Re: Conserving Q/Faraday

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

"John S. Denker" wrote:

> After reading your note (even the parts not cited above) I need more
> information before I can help you.

A lot has already been posted on this thread in December. The topic was
triggered by the "IONS" thread which was started in September.

> 1) Most importantly, can you say what physical principle or process you
> are trying to demonstrate?
>
> This is important because in the scenario you describe, there are many
> processes that could lead to a net charge, and I can't tell which you
> would consider interesting and which you wouldn't.

For the time being the main issue is to be sure that the observed NET Q
(on the dielectric slab) was there before the slab is removed from the
dissectible capacitor.

> Specifically:
>
> 1.1) Suppose there was a huge amount of net charge on the dielectric
> *before* it were put into the capacitor.  To a good approximation that
> charge would be expected to remain.  So measuring the charge after it sits
> in the capacitor for a while doesn't tell us much about the nature of
> capacitors.

     We always verify that the slab is neutral before inserting it.
> 1.2) Similarly, you wrote that
> > We now have a power supply whose terminals are floating.
> Well, if the system is floating it means that the whole system (power
> supply, capacitor plates, dielectric, etc.) is free to pick up stray
> charges during operation.  Some of these net charges could be distributed
> to the dielectric, but I'm not sure why anybody would be interested in
> the details.

Somebody may prefer this to a crossword puzzle. And who knows,
something useful can always be discovered. Students should be
presented laboratory puzzles, when possible.

> 1.3) You mentioned opening the capacitor, and indicated this was
> problematic.  I would say "problematic indeed", including an interesting
> problem you didn't mention.  Unless special precautions are taken
> (including, among others, short-circuiting the capacitor terminals),
> opening the capacitor illustrates the same physical principles as an
> electrophorus.  You can create a huuuge amount of electrostatic energy
> and voltage.

You are correct.

> So.... Step one is to figure out what physical principle or process you
> are trying to demonstrate.  Then it should be possible to design an
> experiment that controls for other factors.

In this particular case the net Q was observed first and the "why" question
was postponed. We just want a clear answer about the reality of the net
Q.
Is it really there or was it introduced by a poorly designed experiment.
Haw can this question be answered by performing a better experiment? The
was
the nature of our call for help.

Ludwik Kowalski