I am having a trouble in understanding this concept. If bodies are at thermal equilibrium, there is no heat transfer. Heat is transferred through three different ways: Conduction, convection and Radiation. Well,according to Prevost theory all bodies radiate thermal radiation at all temperatures. Well, that would include even when they are in thermal equilibrium? If yes, then how so, there shouldn't be any heat transfer. I understand that, if it has to remain in the same temperature the heat radiated out must equal heat radiated in, but I don't understand why in the first place heat is radiated out.
In thermal equilibrium there is no net heat transfer. If to objects are placed inn thermal contact they will generally exchange energy with each other, however in thermal equilibrium the rate of energy transfer is the same in both directions, so there is no net transfer of heat.
Imagine, say, a very large, homogeneous volume of gas at constant temperature. So it's in thermal equilibrium. And radiation's going every which way, i.e., every little volume element is radiating isotropically. Now, just imagine drawing any closed surface you like, and call the interior of that closed surface a "body". Then, regardless of the shape of that surface, the divergence theorem's going to show you the same net flux out as in.
There's no "real body" above, just an imaginary closed surface you drew inside some homogeneous volume of gas at constant temperature. And in "real life", with an "actual body" it's exactly the same thing. If that body's in thermal equilibrium with its surroundings, you can consider its surface as just an imaginarily-drawn boundary surface with respect to your thermal considerations/calculations.