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.

up vote 1 down vote accepted

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.

  • Well, in Wikipedia, it says there has to be no heat transfer. I guess,they have mistaken there? – 123IR Feb 12 at 11:32
  • If for example there's a body C on which heat is given through another body A, and this heat then is transferred to another body B,so temperature of body C remain constant. So in here,the body C aint in thermal equilibrium with anyother right? – 123IR Feb 12 at 11:33
  • In thermodynamics heat is normally implicitly taken to mean the net energy transfer, and with this understanding the Wikipedia article is correct. The point is that if an object is emitting energy thermal radiation at some rate into its surroundings and also absorbing energy from its surroundings at exactly the same rate then there is no heat transfer taking place. – By Symmetry Feb 12 at 14:27
  • That means when an object kept at room temperature, and at thermal.eqm with the atmosphere, transfers some amount of heat and takes exactly the same – 123IR Feb 12 at 16:45
  • By the means of radiations only. – 123IR Feb 12 at 16:57

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.

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.