How various surfaces can be compared by a Leslie cube?
OR
Investigate the absorption of radiation by a black surface and silvery surface using Leslie cube. Also, investigate the emission of radiation by a black surface and silvery surfaces using the Leslie cube.
OR
Explain how the rate of energy transfer is affected by:
- Colour and texture of the surface
- Surface temperature
- Surface area
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:
- A dull black surface is a good absorber of heat as its temperature rises rapidly.
- 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:
Surfaces | Emitter | Absorber | Reflector |
Dull black surface | best | best | worst |
Colored Surface | good | good | bad |
White Surface | bad | bad | good |
Shining silvered surface | worst | worst | best |
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.