Re: IONS on metals/dielectrics & other scary things

[Sam Sampere <sampere@suhep.phy.syr.edu>]

Mary and Bill Allsopp wrote:

> 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.

First let me say that electrons do not jump off a belt in a Van de Graaff
generator.  If this were true, a VdG would work if the inside of the column
were a perfect vacuum, but this is not true.  There are rows of sharp points
at each roller.  The roller becomes charged as the belt 'peels' off, like
scotch tape becomes charged when you peel it off a table.

Because the roller is insulated from ground, it remains charged.  The
electric field around the sharp points (connected to ground, i.e. V=0!)
becomes very large, so large as to ionize the air in the column.  Charges
fly toward the roller, but there is an INSULATING belt in the way.  The
charge lands on this belt, but the turning motor brings in a new piece of
uncharged belt, while further charging the roller.  See the excellent letter
sent yesterday by Bob Sciamonda as to why the charges don't redistribute
themselves on the belt.

The reverse happens at the top roller.  Notice that the two rollers are made
of different materials.  Glass and rubber charge differently when rubbed, so
do these rollers.  Now we need to figure out why the charge inside of the
dome is zero.

> 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.

In a static situation, the current is zero.  If there is a current inside a
conductor, the electric field inside the conductor must be non-zero.  When I
turn off the Van de Graaff, the current into or out of the dome (neglecting
leakage on humid days) is zero.  If there were charges through the thickness
of the metal, or distributed on the inner surface of the dome, then an
electric field would exist inside the metal of the dome.  If this is true,
then there must be an electric current.  If this were true, you would have a
perpetual motion device!  You would detect magnetic fields around your dome.
The dome would get hot.  And this could continue forever.  This of course
cannot happen.  You agreed that the charge was static.

> 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?

I think I answered this earlier, the air gets ionized.  The inside of the
ball is essentially ground, charges move to ground then to the outer surface
of the dome.

> 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.

This is easy.  Get a microammeter.  Connect the top combs through the
ammeter to ground.  Turn on the machine and you will measure the charging
current.  Depending on your machine, you will get between 10 microamps to
100 microamps, give or take some.

> 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.

This is correct.  If you switch the pulleys, the dome will charge
oppositely.  Build 2 machines so the domes charge oppositely and let them
arc to each other.

> Bill:   From a little town in Arkansas



--
Sam Sampere
Syracuse University
Department of Physics
Syracuse, NY 13244
315-443-5999