Question

What is a perfect radiator or black body and why is that important for the equation q/A=sigma(T^4)?

Answer 1

A perfect black body is the body which absorbs all the radiations incidence on its surface. It doesnot reflect back or transmit the incident radiation. The absorptivity if the black body is 1. Further the emissivity which is taken as the ratio of emissive power of body to the emissive power of of black body is also unity.

Now the Total radiation for a unit area of an opaque body of area A, emissivity e and temperature T is given by q/A = e E_b where E_b is emmissive powwr and is equal to T⁴

q/A = eT⁴ .

for black body e =1

q/A = eT⁴