  In this work we study the classical electrodynamics in homogeneous conducting
and nonconducting time-dependent linear media in the absence of charge sources.
Surprisingly, we find that the time dependence of the permittivity gives rise
to an additional term in the Ampere-Maxwell equation and an asymmetry between
the electric and magnetic field wave equations. As special cases we consider a
linear and an exponential growth of the permittivity, as well as a sinusoidal
time-dependent permittivity.
