My textbook says that net rate of heat transfer due to radiation is $\epsilon \sigma A(T^4-T_o^4)$ but i couldn't understand it.

Rate of emission is $\epsilon \sigma A T^4$ and rate of absorption is $a \sigma A T_o^4$ so net rate of heat transfer must be $\epsilon \sigma A T^4 - a \sigma A T_o^4$,it is possible only if $a=e$ which means body is in thermal equilibrium.

But that is not always the case. Please help.



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