VandeGraaff Machine Belt and Rollers

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.

Fig. 1 HOW THE PLASTIC ROLLER BECOMES ELECTRIFIED

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.)

Fig. 2 HOW A GROUNDED NEEDLE ELECTRIFIES THE BELT SURFACE

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 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 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 sphere surface.

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 dirt buildup.