When there is enough matter in a region to allow molecular collisions to occur very much more often than creation or annihilation of photons, for radiation one speaks of local thermodynamic equilibrium.
22.
Milne ( 1928 ), thinking about stars, gave a definition of'local thermodynamic equilibrium'in terms of the thermal radiation of the matter in each small local'cell '.
23.
Thus, when local thermodynamic equilibrium prevails in a body, temperature can be regarded as a spatially varying local property in that body, and this is because temperature is an intensive variable.
24.
This is related to the fact that heat may be transferred from a hotter to a colder physical system even when local thermodynamic equilibrium does not prevail, so that neither system has a temperature.
25.
Mainly developed by the Belgian-Dutch school headed by I . Prigogine, working on a simple hypothesis of local thermodynamic equilibrium, CIT assumes the existence of field laws of the diffusion type.
26.
The nightside upper mesosphere and thermosphere of Venus is also the source of non-local thermodynamic equilibrium emissions of CO 2 and nitric oxide molecules, which are responsible for the low temperature of the nightside thermosphere.
27.
On the other hand, it makes no sense to speak of the internal energy at a point, while when local thermodynamic equilibrium prevails, it makes good sense to speak of the temperature at a point.
28.
Accordingly, the local time rate of entropy production, defined according to the local thermodynamic equilibrium hypothesis, is not an adequate variable for prediction of the time course of far-from-thermodynamic equilibrium processes.
29.
Also it is assumed that the local entropy density is the same function of the other local intensive variables as in equilibrium; this is called the local thermodynamic equilibrium assumption ( see also Keizer ( 1987 ) ).
30.
If these two relaxation times are not well separated, then the classical non-equilibrium thermodynamical concept of local thermodynamic equilibrium loses its meaning and other approaches have to be proposed, see for instance Extended irreversible thermodynamics.
