On the electrodynamics of moving permanent dipoles in external electromagnetic fields

Abstract

The classical theory of electrodynamics is built upon Maxwell's equations and\nthe concepts of electromagnetic field, force, energy and momentum, which are\nintimately tied together by Poynting's theorem and the Lorentz force law.\nWhereas Maxwell's macroscopic equations relate the electric and magnetic fields\nto their material sources (i.e., charge, current, polarization and\nmagnetization), Poynting's theorem governs the flow of electromagnetic energy\nand its exchange between fields and material media, while the Lorentz law\nregulates the back-and-forth transfer of momentum between the media and the\nfields. The close association of momentum with energy thus demands that the\nPoynting theorem and the Lorentz law remain consistent with each other, while,\nat the same time, ensuring compliance with the conservation laws of energy,\nlinear momentum, and angular momentum. This paper shows how a consistent\napplication of the aforementioned laws of electrodynamics to moving permanent\ndipoles (both electric and magnetic) brings into play the rest-mass of the\ndipoles. The rest mass must vary in response to external electromagnetic fields\nif the overall energy of the system is to be conserved. The physical basis for\nthe inferred variations of the rest-mass appears to be an interference between\nthe internal fields of the dipoles and the externally applied fields. We use\ntwo different formulations of the classical theory in which energy and momentum\nrelate differently to the fields, yet we find identical behavior for the\nrest-mass in both formulations.\n