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A Van de Graaff generator uses triboelectricity — the creation of an electric charge through two different materials rubbing together, often referred to as static electricity — to generate a large potential difference that can produce a high-voltage discharge. The machine was developed by Dr. Robert J. Van de Graaff in the early 1930s, although it was preceded by a number of devices that worked on similar principles. The “engine” for the Van de Graaff generator uses two rollers made of different materials — usually metal and plastic — connected by a belt made of rubber or another insulating material, suspended vertically between them and driven by a motor. A metal comb, grounded to the earth, is positioned with the teeth facing the lower roller. At the top of the generator, another metal comb is similarly positioned, with the teeth close to the top roller, and connected to a metal dome.
If the upper roller is metal and the lower roller plastic, the rubbing of the belt against the plastic roller will cause electrons to be stripped from the plastic onto the belt, so that a negative charge builds up on the belt while the roller is left with a positive charge. Electrons from the lower comb are attracted to the positively charged roller and some will jump toward it, but are prevented from reaching it by the insulating belt, which then acquires a still greater negative charge. At the top of the device, electrons in the comb move away from the negatively charged belt and onto the metal dome, to be replaced by electrons from the belt. Thus, there is a continuous transfer of electrons from the lower roller — via the belt and upper comb — to the metal dome, which acquires a large negative charge. If the positions of the rollers are reversed, the dome acquires a positive charge.
The dome shape of the top of the Van de Graaff generator is ideal for achieving an even charge distribution and maximizing the potential. Sharp edges, points or irregularities can cause the charge to leak away into the air. This is why the metal combs are used — the teeth allow electrons to travel easily to or from the comb to achieve a large build-up of charge on the dome. The prototype generator used a tin can instead of a dome, but this was soon improved upon.
Van de Graaff generators are commonly found in school and college physics labs: these can generate potential differences of over 100,000 volts. A popular demonstration involves causing student volunteers’ hair to stand on end when they touch the dome; the hairs acquire the same charge and repel one another. This type of generator can also generate fairly substantial sparks as electrons leap to a nearby object. By pointing a finger close to the dome, a spark up to several inches in length can be made to jump between the dome and the finger, resulting in a mild electric shock. Although very large voltages can be generated by these devices, the current is too small to pose any risk.
It is possible to build generators that produce much larger voltages. A large Van de Graaff generator can produce a potential difference of millions of volts. The biggest demonstration generator ever built stands 40 feet (12.19 m) tall and can generate potential differences of 5 million volts or more, producing sparks resembling lightning that are several feet long.
Van de Graaff generators also have serious applications. They are sometimes employed to generate the huge voltages required for particle accelerators that are used to investigate the fundamental forces of nature. One Van de Graaff generator run by the Australian National University for this purpose produces a potential difference of 15 million volts.
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