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Re: Confused by a derivation.



John D's comments, as usual, strike to the heart of the matter. Just
to summarize them in explicit detail, in case Ludwik or anyone else
is still confused.

Take a metal plate. Each side has area A. Put charge Q onto this
conductor. Let sigma be defined as s = Q/A.

Half the charge goes to either side:

+ + + + + +
===========
+ + + + + +

Each side thus has charge density s/2. Compute the fields everywhere
by superposing the fields (s/2)/(2*eps0) due to the infinite sheet
charge on each side. The result is zero inside the plate and s/2*eps0
outside each sheet. This is the standard textbook result for either
an insulating or a conducting plate.

Now bring a plate of the same dimensions with equal and opposite
charge -Q nearby. Consequently, all of the charge on each plate
redistributes to the internally facing surfaces:

===========
+++++++++++

-----------
===========

The charged sides now have charge density +/-s. We again superpose
the fields (+/-s)/(2*eps0) due to each of these two infinite sheet
charges. The result is s/eps0 between the two sheets and zero
everywhere else (ie. in the bulk of each plate and outside the
capacitor). This is again the standard textbook result.

All clear? Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/