VAN DE GRAAFF MACHINE: Belt and Rollers

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:

• The roller becomes strongly electrified
• The roller attracts opposite charge into the comb teeth
• The electric field between the roller and teeth becomes intense
• The field shreds air molecules into conductive plasma called "corona"
• The air conducts, and charges in the comb leap towards the roller
• The moving charges hit the belt surface and stick to it
• The roller turns, and the charges are carried away on the belt
• The process repeats...

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 negative.)

Fig. 1 HOW THE PLASTIC ROLLER BECOMES ELECTRIFIED

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 CHARGES

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 electric 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 belt.

CHARGES EXIT AT THE TOP

The belt carries charge up the column of the Van de Graaff, then passes 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 metal.

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 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 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 "leapt" 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.

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.

REVERSED POLARITY

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 current 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 the belt. While positive charge moves up 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.

ACTIVE CHARGING SUPPLY

Some expensive Van de Graaff machines eliminate the charging roller altogether. Instead they supply a metal roller connected to a high voltage power supply. The main benefit is to guarantee VDG operation when humidity is so high that a plastic or felt roller would not be charged by contact electrification. And since small amounts of grime will interfere with the process of contact electrification, a VDG with a high voltage supply is much less sensitive to buildup of dirt.