A Van de Graaff generator uses triboelectricity to generate a high voltage discharge. It consists of two rollers of different materials connected by a rubber belt, metal combs, and a metal dome. It can generate potential differences of over 100,000 volts and is commonly found in physics laboratories. It has serious applications, such as generating the enormous voltages required for particle accelerators. The largest demonstration generator ever built can generate potential differences of 12.19 million volts or more.
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 “motor” of the Van de Graaff generator uses two rollers of different materials – usually metal and plastic – connected by a rubber belt or other insulating material, suspended vertically from each other and driven by a motor. A metal comb, fixed to the ground, is positioned with the teeth facing the lower roller. At the top of the generator, another metal comb is similarly positioned, with its teeth close to the top roller, and connected to a metal dome.
If the top roll is metal and the bottom roll is plastic, rubbing the tape against the plastic roll will cause electrons to be stripped from the plastic on the tape, causing a negative charge to build up on the tape while the roll remains with a positive charge. The bottom comb’s electrons are attracted to the positively charged roller and some will jump towards it, but are prevented from reaching it by the electrical tape, which then acquires an even greater negative charge. At the top of the device, the 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 domed shape of the top of the Van de Graaff generator is ideal for achieving uniform charge distribution and maximizing potential. Sharp edges, points or irregularities can cause the charge to disperse into the air. This is why metal combs are used: the teeth allow electrons to travel easily to or from the comb to achieve a large buildup of charge on the dome. The generator prototype used a tin can instead of a dome, but this was soon improved upon.
Van de Graaff generators are commonly found in school and university physics laboratories: these can generate potential differences of over 100,000 volts. A popular demonstration involves making student volunteers’ hair stand on end when they touch the dome; the hairs acquire the same charge and repel each other. This type of generator can also generate fairly large sparks when electrons leap to a nearby object. By pointing a finger near the dome, a spark several inches long can be blown between the dome and the finger, causing a mild electric shock. While these devices can generate very high voltages, the current is too small to pose a 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 largest demonstration generator ever built is 130 feet tall and can generate potential differences of 12.19 million volts or more, producing lightning-like sparks several feet long.
Van de Graaff generators also have serious applications. They are sometimes used to generate the enormous voltages required for particle accelerators which are used to study the fundamental forces of nature. A Van de Graaff generator operated by the Australian National University for this purpose produces a potential difference of 15 million volts.
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