Electromagnetic Momentum Density and Radiation Pressure in Normally Dispersive,Non-Dispersive Media

摘要

Consideration of the electromagnetic momentum density D x B associated with aparallel plate capacitor in the presence of a magnetic field parallel to the plates shows that the momentum is a property of the electromagnetic field in a dielectric (closely related to the polarization of the dielectric) rather than a matter of storing mechanical momentum in the dielectric. This has implications regarding transport of momentum by electromagnetic waves. The physical cause of group velocity in normally dispersive, non-dissipative media is the immobilization of part of the wave energy, the energy transport being due to just part of the energy density propagating at the phase velocity. Something equivalent occurs in the case of momentum flow, and the existence of non-propagating momentum is another characteristic of group velocity. In idealized dielectrics, all of the electromagnetic momentum associated with a wave field flows at the phase velocity; the non-propagating momentum involves an impulse in the direction of propagation delivered to the dielectric upon the establishment of the wave field, and an impulse in the opposite direction delivered to the dielectric upon its decay. Although this appears to describe storage of momentum in the dielectric, the second momentum transfer occurs whether or not the momentum delivered to the dielectric upon establishment of the wave field is retained by the dielectric for the duration of the wave field.