Re: Confused by a derivation.

[Bob Sciamanda <trebor@VELOCITY.NET>]

That pictorial argument takes a bit of selling.  If the two conductors are
unequally charged, charges do appear on the outer surfaces - field lines do
go off to (and come in from) infinity.  You will respond that there must be
charges at infinity because the universe is overall neutral (an added
assumption - from whence?).

Then I should relax the statement that the 2 charged conductors are the
entire universe - they are however infinitely remote (isolated) from the
rest of the universe.  This allows charges at infinity to terminate your
field lines.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "John Mallinckrodt" <ajmallinckro@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, February 05, 2002 9:11 PM
Subject: Re: Confused by a derivation.


> One more thing.  The problem is surely *much* easier than any of
> us have made out if we are willing to specify that there are no
> other charges in the universe. In that case there simply can't be
> any field outside the capacitor since there is no place for the
> field lines to end.  Since there is no place for them to end, they
> can't start.  Hence, no charge is allowed on the outer surfaces.
>
> John Mallinckrodt      mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm
>
> On Tue, 5 Feb 2002, John Mallinckrodt wrote:
>
> > On Tue, 5 Feb 2002, Bob Sciamanda wrote:
> >
> > > 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.
> >
> > Oh.  Oops.  I didn't remember seeing that specification.  I guess
> > I am the only one making mistakes around here!