Thermodynamics of a Fluid Substance in the Electrostatic Field

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Abstract
An ``apparatus'' is described in which, in a thought experiment, the whole of a sample of fluid may be subjected to a uniform electrostatic field under controlled conditions. The work done in such a process can be written into fundamental thermodynamic equations so as to enable the field strength E to be a variable of state in a way which is essentially symmetrical with the way in which P and T are variables of state. This enables a variety of differential coefficients to be obtained, some of which appear to be new. A discussion is given of the relationship between ``electrostriction pressure'' (as derived in electrostatics) and thermodynamic pressure. Application of the formalism to systems of more than one component is illustrated by discussion of the change in composition produced by charging the plates of a small condenser immersed in a large volume of binary fluid mixture. The customary representation of the ``energy density'' in a field as equal to E2K/8π is discussed, and found to be limited in formal validity when the field is nonuniform, when the charging process takes place under conditions which do not hold the volume constant, or when dielectric saturation is taken into account.

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