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Re: Confused by a derivation.



On Sun, 3 Feb 2002, Ludwik Kowalski wrote:

The field inside the conducting plate is zero. For that reason
my expectation would be:

MMMMMMMMMMMMMMMMMMMMMM__________
MMMMMMMMMMMMMMMMMMMMMM
UPPER METALLIC PLATE (no field inside)
++++++++++++++++++++
| | | | | | | | | contributes +sigma/eps-o
v v v v v v v v v


| | | | | | | | | contributes -sigma/eps_o
v v v v v v v v v
- - - - - - - - - - - - - - - - - - - -
LOWER METALLIC PLATE (no field inside)
MMMMMMMMMMMMMMMMMMMMMM__________
MMMMMMMMMMMMMMMMMMMMMM

What is wrong with this reasoning?
Ludwik Kowalski

There is nothing wrong with it except, perhaps, for the word
"contributes" (which seems to imply that the field outside one
plate should be *added* to that outside the other) and, also
perhaps, the slightly enigmatic presence of the + and - signs.

The bottom line any way you look at it is that the field between
the plates has magnitude sigma/eps_o and elsewhere is zero. This
is the only solution that BOTH satisfies Gauss' law AND yields
zero field within the conductors.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm