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Van de Graaff generator
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This article is about the machine used to accumulate electrical charge on a metal globe. For the progressive rock band with a similar name, see Van der Graaf Generator.
Van de Graaff Generator

Van de Graaff Generator.
Uses
Accelerating electrons to sterilize food and process materials, accelerating protons for nuclear physics experiments, driving X-Ray tubes, etc.
Inventor
Robert J. Van de Graaff
Related items
Van de Graaff, linear particle accelerator
pulleys
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe on the top of the stand. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.
Contents
[hide]
∙ 1 Description
∙ 2 History
∙ 3 Van de Graaff generators on display
∙ 4 Comparison with other high voltage generators
∙ 5 Patents
∙ 6 References
∙ 7 See also
∙ 8 External links
[edit] Description
Schematic view of a classical Van de Graaff generator.
1) hollow metal sphere
2) upper electrode
3) upper roller (metal)
4) side of the belt with positive charges
5) opposite side of the belt with negative charges
6) lower roller (for example in acrylic glass)
7) lower electrode (ground)
8) spherical device with negative charges, used to discharge the main sphere
9) spark produced by the difference of potentials
A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two pulleys, one of which is surrounded by a hollow metal sphere.[1] Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, are po
sitioned respectively near to the bottom of the pulley and inside the sphere. (2) is connected to the sphere, and a high DC potential (with respect to earth) is applied to (7); a positive potential in this example.
As the belt is pulled away from the upper roller, it pulls away some of the roller's electrons, leaving the roller with a positive charge, and the belt with a negative charge. This then causes electrons to concentrate at the tips of the lower brush (7), the longer the belt is turning, the higher the concentration of the electrons is. In the end this will cause the air around (7) to ionize, the electrons will then jump through the now conductive air towards the positively charged roller. But instead of hitting the roller, they meet the belt and are carried up to (2), the electrons are then pushed away from the negatively charged belt towards (2), (NOTE: the top roller must be made of an insulating material for this to work) where they are then led to the dome. As the belt continues to move, a constant charging current travels via the belt, and the sphere continues to accumulate negative charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be it
s final potential.
The other method for building Van de Graaff generators is to use the triboelectric effect. The strong e-field from the rollers then induces a corona discharge at the tip of the pointed electrodes. The electrodes then "spray" a charge onto the belt which is opposite in polarity to the charge on the rollers. The remaining operation is otherwise the same as the voltage-injecting version above. This type of generator is easier to build for science fair or homemade projects, since it doesn't require a potentially dangerous high voltage source. The trade-off is that it cannot build up as high a voltage as the other type, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects).
A Van de Graaff generator needs to be sphere shaped in order to work. The fact that electrically charged spheres have no e-field inside makes it possible to keep adding charges from a lower voltage source. Outside the sphere the e-field is very strong and applying charges from the outside is hence impossible.
Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly ideal current source. The maximum achievable potential is approximately equal to the sphere's radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV.
[edit] History
A Van de Graaff generator integrated with a particle accelerator. The generator produces the high fields (in the megavolt range) that accelerate the particles.
The fundamental idea for the friction machine as high-voltage supply, using electrostatic influence to charge rotating disk or belt can be traced back to the 17th century or even before (cf. Friction machines History)

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