Good absorbers of radiation are also good radiaters this statement is quantitatively explained by Kirchoff's law. (i) Emissive Power -
Emissive power denotes the energy radiated per unit area per unit solid angle normal to the area.
E = Δu/ [(ΔA) (Δω) (Δt)]
where, Δu is the energy radiated by area ΔA of surface in solid angle Δω in time Δt.
(ii) Absorptive Power -
Absorptive power of a body is defined as the fraction of the incident radiation that is absorbedby the body
a(absorptive power) = energy absorbed / energy incident
(iii) Kirchoff's Law
"It status that at any given temperature the ratio of emissive power to the absorptive power is constant for all bodies and this constant is equal to the emissive power of perfect B.B. at thesame temperature. E/abody=EB.B.
From kirchoff's law we can say that a body having high emissive power should have high absorptive power and those having low emiesive power should have law absorptive power so as to keep the ratio E/a same.
10. Newton's Law of Cooling
Consider a hot body at temperature T1 is placed in surrounding at temperature T2.
For small temperature difference between the body and surrounding rate of cooling is directly proportional to the temperature difference and surface area exposed i.e.,
dT/dt = - bA (T1 - T2)
This is known a Newton's law of cooling.
b depends on nature of surface involved and the surrounding conditions. Negative sign is to indicate that T1>T2 , dT/dt is negative and temperature decreases with time
According to this law, the rate of cooling is directly prospertional to the excess of temperature.