Intensive quantity

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Intensive quantity

Description

Intensive quantities are partial derivatives of an extensive quantity by the advancement, dtrΞΎX, of an energy transformation tr; example: Force. In contrast to extensive quantities which pertain to the entire system and are additive, extensive quantities 'take well defined values at each point of the system' (Prigogine 1967 Interscience) and are non-additive. Intensive and extensive quantities can be easily discriminated by the units, e.g. [J] for the extensive quantity, in contrast to [JΒ·mol-1] for the corresponding intensive quantity. In the general definition of thermodynamics, intensive quantities are not distinguished from specific quantities (Cohen 2008 IUPAC Green Book). Ergodynamics emphasizes the contrast between specific quantities which are extensive quantities normalized for a variable expressing system size (mass, volume of the system, amount of substance in a system) and intensive quantities which are normalized for the motive unit of a transformation (mass exchanged, volume change of the system, amount of substance reacting in a system; Gnaiger 1993 Pure Appl Chem). Intensive and specific quantities are both non-additive, take well defined values at each point of the system, and both corresponding quantities are expressed in identical units, e.g. the intensive quantity Gibbs force of a catabolic reaction (such as oxidation; 0 = -1 Glc - 6 O2 + 6 CO2 + 6 H2O), Ξ”kGGlc [kJΒ·mol-1], and the specific quantity Gibbs energy per mole glucose contained in a system, GGlc [kJΒ·mol-1] (with respect to an arbitrarily defined reference state, such as the reference state of formation or combustion).


Reference: Gnaiger E (1993) Nonequilibrium thermodynamics of energy transformations. Pure Appl Chem 65:1983-2002. http://dx.doi.org/10.1351/pac199365091983

Force and pressure

Isomorphic forces and pressures are intensive quantities. Pressure, however, is not pure intensity but contains a capacity, since pressure is force (related to the log of a concentration ratio) multiplied by free activity (expressed in units of concentration).

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MitoPedia concepts: MiP concept, Ergodynamics 


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