Electromagnetic forces on point dipoles

Abstract

The forces exerted on classical electric and magnetic dipoles by externally applied electromagnetic fields are derived from first principles. It is found, in accordance with Penfield and Haus (1967) and De Groot and Suttorp (1972), that the force on a magnetic dipole is the same for a perfectly conducting electric-current (Amperian) model and for a magnetic-charge model of the magnetic dipole, provided the sources of the externally applied field lie outside the dipole. However, if the dipoles lie within the polarization densities of the externally applied field, the force they experience depends on the model chosen for the dipoles (and for the polarization densities of the externally applied fields). The difference between the forces on point Amperian and magnetic-charge magnetic dipoles within the polarization densities of an external field has been used to demonstrate experimentally that neutrons scattered by the fields in ferromagnetic materials act like Amperian rather than magnetic-charge magnetic dipoles.