Bodies Prepared With Separately Uniform Temperatures, Then Put Into Purely Thermal Communication Wit
If bodies are prepared with separately uniform temperatures, and are then put into purely thermal communication with each other, by conductive or radiative pathways, they will be in thermal equilibrium with each other just when they have the same temperature, and then there will be no net transfer of heat between them; but if initially they do not have the same temperature, heat will flow from the hotter to the colder, by whatever pathway, conductive or radiative, is available, and this flow will continue until thermal equilibrium is reached and then they will have the same temperature.
One form of thermal equilibrium is radiative exchange equilibrium. Two bodies, each with its own uniform temperature, in solely radiative connection, no matter how far apart, or what partially obstructive, reflective, or refractive, obstacles lie in their path of radiative exchange, not moving relative to one another, will exchange thermal radiation, in net the hotter transferring energy to the cooler, and will exchange equal and opposite amounts just when they are at the same temperature. In this situation, Kirchhoff's law of equality of radiative emissivity and absorptivity and the Helmholtz reciprocity principle are in play.
Read more about this topic: Thermal Equilibrium
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