Re: Conserving Q/Faraday

[William Beaty <billb@ESKIMO.COM>]

On Mon, 25 Jan 1999, Ludwik Kowalski wrote:

> WE NEED ADVICE

> It is likely that
> the net charge is an artifact due to the asymmetry in the plate
> separation process. (One metallic plate is still very close to the
> dielectric slab when another is already further away.) Symmetric
> separation is possible only in a gedanken experiment, not in a real
> experiment (with a thin dielectric foil between two metallic blocks
> attracting it).

Charging during separation is caused by corona discharges as the e-fields
become too strong.  Air breaks down quite easily.  If you perform the
entire experiment in a tank of oil, perhaps the oil will not break down at
all.  Also, eliminate any sharp edges from your metal plates.  Use disks
rather than squares having corners.  Maybe roll up the edges of the metal
disks somehow, in order to eliminate the low-radius edges.  Or maybe use a
pair of stainless-steel mixing bowls which have flat bottoms.

To inject charge, you must form a resistor where the dielectric is the
resistive material.  Therefor you need good, wide-area contact between the
metal and the dielectric, since the charges would tend not to leap across
any tiny air gap.  At least that's what I'd speculate.  On the other hand,
perhaps the air in any air bubble would break down continuously, forming a
conductive plasma bond between the metal and the plastic.

Me, I would use a slightly soft dielectric material, maybe polyethelene,
use polished metal plates, and put the whole thing under a big weight in
order to seal the metal to the plastic.  Maybe roll the weights along at
the start, to drive the air bubbles ahead of the squeezing metal.  Under
oil, the air bubbles might not be such a problem, since any thin layer of
oil would probably be far more conductive than the thin layer of air, as
long as the air between the metal and the dielectric was not breaking down
into a discharge.

If oil does not break down, yet air does break down into a discharge, then
when you withdraw a charged plate from an oil bath, perhaps it would
crackle and glow as it first hits the air.

Another possible way to form charged plastic:  Electrify some adhesive
tape as you wind it onto a spool, and this would trap a net charge in the
volume of the plastic tape.  You'd want to use non-3M tape, since Scotch
tape is said to have antistatic (conductive) compounds mixed into the
adhesive, and this might let the charge migrate back out of the volume too
quickly.

> ASSUME THE NET Q DOES SOMEHOW EXIST IN A DIELECTRIC SLAB OF
> A CAPACITOR. HOW CAN ITS PRESENCE BE DEMONSTRATED WITHOUT
> REMOVING THE SLAB?

I've wondered about this myself.  I cannot see that it is possible.  The
capacitor plates will cause any external fields to be neutralized, and
even if removed, then the opposite and equal charges on the two faces of
the material would still give zero field outside the material, except at
the edges of the slab.  However, if the metal plates were again replaced.
then perhaps some of the surface charges would diffuse back into the
metal, and the voltage measured across the two plates would rise back up
to a large value again.

If you used a slab of clear, optically active material such as
polycarbonate, and viewed the light transmitted through the material
through its flat, polished edges, then perhaps the mechanical stresses
caused by the attracting charges would become visible as color chages if
the material was viewed between crossed polarizers.  Hey, I wonder if this
is how Kerr cells actually function?  Do e-fields cause microscopic
mechanical forces to arise, which tends to align the molecules of the
substance and change the optical activity?

Overall, I think you've discovered something fascinating.  I've heard that
"Electrets,"  materials with permanent "ferroelectric" polarity, are
unstable when first created.  Their surface potential drifts over time,
and then stabilizes in hours or days. Your charge-injection scenario would
explain this, since the "permanent ferroelectrization" phenomenon would be
a mechanizm separate from the injected charge phenomenon.  Over time, the
injected charges would find each other and recombine, while any net
excess would repel itself to the surface and eventually leak to ground,
leaving behind a purely ferroelectric "hysterisis" polarization.

"Charge-injected" plastic slabs might also demonstrate the existence of
polarity of populations of charge carriers.  For example, suppose the
charged metal plates create chemical reactions in the plastic, and so the
capacitor acts like an electrolytic conductor.  If the plates were
removed, then the dielectric would contain equal and opposite populations
of migrating ions, and therefor would have zero net charge.  On the other
hand, suppose that the metal plates emit electrons directly.  The plastic
might not do this.  As a result, the negative plate would fill the
dielectric with electrons, but the positive plate would not fill the
plastic with positive ions.  When the plates were later removed, the
plastic would be found to have an immense net charge, not a cancelled
charge like before.

Another thought: an electret would create a huge external field, since the
trapped flux extends out of the surface of the material.  A capacitor
would be different, since the field lines end upon the surface charges,
and the flux would only be inside the dielectric.  A "charge injected"
plastic slab would not be an electret, it would be a plate-less
capacitor.  Or, if the positive and negative carriers were not present in
equal populations, then the plate would contain an un-cancelled electron
population.  Does my reasoning on this seem sensible?


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