The grand potential is a quantity used in statistical mechanics, especially for irreversible processes in open systems. The grand potential is the characteristic state function for the grand canonical ensemble.
Grand potential is defined by
The change in the grand potential is given by
When the system is in thermodynamic equilibrium, ΦG is a minimum. This can be seen by considering that dΦG is zero if the volume is fixed and the temperature and chemical potential have stopped evolving.
Landau free energy
Grand potential for homogeneous systems (vs. inhomogeneous systems)
In the case of a scale-invariant type of system (where a system of volume has exactly the same set of microstates as systems of volume ), then when we grow the system new particles and energy will flow in from the reservoir to fill the new volume with a homogeneous extension of the original system. The pressure then must be constant with respect to changes in volume: , and the particle and all extensive quantities (particle number, energy, entropy, potentials, ...) must grow linearly with volume, e.g., . In this case we have simply , as well as the familiar relationship for the Gibbs free energy. The value of can be understood as the work we can extract from the system by shrinking it down to nothing (putting all the particles and energy back into the reservoir). The fact that is negative implies that it takes energy to perform this extraction.
Such homogeneous scaling does not exist in many systems. For example, when analyzing the ensemble of electrons in a single molecule or even a piece of metal floating in space, doubling the volume of the space does double the number of electrons in the material. The problem here is that, although electrons and energy are exchanged with a reservoir, the material host is not allowed to change. Generally in small systems, or systems with long range interactions (those outside the thermodynamic limit), .
For an ideal gas,
- Lee, J. Chang (2002). "5". Thermal Physics - Entropy and Free Energies. New Jersey: World Scientific.
- Reference on "Landau potential" is found in the book: D. Goodstein. States of Matter. p. 19.
- McGovern, Judith. "The Grand Potential". PHYS20352 Thermal and Statistical Physics. University of Manchester. Retrieved 5 December 2016.
- Brachman, M. K. (1954). "Fermi Level, Chemical Potential, and Gibbs Free Energy". The Journal of Chemical Physics. 22 (6): 1152–1151. Bibcode:1954JChPh..22.1152B. doi:10.1063/1.1740312.
- Hill, Terrell L. (2002). Thermodynamics of Small Systems. Courier Dover Publications. ISBN 9780486495095.