Re: Confused by a derivation.

[Bob Sciamanda <trebor@VELOCITY.NET>]

Sure, John.  But the statement of this problem says there is no
pre-existing background field.  The four layers of charge are here the
entire universe.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "John Mallinckrodt" <ajmallinckro@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, February 05, 2002 6:57 PM
Subject: Re: Confused by a derivation.


> Well, I'm glad to see that I'm not the only one making mistakes
> although yours is, I think a little more subtle than mine!
>
> On Tue, 5 Feb 2002, Bob Sciamanda wrote:
>
> > While sitting in a doctor's office, I just formulated this hybrid
> > derivation - A lot of Gauss' law with superposition added to taste:
> > 1) s1 + s2 = -s3 -s4  From the problem statement.
> > 2) s2 = -s3 (Both Gaussian caps in conductors)
> > 3) s1 = -s4 From 1) and 2)
> > 4) E = s2/epsilon  the field in the central region. (one cap in
conductor,
> > other cap in central region)
> > 5} GL has told us that the field due to any single charge layer is
> > E=sigma/(2epsilon).
> > 6) The field given in 4) is completely accounted for by the layers s2
and
> > s3.
> > 7) Therefore the contributions of s1 and s4 to E must cancel in the
> > central region.
>
> No.  All that is required is that the combination of s1 and s4
> serve to cancel any preexisting background field.  See, for
> instance, the sketches in my previous post.
>
> John Mallinckrodt      mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm