Thermodynamic state
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In thermodynamics, a thermodynamic state, from Latin status = to stand, is a descriptive term used to characterize a thermodynamic system as described by its properties, i.e. particular thermodynamic parameters. A minimum number of parameters are necessary to specify the state of the system.
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[edit] Overview
The state of the thermodynamic system can be thought of as an optimal ensemble of thermodynamic parameters, namely temperature, pressure, density, composition, etc., which characterize the system, but neither by its surroundings nor by its history. The number of properties required to fix the state of a system is given by the state postulate, which dictates that the state of a simple compressible system is completely specified by two independent, intensive properties.
Every thermodynamic system can be described by a set of thermodynamic parameters; an optimal ensemble of parameters that uniquely specify the macroscopic condition of the system is said to be its state, from these all other parameters can be derived. Depending on the characteristic of a system, a varying number of parameters are needed to describe its state.
[edit] Examples
- Blackbody radiation is an example of a state that is completely described by temperature, although if phase transitions or spontaneous symmetry breaking occur other variables may be needed to discriminate among the phases. (This problem does not arise for blackbody radiation.) Given the internal energy as a function of temperature, we can define A = U - TS.
- Most "pure" nonmagnetic substances fall into this category. Their states are completely described by temperature and pressure, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. Given U and V (or the density ρ) as a function of T and P, we can define the Helmholtz energy as before and the Gibbs energy as G = U - TS + PV and the enthalpy as H = U + PV.
- If there is more than one kind of atom or molecule in a system, its state is described by temperature, pressure, and chemical potentials, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.
- If a substance is a ferromagnet or a superconductor, its state is described by temperature and a magnetic field, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.
[edit] See also
[edit] References
- Black W. & Hartley, J. (1996). Thermodynamics, 3rd Ed. (textbook). New York: Harper Collins.
- Cengel, Yunus, A. (2002). Thermodynamics – an Engineering Approach, 4th Ed. (textbook). New York: McGraw Hill.
- Perrot, Pierre. (1998). A to Z of Thermodynamics (dictionary). New York: Oxford University Press.
