He defined'local thermodynamic equilibrium'in a'cell'by requiring that it macroscopically absorb and spontaneously emit radiation as if it were in radiative equilibrium in a cavity at the temperature of the matter of the'cell '.
32.
In the cases of thermodynamic equilibrium and of local thermodynamic equilibrium, the number densities of the atoms, both excited and unexcited, may be calculated from the Maxwell Boltzmann distribution, but for other cases, ( e . g . lasers ) the calculation is more complicated.
33.
For local thermodynamic equilibrium, the radiation field does not have to be a black-body field, but the rate of interatomic collisions must vastly exceed the rates of absorption and emission of quanta of light, so that the interatomic collisions entirely dominate the distribution of states of atomic excitation.
34.
Since the internal temperature of molecules are usually equal to their kinetic temperature, the distinction is usually of interest only in the detailed study of non-local thermodynamic equilibrium ( LTE ) phenomena such as combustion, the sublimation of solids, and the diffusion of hot gases in a partial vacuum.
35.
"Local thermodynamic equilibrium of matter " ( see also Keizer ( 1987 ) means that conceptually, for study and analysis, the system can be spatially and temporally divided into'cells'or'micro-phases'of small ( infinitesimal ) size, in which classical thermodynamical equilibrium conditions for matter are fulfilled to good approximation.
36.
For example, in the atmosphere, the speed of sound is much greater than the wind speed; this favours the idea of local thermodynamic equilibrium of matter for atmospheric heat transfer studies at altitudes below about 60 km where sound propagates, but not above 100 km, where, because of the paucity of intermolecular collisions, sound does not propagate.
37.
When that state is either one of strict thermodynamic equilibrium, or one of so-called " local thermodynamic equilibrium ", then the distribution of atomic states of excitation ( which includes n _ 2 and n _ 1 ) determines the rates of atomic emissions and absorptions to be such that black-body radiation and is described by Planck's law.
38.
In thermodynamics, exchanges within a system and between the system and the outside are controlled by intensive parameters are homogeneous throughout the whole system, while " local thermodynamic equilibrium " ( LTE ) means that those intensive parameters are varying in space and time, but are varying so slowly that, for any point, one can assume thermodynamic equilibrium in some neighborhood about that point.
