Page 16 - 9-phy-9 Transfer of Heat

9. Transfer of Heat eLearn.Punjab

Why does a cup of hot tea become cold after sometime? Why does a glass of chilled water become
hot after sometime?

All the objects, lying inside a room including the walls, roof and floor of the room are radiating heat.
However, they are also absorbing heat at the same time. When temperature of an object 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 equal to its surroundings. At this stage, the body is giving out the
amount of heat equal to the amount of heat it is absorbing.

When temperature of an object 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.
The rate at which various surfaces emit heat depends upon the nature of the surface. Various
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 as shown in figure 9.15. The four faces
of Leslie’s cube may be as follows:

• A shining silvered surface
• A dull black surface
• A white surface
• A coloured surface

Hot water is filled in the Leslie’s cube and is placed with one Figure 9.15: Radiations from Leslie’s cube.
of its face towards a radiation detector. It is found that 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 figure 9.16 with a candle at the middle of the surface. It is found that:

A dull black surface is a good absorber of heat as its temperature rises rapidly.

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