Re: conserving charges ? /Faraday

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

Brian asked:
> It would be interesting to know the plate dimensions.

Each lexan sheet is about 5 by 7 cm (to fit the soup can which is used
as the Faraday cup) and about 1 mm thick. Will measure on Monday.

> Actually, I merely warned that the proximity of other conductors
> (grounded or not) could complicate the situation into a "more than
> two" conductor situation.

If I understand you correctly, you want us to be sure that all the field
lines originating on the positive plate end on the negative plate. The
geometry is highly favorable for this. Aluminum plates which are in
contact with lexan are smaller than lexan ( about 2 mm margins to
prevent direct air gaps) and the distance between plates is about 2 mm.
Furthermore, the aluminum plates (blocks 6 mm thick) are in contact
with much larger copper plates, one below (grounded) and one above.
Thus pieces of lexan are in the center of a large parallel plate capacitor
and are immersed in the essentially uniform field.

The reason for placing the entire disectable capacitor into the grounded
metallic box was to eliminate a possibility that electric field lines
originating elsewhere may end on the plates of our capacitor. I do not
think that this is possible but a grounded box can not hurt. Can it?

> By grounding the bottom conductor, you no longer have an isolated system;
> an earth ground can provide a source or sink of charge (as when you
> charge a grounded conductor by induction).
> In fact if you are using an AC power supply, one of its output terminals
> (usually the negative) may well be connected to earth ground via the
> third lug safety ground; that is not uncommon.    I should think that you
> would want all parts of your system to be "floating" and isolated (both
> electrically and physically) from earth ground and all else.  Only then
> can you hope for a conservation of charge test.

Will discuss this with Bob Dorner. Our power supply (from Cenco) does
have its negative terminal connected to the third prong of the plug.
I guess we can build a floating power supply (grounding nothing  after
the secondary winding of the transformer) but I am not persuaded that
this is the right thing to do right now. We also have a power supply
from a broken laser; it may have floating terminals. If so we will try it.

> Perhaps in time I may be able to scrounge an electrometer, etc and join
> your efforts - sounds intriguing!

That would be great. I will send you pieces of lexan from the same big
sheet so we will compare data on the same material.  Just let me know
when you are ready.
Ludwik                           P.S.      For those who do not know:

Any TV set, even not working, has a d.c. H.V. power supply good for
this kind of investigation. Also a laser, a geiger counter, etc.
No current is needed so that you can insert a series resistor of 2 or 3
megoms to protect yourself (limiting the current which flows when
two leads are touched).

Instead of a gold leaf electrometer you may consider a "pithball"
suspended in a uniform electric field by a conducting thread and
connected to the Faraday cup. This electrostatic voltmeter may be
more convenient than the gold leaf. No separate measurement of
polarity will be necessary, the suspended piece will turn left or right
from the equilibrium, according to the sign of the charge inside the
Faraday cup. Knowing the weight (and E) you will be able to calculate
coulombs from the angle. And sensitivity will be under your control.
(The voltmeter field can be created by a dedicated H.V. power supply,
or by the power supply used to polarize plastic sheets.) Do a little
calculation of the needed field; it should match the weight of the
"pithball" for the desired sensitivity. Our gold leaf electrometer
deflects about 60 degrees for 1000 volts. To work at 5000 V we must
use lexan whose areas are smaller; otherwise the electrometer is too
sensitive.  It is silly that we can not study the effect of d.o.p. on the
net charge for the same sample.

A commercial electrostatic voltmeter, with several ranges, would
be ideal but such instruments are probably too expensive. The
home-made "pithball" voltmeter will not be linear;  but this is
not important.  Give your student a chance to be creative.