Possibility of a surface charge We note in particular that a surface charge density ρs can exist, in many situations, even
though not all.
It can, for example, exist as a d.c. value on the surface of any material, i.e. a perfect
insulator, (after it has been rubbed with the cat), on a non-insulating imperfect conductor,
or on a perfect conductor.
It can also exist as an a.c. value on the surface of a perfect conductor or an imperfect
conductor, as even when surface currents are outlawed in the latter, volume currents
directed perpendicular to the surface can make a surface charge density change.
The case when ρs is outlawed is when the fields and other variables are all sinusoidal
and both materials are perfect insulators. There can in this situation be no mechanism
of charge transport to change the surface charge density.
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