RE: How many volts ?

[LUDWIK KOWALSKI <kowalskil@alpha.montclair.edu>]

On Tue, 25 Mar 1997 John Mallinckrodt wrote:

> The fringe effect between the plates of a parallel plate capacitor is
> negligible precisely *because* the plates are large relative to their
> spacing. It has nothing whatsoever to do with the amount of charge on
> either plate. This is because the plates are equipotentials and the 
> field near the surface of the plates must, therefore, be perpendicular 
> to the plates.

In this problem Q1=-10 and Q2=+100 nC. Thus the net charge is +90 nC. A 
vary distant observer will see the plates as a positive point-like charge 
whose field lines "radiate" towar infinity. That is what I had in mind by 
"fringe effect". Yes the field near each plate must be perpendicular to 
the surface, both inside, outside and along the sharp rims. The radiating 
flux will depend only on the net charge. It has nothing whatsoever to do 
with the equipotentiality of plates, with the distance between them, with
their relative positions or the voltage generated. I know you will agree 
with this without any reservations. Perhaps the "fringe effect" term 
should not have been used by me for the "uncommon capacitor".

 **********************************************************************
 *   John, how do you justify the assumption that final charges are   *
 *   distributed uniformly? Isn't it true that your 62 V prediction   *
 *   depends on that assumption?                                      *
 **********************************************************************
                                          Ludwik