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Re: IONS on metals/dielectrics & other scary things



Well, as long as the topic is "charge" and the strange things that
charges do, I will recite a short story of an adventure into the never
world that I recently had.

I have a friend who is a great man of science, and a college physics
professor, who has been my mentor on several occasions.

I could not figure out why electrons jumped off the top of the belt on
the VandeGraph machine, so I asked him. Without blinking an eye he said
the there was no charge inside of the sphere and therefore the electrons
jumped off the belt to a much lower potential and then quickly run to
the outside where they add to the much higher potential surface of the
dome.

Well, that flew in the face of my experience, so I asked him "why is the
charge on the inside of the dome zero"? He took an old book from his
shelf and turned to the page where it said the same thing that he had
just said, and asked "see"? I still didn't buy it, so I went home and
built an "EXTERNAL BELT" VandeGraph machine where the collector brush's
wire connected to the outside surface of the dome. It threw sparks just
as well as its professional, and much more handsome, brother, and my
friend was quite perplexed. We haven't spoken of this since.

Can anyone shed light on this subject? In my ignorance, I can see no
reason that the charge on the inside surface of the dome should not be
essentially the same as it is on the outside. After all, the charge is
D.C., both surfaces are conductors and they are connected together by a
conductor. In fact, at D.C. there is no "skin effect" and the charge
should be able to flow freely through the metal in the dome. As each
point inside of the dome is looking at every other point, there may be
some repulsion of electrons near the inside surface due to like charge
repulsion, but if the dome is large, the effect should be minimal.

Why do the electrons, which are presumably deposited on the belt by the
non-touching brushes at the bottom of the belt, stick to the belt (which
is a dielectric, and is the subject of our current topic) then decide to
get off at the top roller and jump through an intervening space into the
pointy ends of several sewing needles?

As the dome becomes highly charged, the belt electrons seem oblivious to
the fact that they are going from, what would appear to me, a lesser
negative charge on the belt to a much higher negative charge on the
dome. This has caused me considerable quandary. I don't even know how
to set up the equipment to measure what is happening.

What is the theoretical limit to the charge that can be deposited on the
dome with the VandeGraph method, (assuming that we disallow leakage in
all of its many forms)?

NOTE: I am assuming that the dissimilar pulley materials are arranged so
that electrons go up the belt and not down. Though I have not tried the
inverse experiment, I fully believe that charging the dome positively
will work with equal success.

I hope that this clears up some of the murky water that we are wading
around in.

Bill: From a little town in Arkansas

P.S. I wish to thank Ludwik Kowalski for broaching a very interesting
question.