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Re: Gauss' law and displacement current



John Denker has used some lateral thinking to warn us all of mistakes we can
make if we do not explicitly state any restrictions that apply when
"general" and sweeping statements are made.

In the thread on Gauss' law and displacement current Carl Mungan proposed a
situation with the "usual RC series circuit consisting of a battery V,
switch, resistor R, and parallel-plate capacitor C". So all of the
responders, including Eugene Mosca yesterday, were thinking in terms of
circuit currents in wires attached to batteries and looking at the source of
the electric field responsible for the current. As Eugene pointed out (and
referenced to the excellent text by Chabay and Sherwood) the field arises
from the gradient of surface charge on the wires.

John has cited a different situation, where the current arises as a result
of the electric field associated with a changing magnetic field. The rest
of us were thinking, in a blinkered fashion, of currents and batteries.
Statements which are true for one physical situation (batteries) need be
true for a different physical situation (changing magnetic flux).

Thanks, John, for pointing this out. However your comments do not change
the physics of the situation that Eugene (and Chabey and Sherwood) were
discussing; some physics that is not appreciated by most students (and, dare
I suggest, quite a few teachers and physicists).

Brian McInnes