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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. These conditions are unfulfilled, for example, in very rarefied gases, in which molecular collisions are infrequent; and in the boundary layers of a star, where radiation is passing energy to space; and for interacting fermions at very low temperature, where dissipative processes become ineffective. When these 'cells' are defined, one admits that matter and energy may pass freely between contiguous 'cells', slowly enough to leave the 'cells' in their respective individual local thermodynamic equilibria with respect to intensive variables.

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. These conditions are unfulfilled, for example, in very rarefied gases, in which molecular collisions are infrequent; and in the boundary layers of a star, where radiation is passing energy to space; and for interacting fermions at very low temperature, where dissipative processes become ineffective. When these 'cells' are defined, one admits that matter and energy may pass freely between contiguous 'cells', slowly enough to leave the 'cells' in their respective individual local thermodynamic equilibria with respect to intensive variables.

物质的局部热力学平衡(参见 Keizer (1987))意味着，从概念上来说，为了研究和分析，系统可以在空间和时间上分割为小(无限小)尺寸的‘细胞’或‘微相’ ，每个微元中物质的经典热力学平衡条件得在很好的近似下得以满足。经典热力学平衡条件对系统整体可能不能满足，例如在非常稀薄的气体中，分子碰撞很少发生; 在恒星的边界层中，辐射将能量传递到空间; 在非常低的温度下相互作用的费米子中，耗散过程变得无效。但是当我们定义这些“细胞”时，人们承认物质和能量可以在相邻的“细胞”之间自由通过，慢到足以在它们各自关于强度量的局部热力学平衡中离开“细胞”。

物质的局部热力学平衡(参见 Keizer (1987))意味着，从概念上来说，为了研究和分析，系统可以在空间和时间上分割为小(无限小)尺寸的‘单元’或‘微相’ ，每个单元中物质的经典热力学平衡条件得在很好的近似下得以满足。经典热力学平衡条件对系统整体可能不能满足，例如在非常稀薄的气体中，分子碰撞很少发生; 在恒星的边界层中，辐射将能量传递到空间; 在非常低的温度下相互作用的费米子中，耗散过程变得无效。但是当我们定义这些“单元”时，人们承认物质和能量可以在相邻的“单元”之间自由通过，慢到足以在它们各自关于强度量的局部热力学平衡中离开“单元”。

One can think here of two 'relaxation times' separated by order of magnitude.<ref name="Zubarev 1971/1974">[[Dmitry Zubarev|Zubarev D. N.]],(1974). ''[https://books.google.com/books?id=SQy3AAAAIAAJ&hl=ru&source=gbs_ViewAPI Nonequilibrium Statistical Thermodynamics]'', translated from the Russian by P.J. Shepherd, New York, Consultants Bureau. {{ISBN|0-306-10895-X}}; {{ISBN|978-0-306-10895-2}}.</ref> The longer relaxation time is of the order of magnitude of times taken for the macroscopic dynamical structure of the system to change. The shorter is of the order of magnitude of times taken for a single 'cell' to reach local thermodynamic equilibrium. If these two relaxation times are not well separated, then the classical non-equilibrium thermodynamical concept of local thermodynamic equilibrium loses its meaning<ref name="Zubarev 1971/1974"/> and other approaches have to be proposed, see for instance [[Extended irreversible thermodynamics]]. 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&nbsp;km where sound propagates, but not above 100&nbsp;km, where, because of the paucity of intermolecular collisions, sound does not propagate.

One can think here of two 'relaxation times' separated by order of magnitude.<ref name="Zubarev 1971/1974">[[Dmitry Zubarev|Zubarev D. N.]],(1974). ''[https://books.google.com/books?id=SQy3AAAAIAAJ&hl=ru&source=gbs_ViewAPI Nonequilibrium Statistical Thermodynamics]'', translated from the Russian by P.J. Shepherd, New York, Consultants Bureau. {{ISBN|0-306-10895-X}}; {{ISBN|978-0-306-10895-2}}.</ref> The longer relaxation time is of the order of magnitude of times taken for the macroscopic dynamical structure of the system to change. The shorter is of the order of magnitude of times taken for a single 'cell' to reach local thermodynamic equilibrium. If these two relaxation times are not well separated, then the classical non-equilibrium thermodynamical concept of local thermodynamic equilibrium loses its meaning<ref name="Zubarev 1971/1974"/> and other approaches have to be proposed, see for instance [[Extended irreversible thermodynamics]]. 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&nbsp;km where sound propagates, but not above 100&nbsp;km, where, because of the paucity of intermolecular collisions, sound does not propagate.

One can think here of two 'relaxation times' separated by order of magnitude. The longer relaxation time is of the order of magnitude of times taken for the macroscopic dynamical structure of the system to change. The shorter is of the order of magnitude of times taken for a single 'cell' to reach local thermodynamic equilibrium. 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. 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&nbsp;km where sound propagates, but not above 100&nbsp;km, where, because of the paucity of intermolecular collisions, sound does not propagate.

One can think here of two 'relaxation times' separated by order of magnitude. The longer relaxation time is of the order of magnitude of times taken for the macroscopic dynamical structure of the system to change. The shorter is of the order of magnitude of times taken for a single 'cell' to reach local thermodynamic equilibrium. 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. 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 60km where sound propagates, but not above 100km, where, because of the paucity of intermolecular collisions, sound does not propagate.

Edward A. Milne, thinking about stars, gave a definition of 'local thermodynamic equilibrium' in terms of the thermal radiation of the matter in each small local 'cell'. 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'. Then it strictly obeys Kirchhoff's law of equality of radiative emissivity and absorptivity, with a black body source function. The key to local thermodynamic equilibrium here is that the rate of collisions of ponderable matter particles such as molecules should far exceed the rates of creation and annihilation of photons.

Edward A. Milne, thinking about stars, gave a definition of 'local thermodynamic equilibrium' in terms of the thermal radiation of the matter in each small local 'cell'. 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'. Then it strictly obeys Kirchhoff's law of equality of radiative emissivity and absorptivity, with a black body source function. The key to local thermodynamic equilibrium here is that the rate of collisions of ponderable matter particles such as molecules should far exceed the rates of creation and annihilation of photons.

==Entropy in evolving systems==

==Entropy in evolving systems==