VAN DE GRAAFF MACHINE: Belt and RollersThe standard classroom Van de Graff machine contains no power supply. Open one up, and you'll find that its AC cord leads to a simple electric motor. If you had a gearbox and a crank, you could build a hand-cranked Van de Graaff machine with no electric cord at all!
Although they look simple at first glance, the belt, combs, and
rollers invisibly combine to form an electrostatic device called a
Continuously Operating Electrophorus. This device harnesses an effect
called Electrostatic Induction in order to pump electric charge between
the metal comb and the surface of the moving belt. Overall, the
"electrophorus" works like this:
Fig. 1 The plastic roller becomes electrified
CHARGING THE ROLLER
In the first stage of operation the roller's surface becomes strongly
electrified. This happens as it contacts the belt surface, and it occurs
for the same reason that a balloon becomes electrified when rubbed upon
hair. Notice that the belt and the roller are made from two different
materials. When the rubber belt touches the plastic roller, chemical
bonds form and the charges in the surface atoms of the two materials share
unequally. As the roller rotates, the belt peels away and the surfaces
separate again. The belt and roller surfaces take their equal and
opposite charges with them. This whole process is called "frictional
charging", but since no friction is actually required, it's more accurate
to call it "electrification by contact." (Note: in the example below, the
roller receives a positive surface charge, but this is not always true.
The roller's polarity depends on the materials used for the belt and
roller, and in some Van de Graaff machines the roller becomes
After operating for awhile the belt will become weakly negative
and the roller will be strongly positive. Areas of equal and
opposite charges were created, but since the negative charge is
spread widely on the belt, it is much weaker than the concentrated
charge on the roller. The weak charge on the belt can be ignored
for now, since it doesn't figure into the next step.
CHARGES LEAP THROUGH THE AIR
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 negative 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, and like a metal it's a fairly good conductor.
Fig. 2 POSITIVE OBJECT MAKES GROUNDED NEEDLE SPEW CHARGED WIND
THE BELT INTERCEPTS THE LEAPING CHARGESThe negatively charged wind is strongly attracted to the positively charged roller surface. However, the rubber belt is in the way. The negative air moves towards the positive roller and coats the surface of the belt, which partially shields and cancels the roller's charge. But then the roller rotates and the belt surface moves upwards, carrying the negative charge with it. Fresh rubber surface is continually re-exposed, which keeps attracting more negative charge from the needle.
Notice that no matter how much negative charge spews from the needle, the
belt always intercepts it before it cancels the positive charge on the
roller. The roller never loses its positive 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 on the tip of the needle. This is also
called "charging by corona wind", since the "corona discharge" plasma
allows charges to flow from needle and into the air.
Fig. 3 THE CHARGED ROLLER FORCES THE COMB TO CHARGE THE BELT
The other end of the needle is connected by a wire to the ground or to
a large metal object. As negative charges spew from the needle and are
attracted towards the positive roller, more are drawn in through the wire.
As the roller rotates it maintains its positive charge, which causes the
needle to spew negative charge onto the belt, which causes a small
current to flow from ground and into the needle. Are there charges in the
ground? Yes, because the Earth is conductive. It contains equal amounts
of opposite charge, and so the wire can suck one polarity of charges out
of the Earth. Overall, the system acts as a miniature charge-pump by
forcing charge to flow from the neutral earth and onto the surface of the
CHARGES EXIT AT THE TOP
The belt carries charge up the column of the Van de Graaff, then
by 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 must 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 exposes the positive metal atom nucleii. The surface
charge at the tip of the needle is intensely positive, and the electrical
attraction/repulsion forces tear apart the nearby air molecules into
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 plasma electrons into
the needle, the "icepail effect" sucks the excess charge to the outside of
the sphere. On average, the negative surface charge on the belt has
onto the needle and flowed to the surface of the Van de Graaff sphere.
Fig. 4 A BELT, TWO COMBS, AND TWO DIFFERING ROLLERS
Overall, the VDG machine looks very simple to the eye. A belt passes over
two rollers. The rollers must be made from two different materials (e.g.
plastic and aluminum.) Two "combs" of wire are held near the belt
surface. Add a hand-crank or a motor and a couple of hollow spheres, and
that's everything. Or simplify it further by connecting one end to the
earth and put a hollow sphere over the other end.
NEXT: THE HOLLOW SPHERE
Above is the basic explanation of the VDG operation. Real
Van de Graaff generators have added complexity, and commercial units will
often have differing details.
For example, a plastic belt and a rubber roller could be used. This
would reverse the polarity and paint the belt with positive rather than
negative charge. This reverses both the overall direction of electric
current and the polarity of the imbalance of
charge on the upper sphere. Or, the entire column assembly could be built
upside-down, with the plastic roller up in the hollow sphere and the metal
roller inside the base. This works
fine, and just as you might expect, it reverses both the direction of
and the charge polarity of the sphere.
Or, roller and belt materials could be chosen so that both rollers
develop a charge, with one roller becoming positive and the other being
negative. This would send equal and opposite charge to the two ends of
belt. While positive charge moves up one half of the belt, negative
runs down the other. This doubles the overall electric current and makes
the VDG work better in humid weather.
ACTIVE CHARGING SUPPLY
Some expensive Van de Graaff machines eliminate the charging roller
Instead they supply a metal roller connected to a high voltage power
The main benefit is to guarantee VDG operation when humidity is so high
a plastic or felt roller would not be charged by
contact electrification. And since small amounts of grime will interfere
the process of contact electrification, a VDG with a high voltage supply
much less sensitive to buildup of dirt.