes, and fiberglass are also bad conductors. Some of these materials are used for laggings to insulate water pipes, hot water cylinders, ovens, refrigerators, walls, and roofs of houses. Woolen cloth is used to make warm winter clothes.

                Soft insulation boards are used between the external brick walls of a house.

The thermal conductivities of some substances are given in the table.

FOR YOUR INFORMATION

Water is a poor conductor. Water at the top of the test tube starts boiling after getting heat from the burner without melting ice.

DO YOU KNOW

Sauce pans are made of metal for quick heat transfer.

DO YOU KNOW

Feathers give good thermal insulation especially when fluffed up.

Convection:

            Transfer of heat by actual movement of molecules from a hot place to a cold place is known as convection.

             Liquids and gases are poor conductors of heat. However, heat is transferred through fluids (liquids or gases) easily by another method called convection.

Experiment:

             Take a beaker and fill two-thirds of it with water. Heat the beaker by keeping a burner below it. Drop two or three crystals of potassium permanganate in the water. It will be seen that colored streaks of water formed by the crystals move upwards above the flame and then move downwards from sideways. These colored streaks show the path of currents in liquid. When the water at the bottom of the beaker gets hot, it expands, becomes lighter, and rises. While the cold but denser water moves downward to take its place.

Convection currents in the air:

              Gases also expand on heating; thus, convection currents are easily set up due to differences in the densities of air at various parts of the atmosphere.

Use of convection currents:

              Convection currents set up by electric gas or coal heaters help to warm our homes and offices. The central heating systems in buildings work on the same principle by convection. Convection currents occur on a large scale in nature. The day-to-day temperature changes in the atmosphere result from the circulation of warm or cold air that travels across the region. Land and sea breezes are also examples of convection currents.

A liquid or a gas becomes lighter (less dense) as it expands on heating. Hot liquid or gas rises above the heated area. The cooler liquid or gas from the surroundings fills the place which in turn is heated up. In this way, all fluid is heated up. Therefore, the transfer of heat through the fluids takes place by the actual movement of heated molecules from hot to cold parts of the fluid.

Land and sea breezes:

Land and sea breezes are the result of convection. On a hot day, the temperature of the land increases more quickly than the sea. It is because the specific heat of the land increases more quickly than the sea. It is because the specific heat of land is much smaller as compared to water. The air above land gets hot and rises. Cold air from the sea begins to move towards the land it is called the sea breeze.

At night, the land cools faster than the sea. Therefore, the air above the sea is warmer, and rises and the cold air from the land begin to move towards the sea it is called the land breeze.

A glider looks like a small airplane without an engine. Glider pilots use the upward movement of hot air currents due to the convection of heat. These rising currents of hot air are called thermals. Gliders ride over these thermals. The upward movement of air currents in thermals helps them to stay in the air for a long period.

The birds stretch out their wings and circle in these thermals. The upward movement of air helps birds to climb up with it. Eagles, hawks, and vultures are expert thermal climbers. After getting a free lift, birds can fly for hours without flapping their wings. They glide from one thermal to another and thus travel through large distances and hardly need to flap their wings.

Radiations:

             Radiation is the mode of transfer of heat from one place to another in the form of waves called electromagnetic waves.

Transfer of heat energy from the sun to earth:

              Our Sun is the major source of heat energy. Sunlight reaches us neither by conduction nor by convection because the space between the Sun and the Earth's atmosphere is empty. There is a third mode called radiation by which heat travels from one place to another. It is through radiation that heat reaches us from the Sun.

When the temperature of an object (cup of hot tea) is higher than its surroundings then it is radiating more heat than it is absorbing. As a result, its temperature goes on decreasing till it becomes less than its surroundings.

Therefore, a cup of hot tea becomes cold after some time.

When the temperature of an object (chilled water) is lower than its surroundings, then it is radiating less heat than it is absorbing. As a result, its temperature goes on increasing till it becomes equal to its surroundings. That is why a glass of chilled (frozen) water become hot sometimes.

The rate at which various surfaces emit heat depends upon the nature of the surface. These surfaces can be compared using Leslie’s cube.

Emission and absorption of radiation:

A Leslie cube is a metal box having faces of different nature. The four faces of Leslie's cube may be as follows:

• A shiny silvered surface
• A dull black surface
• A white surface
• A colored surface

Hot water is filled in Leslie's cube and placed with one of its faces toward a radiation detector. It is found that a black dull surface is a good emitter of heat.

The rate at which various surfaces absorb heat also depends upon the nature of those surfaces. For example, take two surfaces, one is dull black and the other is a silver polished surface as shown in the figure with a candle in the middle of the surface. It is found that:

1. A dull black surface is a good absorber of heat as its temperature rises rapidly.
2. A polished surface is a poor absorber of heat as its temperature rises very slowly. The observations made from the setup are shown in the table given below:

It is also found that the transfer of heat by radiation is also affected by the surface area of the body emitting or absorbing heat. The larger the area, the greater will be the transfer of heat. It is due to the reason of large numbers of slots are made in radiations to increase their surface area.

Heat does not reach us by conduction through the air from a fireplace because air is a poor conductor of heat. Heat does not reach us by convection because the air getting heat from the fireplace does not move in all directions. Hot air moves upward from the fireplace. The heat from the fireplace reaches us directly by a different process in the form of waves called radiation. A sheet of paper or cardboard kept in the path of radiation stops these waves to reach us.

Greenhouse effect:

The warming results when solar radiation is trapped by the atmosphere, caused by atmospheric gases that allow sunshine to pass through but absorb heat that is radiated back from the warmed surface of the earth.

Explanation:

Light from the Sun contains thermal radiations of long wavelengths as well as light and ultraviolet radiations of short wavelengths. Glass and transparent polythene sheets allow radiations of short wavelengths to pass through easily but not long wavelengths of thermal radiations. Thus, a greenhouse becomes a heat trap.

Radiations from the Sun pass easily through the glass and warms up the objects in a greenhouse. These objects and plants give out radiations of much longer wavelengths. Glass and transparent polythene sheets do not allow them to escape out easily and are reflected in the greenhouse. Thus, maintaining the inside temperature of the greenhouse. The greenhouse effect promises better growth of some plants.

Global warming:

Carbon dioxide and water also behave similarly to radiations as glass or polythene Earth's atmosphere contains carbon dioxide and water vapors. It causes the greenhouse effect and thus maintains the temperature of the Earth.

In recent years, the percentage of carbon dioxide has increased considerably. This has caused an increase in the average temperature of the Earth by trapping more heat due to the greenhouse effect. This phenomenon is known as global warming. It has serious implications for the global climate.

Application and consequences of radiation:

A black and rough surface absorbs more heat than a white or polished surface. Since good absorbers are also good radiators of heat. Thus, a black-colored body gets hot quickly absorbing heat reaching it during a sunny day, and also cools down quickly by giving out its heat to its surroundings.

The bottoms of cooking pots are made black to increase the absorption of heat from the fire. White surfaces reflect more than colored or black surfaces. Similarly, polished surfaces are good reflectors than rough surfaces, and the reflection of heat radiations is greater from polished surfaces. Hence, we wear white or light-colored clothes in summer which reflect most of the heat radiation reaching us on a hot day.

We polish the interior of the cooking and hot pots for reflecting most of the heat radiation within them.

FOR YOUR INFORMATION

In a thermos flask, most of the heat is prevented to enter or leave the flask. This is done by suitable measures to reduce the transfer of heat due to conduction, convection, and radiation. Thus, anything kept in it maintains its temperature for a long time.