Van de Graaff generator

This article covers the physical device Van de Graaff generator . For the rock band see Van der Graaf Generator .

Van de Graaff generator for demonstration and teaching purposes

Van de Graaff generator schematically
1. hollow metal ball (positively charged)
2. upper decrease comb (with a brush, with a small distance from the tape)
3. upper deflection roller (metal)
4. positively charged hinge side (tape dielectric)
5. negative loaded tape page
6. lower pulley (plastic. for example acrylic glass )
7. decrease lower comb (earth, counter electrode for negative charge)
8. ball having a negative charge (for the discharging of the main ball)
9. spark gap or spark discharge

Disused 1-MV-Van-de-Graaff generator in the foyer of the Mathematics Museum of the Eötvös Loránd University in Budapest (designer: Károly Simonyi , Sopron , Hungary 1951)

A Van de Graaff generator , also known as a band generator , is an apparatus for generating high electrical DC voltages . The generator was named after the American physicist Robert Van de Graaff , who developed belt generators from 1929.

The belt generator is one of the electrical generators and converts mechanical energy into electrical energy, but with a very low level of efficiency . It is next to the Influenzmaschine and the Tesla transformer , the device most commonly used for physical training experiments for high-voltage generation . It is also used in the Van de Graaff accelerator .

construction

A circumferential electrically insulating band, for example a rubber band - recognizable in the picture as a dark vertical stripe - can be electrically charged by friction or by spraying the charge (see corona discharge or) from an external voltage source. The movement of the belt transports the charge into the interior of the large metallic hollow sphere visible in the picture, where it is "stripped" from the belt by a brush that is conductively connected to the sphere. As a result, the sphere can be charged to ever higher levels of tension in relation to the environment; the voltage is only limited by spark breakdowns when the field strength is too high.

The second smaller ball visible in the picture is connected to the foot (earth potential) and can be pivoted and serves as the opposite pole to determine the range of the spark discharge and to discharge the equipment safely. It has no meaning for the function of the generator itself.

functionality

Electric charge of the desired polarity, positive or negative, is applied to the tape as frictional electricity (unrolling the tape from the lower roller) or from an external voltage source as a corona discharge by spraying a metal comb onto the tape and transporting it into the hollow sphere. There it is transferred to the ball via the bridge located inside. Depending on the materials used for the rollers and the belt or the configuration of the voltage source, the hollow sphere is charged positively or negatively. The inside of the ball is field-free like a Faraday cage , which is why electrical charge from the upper brush can be more easily transferred to the belt. If the upper brush were attached outside the sphere, an ever stronger electric field would counteract the transfer of charge carriers to the sphere.

Due to the mutual repulsion of charges of the same name, each new amount of charge brought into the interior of the sphere migrates to the surface of the sphere, regardless of the charge already present there. In this way, more and more charge can be transported into the sphere, and high tension can arise between the sphere and the environment. This voltage difference has to be overcome by the charge on the belt on its way into the sphere. This is achieved by doing mechanical work when turning against the electrostatic attraction that exists between the charge and earth potential: the change in distance leads to increasing voltage on the strip and the charge can be deposited as electrical field energy in the upper sphere. Similar to a charged plate capacitor, the plates of which are removed from each other (reduction in capacitance ), the voltage or the potential difference to earth potential increases if the charge on the strip remains the same : ${\displaystyle C}$ ${\displaystyle Q}$ ${\displaystyle U}$

${\displaystyle U={\frac {Q}{C}}}$

With

${\displaystyle U}$	- increasing potential difference compared to earth potential
${\displaystyle Q}$	- constant charges on the belt
${\displaystyle C}$	- decreasing capacity of a belt section with respect to earth when running up

After the return, the tape is ready to accept new charges again, since the potential decreases again during return (approaching the earth potential). The work is deposited as electrical field energy between the upper sphere and the environment / earth.

As the ball continues to turn, an equilibrium is established between the charge supplied and the charges flowing away through the air and other paths. Since the mechanically generated charging current is comparatively small, the achievable voltage is limited by the discharging current through the weakly conductive air. Therefore, band generators work very poorly in humid climates.

With correspondingly large devices, voltages of several million volts can be generated. The large generators used for accelerators are usually built into a pressure tank that is filled with a suitable, dry gas (e.g. sulfur hexafluoride ).

The spherical or at least rounded shape with smooth surfaces minimizes the loss of charge by avoiding or reducing the peak discharges occurring at sharp edges and points .

Belt generators that work without an external voltage source require suitable material pairings (see frictional electricity ) between the lower or upper roller and the belt in order to generate enough static electricity in the belt when it is lifted from the lower roller.

Externally powered band generators often have metal bars that are isolated from one another on the band and that are charged via a tip discharge via a brush or a metal comb located just above the lower roller. With them, one of the rollers does not have to be made of insulating material. From this the Pelletron was developed, which uses pieces of metal pipe or balls isolated from one another by an insulating chain.

Hazard warning

With small band generators, the discharge current flowing through the human body when approaching the upper sphere is only briefly very high and therefore usually harmless. However, band generators are a source of danger in particular when high-voltage-resistant capacitors (e.g. so-called Leiden bottles ) are charged with them. The stored energy can then become so high that an electric shock is life-threatening.

photos

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  Van de Graaff generator for schools

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  Van de Graaff generator without a hollow sphere

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  Upper acceptance comb

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  Lower pick comb

Trivia

The British progressive rock band Van der Graaf Generator named themselves after this device.

literature

• Wolfgang Finkelnburg: Introduction to Atomic Physics . Ed .: Springer-Verlag Berlin Heidelberg. Springer-Verlag Berlin Heidelberg, 1954, ISBN 978-3-662-01432-5 .

• Manfred von Ardenne , Gerhard Musiol, Uwe Klemradt: Effects of physics and their applications . 3. Edition. Harri Deutsch, 2005, ISBN 978-3-8171-1682-9 .

Individual evidence

1. ^ RJ Van de Graaff, KT Compton, LC Van Atta: The Electrostatic Production of High Voltage for Nuclear Investigations . In: American Physical Society (Ed.): Physical Review . 43, No. 3, February 1933, pp. 149-157. bibcode : 1933PhRv ... 43..149V . doi : 10.1103 / PhysRev.43.149 . Retrieved May 1, 2018.
2. ↑ Bandgenerator - Spektrum.de, 1998, accessed May 1, 2018

Web links

Commons : Van-de-Graaff generator  - collection of images, videos and audio files

Wiktionary: Bandgenerator  - explanations of meanings, word origins, synonyms, translations

Wiktionary: Van de Graaff generator  - explanations of meanings, word origins, synonyms, translations