Re: Confused by a derivation.

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Tue, 5 Feb 2002, Bob Sciamanda wrote:

> That pictorial argument takes a bit of selling.  If the two conductors are
> unequally charged,

But Bob, I KNOW that we agreed that the plates are equally
charged.  See

 <http://mailgate.nau.edu/cgi-bin/wa?A2=ind0202&L=phys-l&D=0&P=17688>

and

 <http://mailgate.nau.edu/cgi-bin/wa?A2=ind0202&L=phys-l&D=0&P=17999>

Obviously, if the net charge is not zero, some field lines MUST go
off to infinity implying the existence of charges "at infinity"
for those field lines to "end on." But that violates the
assumption that there are no other charges in the universe which
you told me that the problem specified just a couple of messages
ago.

> ... You will respond that there must be charges at infinity
> because the universe is overall neutral

I guess I did, huh?  Not, however, because the universe is
neutral, but because those field lines have to "end" on charges
"at infinity"  or otherwise.  (I know it amounts to the same
thing!)

> 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.

If you do then we are back to my previous arguments.  See

 <http://mailgate.nau.edu/cgi-bin/wa?A2=ind0202&L=phys-l&D=0&P=19037>

and

 <http://mailgate.nau.edu/cgi-bin/wa?A2=ind0202&L=phys-l&D=0&P=20313>

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