Electromagnetic momentum density and the Poynting vector in static fields

Many static configurations involving electrical currents and charges possess angular momentum in electromagnetic form; two examples are discussed here, an electric charge in the field of a magnetic dipole, and an electric charge in the vicinity of a long solenoid. These provide clear evidence of the physical significance of the circulating energy flux indicated by the Poynting vector, as the angular momentum of the circulating electromagnetic energy can be converted to mechanical angular momentum by turning off the magnetic field. Electromagnetic momentum is created whenever electric fields change in the presence of a magnetic field and whenever magnetic fields change in the presence of an electric field. When simple dielectrics are involved, the momentum density can be resolved into two components, a pure-field component ε0E×B and a component χeε0E×B associated with the polarization of the dielectric, the sum being εrε0E×B=D×B. It is argued that the latter component should be considered to be part of the electromagnetic momentum density, whose value then is D×B.