Thermodynamics in the presence of electromagnetic fields.

Abstract

The physical significance of the internal energy as a state function and of thermodynamic variables in the presence of fields are considered. Intensive variables are formulated in the presence of fields and it is shown that, due to the fact that their form depends on field constraints, they are not unique. Four different pairs of pressure and chemical potential corresponding to four different field constraints are identified for the case of a uniformly magnetized continuum. This multiplicity of form of the pressure and chemical potential is a consequence of the effect of fields on thermodynamic systems and their environment. It is shown that the differential expressing the net effect of fields on the internal energy of a thermodynamic system must be exact. This is shown to apply for the analysis of electrostatic and magnetoquasistatic fields, regarding their thermodynamic properties and conditions of equilibrium. Discrete systems are defined as those involving field lines that cross their boundaries. The magnetic field which is energized by a polarized discrete system, outside its boundaries, is defined as pertaining to the system. It is claimed that internal energy, i.e., in its literal thermodynamic meaning, does not exist in discrete magnetized systems where part or all of their magnetic energy may be stored outside their boundaries. Once this externally stored energy is defined as having a source which is internal to the system, then the concept of internal energy can still be used. Finally, intensive thermodynamic variables are formulated for discrete systems. These variables are shown to have the same multiplicity of form under different field constraints.