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Van de Graaff generators in the classroom: Theory, operation, and safety

20 April 1998


The generator uses a Teflon pulley at the lower end of the machine, attached to an electric motor. A rubber belt passes over the pulley. As the pulley turns, the pulley acquires a negative charge while the inside surface of the belt near the pulley acquires an equal amount of positive charge. The outside surface of the belt acquires an equal amount of negative charge by induction. An electrode in the form of a wire screen (called the lower brush) drains away these negative charges to the ground. A similar screen electrode (called the upper brush) at the top of the belt removes the positive charges and deposits them on the collector dome. The Teflon pulley retains the negative charges that it acquires.

Positive charges stay on the inside surface of the belt and travel upwards as the belt moves. At the top, it runs over an aluminum pulley that conducts the positive charges and retains them. Free electrons from the metallic pulley flow on to the belt and are carried down to the the lower plastic pulley. As the belt keeps running, more charges are deposited on both pulleys. The positive charges are transferred to the collector dome and the negative charges are drained into the ground. The belt plays an important role in transporting negative charges from the upper comb to the lower comb and positive charges from the lower comb to the upper comb. There is no transfer of charge from the electrical lines. This device would work exactly the same if it were powered by a hand crank.

On the metallic collector dome, the positive charges spread out due to electrostatic repulsion and become uniformly distributed due to the dome's spherical shape. The buildup of positive charges on the dome continues until ionization intensity is reached. At equilibrium, the potential difference between the collector dome and the generator housing can reach one-half million volts. The air between the collector experiences dielectric breakdown and the generator discharges the accumulated static charge in the form of a spark. This discharge causes the potential difference to drop below the ionization threshold but is brought up again in a matter of seconds. This process continues as long as the generator is running.

Source: Zafar A Ismail, Van de Graaf Generator: Instructions & Applications

Operation & safety