Because of the dependency between the electric and magnetic fields, we should use the term electromagnetic field when
dealing with time-dependent fields. This term indicates that the two fields cannot be treated separately. The remaining part of
this books deals with the electromagnetic field. In some cases, we will find it easier or more useful to treat the electric field or
the magnetic field alone, but whichever field we choose to emphasize, it should be remembered that the other field can
always be derived if necessary and, more importantly, it always exists.