Free energy of electrets

In this paper we present a calculation of the free energy of an electret placed between two plane electrodes. By making use of the second law of thermodynamics we describe the behavior of a noncompressible electret free to move between two electrodes and of a compressible electret with two contacting electrodes. This latter case corresponds to very usual conditions for the study of piezo- or pyroelectric properties of polymers. In the last section we use the expression of the free energy to derive the forces acting on an electret. The equation of motion is then developed by applying the Lagrangian formalism to both the mechanical and the electrical variables, which are coupled.