Although it looks simple, the belt, combs, and rollers within a VDG
machine form an interesting device called a Continuously Operating
Electrophorus, which creates charge separation by electrostatic induction.
The end result of this mechanism is to cause the sharp points of the metal
comb to deposit a charge imbalance upon the surface of the belt.
First the roller surface becomes strongly charged. This happens as the
belt touches the roller and then separates again. When the belt touches
the roller, chemical bonds form and the surfaces adhere together.
(Chemical bonds form when *any* object touches another.) Chemical bonds
can alter the balance of charge on the surface of the roller because atoms
are composed of equal amounts of positive and negative charge, and when
the atoms of the belt and roller temporarily bond together, they share
their outer shells of negative charge. Because the belt material and the
roller material are not the same, many of the chemical bonds will be
asymmetrical; the shared negative charge in the bonded atoms will stay
more with the rubber belt than with the plastic roller. In this way a
charge-imbalance instantly appears when the surfaces touch. The rubber
gets more electrons than protons and is negative, while the plastic roller
has fewer electrons than protons and is positive. When the belt and
roller surfaces peel apart, much of the electric charge returns to its own
surface, but some of it does not. The belt is left with more negative
than positive charges, so it is negative overall. The roller has less
negative than positive, so it is positive overall. This whole process is
called "contact electrification," also called "frictional charging",
though of course no friction is actually required. (Note: it is possible
to reverse these polarities. If the belt was plastic and the roller was
coated with rubber, the roller would end up positive instead of negative.)
As the belt continuously passes over the roller, the inner surface of
the belt becomes weakly charged negatively, while the roller is strongly
charged positive. This charging effect is actually quite feeble, but this
is OK because it only needs to maintain the charge on the roller, and the
charge on the roller is not intentionally removed by other processes in
the VDG operation.
The small charge on the belt shown above is actually not important in
the VDG functioning. But the charge on the roller is essential to the
next stage. A metal needle is held near the surface of the belt at the
place where the belt passes over the roller. Metals are composed of a
solid grid of positive atoms immersed in a movable "fluid" of negative
electrons, and when the metal needle comes close to the roller, the
positive surface charge on the roller attracts the electron-fluid of the
metal. But no electrons leave the metal yet.
The electron fluid of the metal migrates toward the tip of the needle.
The needle tip acquires an intensely strong negative charge, and this
negative charge affects the air. Any air molecules which come near the
needle tip are torn into separate electrons and positive atomic nucleii by
the intense electric attraction/repulsion forces. The freed electrons of
the air are strongly repelled, and they strike other air molecules and
rupture them as well. A mass of shattered air and free electrons forms at
the needle tip. This stuff is called "corona discharge" or "St. Elmo's
Fire", also "plasma", the fourth state of matter. Plasma has movable
electrons like metals do, and like metals it's a fairly good conductor.
Next, positive air molecules from the plasma collide with the metal needle
and steal electrons from it. The plasma forms a conductive bridge between
the metal needle tip and the insulating air. It allows electrons from the
needle to spew into the air where they are captured by nearby air
molecules, with the result that a "wind" of negatively charged air flows
from the needle tip. (Note that the VDG needs air on its needle tips in
order to operate. It wouldn't work right if it was operated in a vacuum.)
The negatively charged wind is strongly attracted to the positively
charged roller surface. However, the rubber belt is covering the roller.
The charged air moves towards the roller and coats the surface of the
belt, which partially shields and cancels the roller's charge. But then
the roller turns and the belt surface moves upwards, carrying the negative
charge with it. This continually re-exposes fresh rubber surface, which
keeps attracting more negative charge from the needle.
Here's a strange fact: no matter how much negative charge spews from the
needle, the belt always intercepts it before it can cancel the positive
charge on the roller. The roller never loses its charge, yet the roller
forces charge to flow from the needle to the belt. It almost seems like
perpetual motion. This is called "charging by induction", since the
positive roller "induces" a charge to flow from needle to belt, yet the
positive roller itself never loses its charge.
The other end of the needle is connected by a wire to the ground or to
a large metal object, so as electrons spew from the needle and are
attracted towards the positive roller, more are drawn in through the wire.
As the roller turns it maintains its positive charge, causes the needle to
spew negative charge on the belt, and causes a small electric current to
flow from ground and into the needle. Overall, the system acts as a
miniature charge-pump by forcing charge to flow from the neutral earth and
onto the surface of the belt.
The belt carries charge up the column of the VandeGraaff then passes
through another roller and needle assembly. This second roller acts in
reverse to the first, and the charge on the belt is dumped into the upper
needle tip. This second roller must *not* be positively charged. In order
to work in reverse, it either must have a negative charge, or it can be
neutral. In many classroom VDG devices this second roller is neutral
As the negatively charged belt passes over the upper roller, it repels
the "fluid" electrons of the metal needle tip and pushes them away from
the tip. This leaves behind positive metal atom nucleii. The charge
right at the
tip is intensely positive, and the electrical attraction/
repulsion forces again tear the nearby air molecules into conductive
glowing plasma. This time the free electrons of the plasma are attracted
into the needle, leaving behind positively charged air molecules which
rush away. The positive air is attracted to the negative charge on the
rubber belt, and it combines with the belt charge and mostly cancels it
out. The needle is connected to a wire, which is connected to the inside
of the hollow VDG sphere. As the belt repels the needle's electrons, the
"icepail effect" sucks them to the outside of the sphere. Overall, the
belt's negative charge has "leapt" onto the needle and flowed to the VDG
Above is the explanation of the VDG basic operation. Real VDGs have
added complexity, and many will have differing details.
For example, a rubber roller and plastic belt could be used, and this
would paint the belt with positive rather than negative charge. It would
reverse the overall direction of electric current and the polarity of the
charge on the upper sphere. Or, the entire column assembly could be built
upside-down, with the charging roller up in the hollow sphere. This works
fine, and also reverses the direction of current and the charge polarity.
Or, roller and belt materials could be chosen so that each roller sends
opposite charge to the belt, and while positive charge moves along one
half of the belt, negative charge runs down the other. This doubles the
overall electric current and makes the VDG work better in humid weather.
One way to do this is to use a rubber belt with a plastic roller on one
end and a felt-covered roller on the other.
Some expensive VDGs eliminate the charging roller altogether, and
instead supply a metal roller with a high voltage power supply. The main
benefit of this is that the VDG will still be able to operate where
humidity is so high that a plastic or felt roller will not be charged by
contact electrification. Also, small amounts of grime will interfere with
the contact electrification process. A VDG with a high voltage supply is
much less sensitive to used instead, the machine becomes fairly immune to