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Thermodynamic equilibrium is an axiomatic concept of classical thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by permeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems. In a system in its own state of internal thermodynamic equilibrium, no macroscopic change occurs. Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria. Systems can be in one kind of mutual equilibrium, though not in others. In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation. In a macroscopic equilibrium, almost or perfectly, exactly balanced microscopic exchanges occur; this is part of the notion of macroscopic equilibrium. An isolated thermodynamic system in its own state of internal thermodynamic equilibrium has a uniform temperature. If its surroundings impose some unchanging long range force field on it, it may consist of one phase or may exhibit several spatially unchanging internal phases. If its surroundings impose no long range force field on it, then either (1) it is spatially homogeneous, with all intensive properties being uniform; or (2) it has several internal phases, which may exhibit indefinitely persistent continuous spontaneous microscopic or mesoscopic fluctuations. In non-equilibrium systems, by contrast, there are net flows of matter or energy. If such changes can be triggered to occur in a system in which they are not already occurring, it is said to be in a metastable equilibrium. It is an axiom of thermodynamics that when a body of material starts from a non-equilibrium state of non-homogeneity or chemical non-equilibrium, and, by a thermodynamic operation, is then isolated, it spontaneously evolves towards its own internal state of thermodynamic equilibrium. This axiom is presupposed by the second law of thermodynamics, which restricts what can happen when a system, having reached thermodynamic equilibrium, with a well defined entropy, is subject to a thermodynamic operation. == Overview == Classical thermodynamics deals with states of dynamic equilibrium. The state of a system at thermodynamic equilibrium is the one for which some thermodynamic potential is minimized, or for which the entropy (''S'') is maximized, for specified conditions. One such potential is the Helmholtz free energy (''A''), for a system with surroundings at controlled constant temperature and volume: : Another potential, the Gibbs free energy (''G''), is minimized at thermodynamic equilibrium in a system with surroundings at controlled constant temperature and pressure: : where ''T'' denotes the absolute thermodynamic temperature, ''P'' the pressure, ''S'' the entropy, ''V'' the volume, and ''U'' the internal energy of the system. Thermodynamic equilibrium is the unique stable stationary state that is approached or eventually reached as the system interacts with its surroundings over a long time. The above-mentioned potentials are mathematically constructed to be the thermodynamic quantities that are minimized under the particular conditions in the specified surroundings. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thermodynamic equilibrium」の詳細全文を読む スポンサード リンク
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