A material is said to be ohmic if an electric field, \(\vec{E}\), in the
material gives rise to current density \(\vec{J}=\sigma \vec{E},\) where the
conductivity, \(\sigma\) is a constant independent of \(\vec{E}\) or \(\bar{J}\).
(This is the precise form of Ohm's Law, ) Suppose in some material an electric
field, \(\vec{E}\), produces current density, \(\bar{J},\) not necessarily related
by Ohm's Law; that is, the material may or may not be ohmic.
a) Calculate the rate of energy dissipation (sometimes called ohmic heating or
joule heating) per unit volume in this material, in terms of \(\vec{E}\) and
\(\vec{J}\).
b) Express the result of part (a) in terms of \(\vec{E}\) alone and \(\vec{J}\)
alone, for \(\overrightarrow{\boldsymbol{E}}\) and \(\vec{J}\) related via Ohm's
Law, that is, in an ohmic material with conductivity \(\sigma\) or resistivity
\(\rho\).