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Re: Confused by a derivation.



From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
. . .
(5) Put one end inside the gap and one end within one of the plates.
This
gets interesting. In hindsight we see we will get the wrong answer for
the
field between the two plates if we assume the charge density on the
surface
contained within the gaussian surface is Q/2A where A is the surface
area of
just the inner surface, but Q is the total charge. In order to account
for
the electric field in the gap that is created by the charges on the
capacitor plate that is NOT currently contained on our gaussian surface,
we
have to assume the charge on the plate which contains the end of our
cylinder has all its charge on the inner side so that the charge density
on
the inner surface is Q/A rather than Q/2A.

There is no confusion here. I suggest you do the general problem of three
(or more) parallel conducting plates carrying arbitrary net charges Qi.
Use only Gauss' law and the constraint of zero field inside conductors -
find the charge density on each plate face. This was discussed to death
some time ago on this list.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor