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For thermodynamics, a thermodynamic state of a system is fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables. Once such a set of values of thermodynamic variables has been specified for a system, the values of all thermodynamic properties of the system are uniquely determined. Thermodynamics sets up an idealized formalism that can be summarized by a system of postulates of thermodynamics. Thermodynamic states are amongst the fundamental or primitive objects or notions of the formalism, in which their existence is formally postulated, rather than being derived or constructed from other concepts.〔Callen, H.B. (1960/1985). ''Thermodynamics and an Introduction to Thermostatistics'', (1st edition 1960) 2nd edition 1985, Wiley, New York, ISBN 0-471-86256-8, p. 13.〕〔Carathéodory, C. (1909).〕 A thermodynamic system is not simply a physical system.〔Jaynes, E.T. (1965), p. 397.〕 Rather, in general, indefinitely many different alternative physical systems comprise a given thermodynamic system, because in general a physical system has vastly many more detailed characteristics than are mentioned in a thermodynamic description. A thermodynamic system is a macroscopic object, the microscopic details of which are not explicitly considered in its thermodynamic description. The number of state variables required to specify the thermodynamic state depends on the system, and is not always known in advance of experiment; it is usually found from experimental evidence. Always the number is two or more; usually it is not more than some dozen. Though the number of state variables is fixed by experiment, there remains choice of which of them to use for a particular convenient description; a given thermodynamic system may be alternatively identified by several different choices of the set of state variables.〔Prigogine, I., Defay, R. (1950/1954), p. 1.〕〔Zemanksy, M.W., Dittman, R.H. (1937/1981), p. 6.〕 For equilibrium thermodynamics, in a thermodynamic state of a system, its contents are in internal thermodynamic equilibrium, with zero flows of all quantities, both internal and between system and surroundings. For Planck, the primary characteristic of a thermodynamic state of a system that consists of a single phase, in the absence of an externally imposed force field, is spatial homogeneity.〔Planck, M., (1923/1927), p. 3.〕 For non-equilibrium thermodynamics, a suitable set of identifying state variables includes some macroscopic variables, for example a non-zero spatial gradient of temperature, that indicate departure from thermodynamic equilibrium. Such non-equilibrium identifying state variables indicate that some non-zero flow may be occurring within the system or between system and surroundings.〔Eu, B.C. (2002).〕 ==State functions== (詳細はthermodynamic process; it typically involve transfers of matter or energy between system and surroundings. In any thermodynamic process, whatever may be the intermediate conditions during the passage, the total respective change in the value of each thermodynamic state variable depends only on the initial and final states. For an idealized continuous process, this means that infinitesimal incremental changes in such variables are exact differentials. Together, the incremental changes throughout the process, and the initial and final states, fully determine the idealized process. In the most commonly cited simple example, an ideal gas, the thermodynamic variables would be any two variables out of the following four: entropy, pressure, temperature, and volume. Thus the thermodynamic state would range over a two-dimensional state space. The remaining two variables, as well as other quantities such as the internal energy, would be expressed as state functions of these two variables. The state functions satisfy certain universal constraints, but ultimately they depend on the materials involved in the concrete system. Various thermodynamic diagrams have been developed to model the transitions between thermodynamic states. 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 state」の詳細全文を読む スポンサード リンク
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