10.4 Radiation

Radiation is the method of heat transfer that does not require a material medium.


It is the transfer of heat in the form of electromagnetic radiation






Transfer of heat from Sun to Earth through Radiation, which takes place in a vacuum.

The Sun transfer heat to the Earth by radiation. Heat is transferred by electromagnetic waves called infra red waves. Heat transferred by infared waves are called radient heat. Conduction and convection is not possible here because of the vacuum between Sun and Earth.





Absorption and Emission of Radiation

Infared radiation is absorb and emitted by all objects and surfaces and absorbing radiant heat will result in a rise in temperature, while emitting it will result in a drop in temperature. How Hot a surface becomes depends mainly upon the amount of infrared radiation is absorbed by the surface.




If we stand near an open fire, example a fireplace or campfire, we will notice that the side of us that is facing the fire becomes much warmer than the other side overtime, even when there is no wind blowing. The hotter the object is, the more energy it radiates.




The rate of heat transfer by radiation from a hot body is affected by:

The colour and texture of the surface of a body

The surface temperature of the body

The surface area of the body

Colour and Texture of the surface of a body

A dull black surface is a better radiator of heat than a shiny surface.

Refer to the picture above, the block that is Lampblack-coated absorbed 0.97E compared to the Silver-coated block, which had only absorbed 0.10E.Hence, the darker the object is, the more radiation it will absorb.

However from the same picture as used above, we can also see that no matter which factors are affecting the amount of radiation transferred, the amount emitted and absorbed are the same.

Hence we can say that,

A rough dull black surface is both a good emitter and a good absorber.

A smotth polished surface is both a poor emitter and a poor absorber.






Surface Temperature



An object with a higher temperature will radiate heat faster. At a room temperature, a cup of water at 80 degrees (C) will radiat heat faster to the surroundings than another cup at 40 degrees(C)

Therefore,
The rate of radiation increases as the temperature increases.




Surface area



Large surface area will mean that more space will absorb heat, which will also emit the same amount of radiation. Hence, the rate of radiation increases as the surface area increases.