> The term "lossy dielectric" implies an energy loss or joule heating in
> the dielectric material. Could someone out there explain the actual
> loss mechanism? Is is a paramagnetic or molecular vibration?, or?.. I
There are two fundamental mechanisms that contribute to dielectric loss.
One is nonzero dielectric conductivity that Nirmal mentioned. A lossy
dielectric may "leak" current resulting in dissipated power v(t)*i_leaked(t).
This is due to some microscopic charge that is (more or less) free
to move around in the dielectric, resulting in both DC and AC losses.
The other contributor to dielectric loss is an effect called "dielectric
hysteresis". It occurs only for changing (AC) fields. Atoms or molecules
of the dielectric can be viewed as electric dipoles (classical mechanics
and classical field theory). These dipoles oscillate at the frequency
of the driving electric field. In the process of oscillation they bump
into one another resulting in damping (damped oscillations). This damping
do to collisions (interaction) between different dipoles is what causes
losses. The simplest way these oscillating dipoles are modeled is by writing
down the equation of motion for a pair of charges connected with a spring
that has some spring coefficient k_spring=Force/displacement and some some
damping coefficient k_damping=Force/(d(displacement)/dt). The solution
of the resulting second order ordinary differential equation can be tied
with the polarization (dipole moment per unit volume) to get the equations
for the dielectric constant as a function of frequency.
So the loss mechanism that you were looking for is due to damped atomic
or molecular oscillations.