If two bodies are in thermal equilibrium with each other does it mean that there is no heat transition between them or is it that there is no NET heat transit between them ? I think that the heat exchange occurs still but the net transit is zero, that is for a body , incoming heat= outgoing heat . Just want to know whether I am correct in this context and if not then what is the actual scenario.


1 Answer 1


Imagine a block of metal surrounded by air at exactly the same temperature. As there is no temperature gradient between the objects, there's no net transfer of heat fron one to the other.

But the atoms/molecules that make up the objects have a broad kinetic energy spectrum. Through collisions, more energetic atoms/molecules of one object can still transfer momentum, i.e. energy, to the less energetic atoms/molecules of the other object and vice versa. Heat is still being exchanged between them but the net flow of heat is zero.

  • $\begingroup$ I would add that in thermal equilibrium there is no heat flow between the bodies on anymacroscopic scales not just the net being zero; energy exchange at microscopic scales is not heat flow, it is molecular collisions and some such. $\endgroup$
    – hyportnex
    Nov 23, 2016 at 15:41
  • $\begingroup$ At least in the case of conduction and convection heat flow is due to molecular collisions. Heat flow is macroscopic because it's the sum of huge amounts of such collisions. $\endgroup$
    – Gert
    Nov 23, 2016 at 15:59
  • $\begingroup$ that is just the point. $\endgroup$
    – hyportnex
    Nov 23, 2016 at 16:01
  • 1
    $\begingroup$ Or consider radiation: an object in equilibrium in an oven is radiating and absorbing, the net being zero. $\endgroup$
    – user137289
    Nov 27, 2016 at 1:40

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