Electromagnetic fields in matter revisited

Abstract: The force density on matter and the kinetic energy-momentum tensor of the electromagnetic field in matter are obtained starting from Maxwell equations and Lorentz force at microscopic level and averaging over a small region of space-time. The macroscopic force density is taken to depend linearly on the average fields and their first derivatives and is shown to be determined by two phenomenological fields which are subsequently identified with the free current density and the polarization density tensor. It is shown that as expected, the average current density is the sum of the free current density and a dipolar contribution and that the average field satisfy the Maxwell equations. The macroscopic energy-momentum tensor of the field is shown to be equal to the standard empty-space energy-momentum tensor built with the macroscopic fields plus a dipolar correction. The density of momentum of the field is confirmed to be given by Minkowski's expression. The energy-momentum tensor of macroscopic matter is equal to the average of the microscopic energy-momentum tensor of matter plus the difference between the average tensor of microscopic fields and the macroscopic tensor of fields.