The sources of electromagnetic fields are electric charge, electric current, polarization
and magnetization. The relationships among the fields and their sources, all of which
represented by functions of space and time, are described by Maxwell’s macroscopic equations.
The fields carry energy, whose rate-of-flow at each point in space at any instant of time is given
by the Poynting vector. At any location where one or more fields and one or more sources
reside simultaneously, there could occur an exchange of energy between the fields and the
sources. The time-rates of such exchanges are uniquely specified by the Poynting theorem,
which is a direct consequence of Maxwell’s macroscopic equations in conjunction with the
definition of the Poynting vector. Electromagnetic fields also carry momentum and angular
momentum, whose densities at all points in space-time are simple functions of the local
Poynting vector. A generalized version of the Lorentz law of force dictates the time-rate of
exchange of momentum between the fields and the sources in regions of space-time where they
overlap. There also exists a simple expression for the torque exerted by the fields on the
sources, which defines the time-rate of exchange of angular momentum between them. This
chapter is devoted to a precise and detailed description of the relations among the fields and
their sources, as well as their interactions involving electromagnetic force, torque, energy,
momentum, and angular momentum.
