# Unclear formula for radiative energy transfer

by kontejnjer
Tags: energy, formula, radiative, transfer, unclear
 P: 42 Right, so I was going over the formula sheet for my upcoming exam in thermodynamics, and I've stumbled upon this formula: $q=\frac{\sigma(T^{4}_{2}-T^{4}_{1})}{\frac{1}{\epsilon_1}+\frac{1}{\epsilon_2}-1}$ with a description that (I think) translates as heat flux density. I'm currently puzzled as to where this equation is applied, as our assistant didn't say anything about it or how it was derived in the first place. I'm guessing that we have two bodies with temperatures $T_1$ and $T_2$ with emissivity factors $\epsilon_1$ and $\epsilon_2$, so q is supposed to be the energy (heat) transferred in unit time over a unit surface from one body to the other, but I still haven't the slightest idea as to how this equation is derived. Are the shape of the bodies and their mutual position relevant here? What are the conditions under which the equation is applicable? I'm somewhat puzzled so any help would be much appreciated.