Re: Conserving Q ?

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

Bob Sciamanda wrote:

> Couple of  uncooked thoughts:
> 1) The dielectric can acquire a static charge just by contact with other
> stuff (triboelectricity - not a new phenomenon).  Please clarify: Did
> both conductors and the dielectric retain a final net charge? (You spoke
> only of two retained charges.)

In most case we first discharge the metallic plates (by grounding
them with a wire) then dissect the capacitor and bring the plastic
pieces into the electrometer. First one, then another, then both
together. Experiments with zero voltage (everything else exactly
the same) show no charges, even when we deliberately rub plastic
pieces against each other. At one point the piezoelectric effect
was we suspected (yes lexan is not a crystal, but who knows). A
zero voltage test, with a load of 4 kg, eliminated this suspicion.
Yes, there are factors of which we are not aware and we are
trying to identify them. The reproducibility is not good. What
I reported is certainly correct but the actual numbers may differ by
a factor of two or three under practically identical conditions.
I wish we had a modern "clean room lab".

> 2) Perhaps this is not a simple two conductor situation:
> Consider a battery connected to a capacitor made of two separated
> conductors (as you have).  Now, with the battery still connected,
> bring a third (perhaps grounded) conductor close to ONE of the
> original capacitor conductors.  . . . ?

A good point. We should repeat experiments in a grounded metallic
box to verify this. I do not expect any difference because at present the
setup rests on a wide grounded copper plate (lower Al sits on it). It is
the same ground as in the power supply (not a battery). But who
knows? We will check it.

If somebody there is planning to conduct similar experiments it
may be important to specify the sensitivity of the electrometer on
which the Faraday cup (a soup can) is mounted. The leaf deflection
approaches "90 degrees" when d.o.p. is close to 1500 V. Surprisingly,
the angle-voltage graph is quite linear. This may be due to the
fact that the center of the scale (taped to the glass window) is not
exactly at the pivoting point of the gold leaf. Our "divisions" are
not exact angles. The electrometer is from Cenco, it has flat
windows on both sides. The original leaf was missing and I replaced
it many years ago. I also had to clean the insulating "cork" supporting
the gold leaf rod. A student who works with us repaired another
old electrometer using the aluminum leaf. It is less sensitive but
works fine. We use it when charges are too big for my electrometer.

I would prefer to use a commercial electrostatic voltmeter but we do
not have one. Anything that can measure the d.o.p. between the
Faraday cup and the ground (without discharging !) will work. What
about modern electronic electrometers which detect charges without
touching? I saw one of them at the AAPT meeting several years ago.
Can somebody tell us more about these devices.  How reliable are
they? Are they commercially available?

Ludwik Kowalski