Thermodynamic state

A thermodynamic state is the macroscopic condition of a thermodynamic system as described by its particular thermodynamic parameters. The state of any thermodynamic system can be described by a set of thermodynamic parameters, such as temperature, pressure, density, composition, independently of its surroundings or history.

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The parameters required to specify the state depend on the characteristics of the system. There is an optimal ensemble of parameters that uniquely specify the state, and all other parameters can be derived from these. The state postulate says that the state of a simple compressible system is completely specified by two independent, intensive properties.

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.

See also

• Thermodynamic system
• Thermodynamics

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.
• Cengel, Yunus, A. (2007). Thermodynamics – an Engineering Approach, 6th Ed. (textbook). New York: McGraw Hill.
• Perrot, Pierre. (1998). A to Z of Thermodynamics (dictionary). New York: Oxford University Press.