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A thermodynamic potential is a scalar quantity used to represent the thermodynamic state of a system. The concept of thermodynamic potentials was introduced by Pierre Duhem in 1886. Josiah Willard Gibbs in his papers used the term ''fundamental functions''. One main thermodynamic potential that has a physical interpretation is the internal energy . It is the energy of configuration of a given system of conservative forces (that is why it is a potential) and only has meaning with respect to a defined set of references (or data). Expressions for all other thermodynamic energy potentials are derivable via Legendre transforms from an expression for . In thermodynamics, certain forces, such as gravity, are typically disregarded when formulating expressions for potentials. For example, while all the working fluid in a steam engine may have higher energy due to gravity while sitting on top of Mount Everest than it would at the bottom of the Mariana Trench, the gravitational potential energy term in the formula for the internal energy would usually be ignored because ''changes'' in gravitational potential within the engine during operation would be negligible. == Description and interpretation == Five common thermodynamic potentials are:〔Alberty (2001) p. 1353〕 where = temperature, = entropy, = pressure, = volume. The Helmholtz free energy is often denoted by the symbol , but the use of is preferred by IUPAC,〔Alberty (2001) p. 1376〕 ISO and IEC.〔ISO/IEC 80000-5:2007, item 5-20.4〕 is the number of particles of type in the system and is the chemical potential for an -type particle. For the sake of completeness, the set of all are also included as natural variables, although they are sometimes ignored. These five common potentials are all energy potentials, but there are also entropy potentials. The thermodynamic square can be used as a tool to recall and derive some of the potentials. Just as in mechanics, where potential energy is defined as capacity to do work, similarly different potentials have different meanings. Internal energy ( ) is the capacity to do work plus the capacity to release heat. Gibbs energy is the capacity to do non-mechanical work. Enthalpy is the capacity to do non-mechanical work plus the capacity to release heat. Helmholtz free energy is the capacity to do mechanical work (useful work). From these definitions we can say that is the energy added to the system, is the total work done on it, is the non-mechanical work done on it, and is the sum of non-mechanical work done on the system and the heat given to it. Thermodynamic potentials are very useful when calculating the equilibrium results of a chemical reaction, or when measuring the properties of materials in a chemical reaction. The chemical reactions usually take place under some simple constraints such as constant pressure and temperature, or constant entropy and volume, and when this is true, there is a corresponding thermodynamic potential that comes into play. Just as in mechanics, the system will tend towards lower values of potential and at equilibrium, under these constraints, the potential will take on an unchanging minimum value. The thermodynamic potentials can also be used to estimate the total amount of energy available from a thermodynamic system under the appropriate constraint. In particular: (see principle of minimum energy for a derivation)〔Callen (1985) p. 153〕 * When the entropy ( ) and "external parameters" (e.g. volume) of a closed system are held constant, the internal energy ( ) decreases and reaches a minimum value at equilibrium. This follows from the first and second laws of thermodynamics and is called the principle of minimum energy. The following three statements are directly derivable from this principle. * When the temperature ( ) and external parameters of a closed system are held constant, the Helmholtz free energy ( ) decreases and reaches a minimum value at equilibrium. * When the pressure () and external parameters of a closed system are held constant, the enthalpy ( ) decreases and reaches a minimum value at equilibrium. * When the temperature ( ), pressure () and external parameters of a closed system are held constant, the Gibbs free energy ( ) decreases and reaches a minimum value at equilibrium. 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 potential」の詳細全文を読む スポンサード リンク
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