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    Van de Graaff Generator
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    Van de Graaff Generator Experiments

    Van de Graaff Generator

    Definition

    A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe.

    Basics

    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. Invented in 1929 by American physicist Robert J. Van de Graaff, 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.

    Description

    A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two metal pulleys, one of which is surrounded by a hollow metal sphere. Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the lower pulley and inside the sphere, over the upper pulley. Comb (2) is connected to the sphere, and comb (7) to the ground. A high DC potential (with respect to earth) is applied to roller (6); a positive potential in this example.

    As the belt passes in front of the lower comb, it receives negative charge that escapes from its points due to the influence of the electric field around the lower pulley, that ionizes the air at the points. As the belt touches the upper roller (3), it transfers to it some electrons, leaving the roller with a negative charge (if it is insulated from the terminal), which added to the negative charge in the belt generates enough electric field to ionize the air at the points of the upper comb. Electrons then leak from the belt to the upper comb and to the terminal, leaving the belt positively charged as it returns down and the terminal negatively charged. The sphere shields the upper roller and comb from the electric field generated by charges that accumulate at the outer surface of it, causing the discharge and change of polarity of the belt at the upper roller to occur practically as if the terminal were grounded. 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 its final potential.

    Another method for building Van de Graaff generators is to use the triboelectric effect. The friction between the belt and the rollers, one of them now made of insulating material, or both made with insulating materials at different positions on the triboelectric scale, one above and other below the material of the belt, charges the rollers with opposite polarities. The strong e-field from the rollers then induces a corona discharge at the tips of the pointed comb 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, that cannot also be easily regulated, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects).

    A Van de Graaff generator terminal doesn't need to be sphere shaped in order to work, and in fact the optimum shape is a sphere with an inward curve around the hole where the belt enters. The fact that electrically charged conductors of any shape have no e-field inside makes it possible to keep adding charges continuously. A rounded terminal minimizes the electric field around it, allowing greater potentials to be achieved without ionization of the surrounding air, or other gas. Outside the sphere the e-field quickly becomes very strong and applying charges from the outside would soon be prevented by the field.

    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.

    History

    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)

    The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff at Princeton University. The first model was demonstrated in October 1929. The first machine used a silk ribbon bought at a five and dime store as the charge transport belt. In 1931 a version able to produce 1,000,000 volts was described in a patent disclosure. This version had two 60 cm diameter charge accumulation spheres mounted on Pyrex glass columns 180 cm high; the apparatus cost only $90.

    Van de Graaff applied for a patent in December 1931, which was assigned to MIT in exchange for a share of net income. The patent was later granted.

    In 1933 Van de Graaff built a 40-foot (12 m) model at MIT's Round Hill facility, the use of which was donated by Colonel Edward H. R. Green.

    A more recent development is the tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged ions are accelerated through one potential difference before being stripped of two or more electrons, inside a high voltage terminal, and accelerated again.

    One of Van de Graaff's accelerators used two charged domes of sufficient size that each of the domes had laboratories inside - one to provide the source of the accelerated beam, and the other to analyze the actual experiment. The power for the equipment inside the domes came from generators that ran off the belt, and several sessions came to a rather gruesome end when a pigeon would try to fly between the two domes - causing them to discharge (The accelerator was set up in an airplane hangar).

    By the 1970s, up to 14 million volts could be achieved at the terminal of a tandem that used a tank of high pressure sulfur hexafluoride (SF6) gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of megaelectronvolts, sufficient to study light ion direct nuclear reactions. The highest potential sustained by a Van de Graaff accelerator is 25.5 MV, achieved by the tandem at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory.

    A further development is the pelletron, where the rubber or fabric belt is replaced by a chain of short conductive rods connected by insulating links, and the air-ionizing electrodes are replaced by a grounded roller and inductive charging electrode. The chain can be operated at much higher velocity than a belt, and both the voltage and currents attainable are much higher than with a conventional Van de Graaff generator.

    The Nuclear Structure Facility, or NSF at Daresbury Laboratory, was proposed in the 1970s, commissioned in 1981 and opened for experiments in 1983. It consisted of a tandem Van de Graaff operating routinely at 20 MV, housed in a distinctive building 70 metres high. During its lifetime it accelerated 80 different ion beams for experimental use, ranging from protons to uranium. A particular feature was the ability to accelerate rare isotopic and radioactive beams. Perhaps the most important discovery made on the NSF was that of super-deformed nuclei. These nuclei, when formed from the fusion of lighter elements, rotate very rapidly. The pattern of gamma-rays emitted as they slow down provided detailed information about the inner structure of the nucleus. Following financial cutbacks, the NSF closed in 1993.

    The largest Van de Graaff generator in the world, built by Dr. Van de Graaff himself in the 1930s, is now on permanent display at Boston's Museum of Science. With two conjoined 4.5 meter (15 foot) aluminium spheres standing on columns 22 feet tall, this generator can often reach 2 MV (2 million volt). Shows using the Van de Graaff generator and several Tesla coils are conducted 2-3 times a day. Many science museums, such as the American Museum of Science and Energy, have small-scale Van de Graaff generators on display, and exploit their static-producing qualities to create "lightning" or make people's hair stand on end.

    Other classical electrostatic machines like a Wimshurst Machine or a Bonetti machine can easily produce more current than a Van de Graaff generator for experiments with electrostatics, and have positive and negative output. The less insulated structures, however, result in smaller voltages.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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