Lecture - 85
Topic : Kirchhoff's Law
Statement + Proof
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Kirchhoff's Law :The law states that at any temperature the ratio of total emissive power to the total absorptivity
is a constant for all substances which are in thermal equilibrium with their surroundings.
Proof :Let us consider a large radiating body of surface area A which encloses a small body of surface
area A1. Let the energy fall on the unit surface of the small body at the rate Eb. Of this energy, a
fraction
will be absorbed by the small body.
When thermal equilibrium is attained, the energy absorbed by the small body must be equal
to energy emitted by the large body.
Lecture- 87 Page 1
Thus, Kirchhoff's law also states that the emissivity of body is equal to the absorptivity
when body is in thermal equilibrium with their surroundings.
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