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Existence of statistical ensemble with fixed energy but varying volume

Physics Asked by Quantumwhisp on September 26, 2020

To me, every statistical ensemble in statistical physics was introduced beginning with the microcanonical ensemble, in which every microstate is equally probable. A canonical ensemble is described by combining two ensembles, who together shall form a microcanonical ensemble. The microstates in system 1 shall then form the canonical ensemble, system 2 is said to be large compared to system 1, so that it’s temperature $T = frac{partial S}{partial E}$ doesn’t change when the two systems interchange energy.
If one still requires every possible microstate of the whole system to be equally probable, then the probablilities for microstates $Gamma$ in the small system scale with a factor $e^{-frac{H(Gamma)}{k_b T}}$. The canonical ensemble is no longer described by its energy, but by it’s temperature ( and volume V, particlenumber N ….)

My question: Is there also a way to describe an ensemble that has a fixed Energy, E, but varying Volume, that means, a EpN Ensemble, or a SpN Ensemble?
That would mean that I look at two systems that can interchange Volume, but don’t interchange energy, in the same way I described it above for the canonical ensemble.

I am asking because the Enthalpie H(S,p,N) exists, and is a thermodynamical potential that dependes on S,p,N, which suggests that such an ensemble exists.

One Answer

Yes. It is called the Isoenthalpic-isobaric Ensemble. See my answer for more details, but remember that moving between different ensembles is as simple as a Laplace transform or a Legendre transform (that answer will show more details). So you can really construct as many "ensembles" as you have Thermodynamic variables to do so.

Answered by aidan.plenert.macdonald on September 26, 2020

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