NationMaster - Encyclopedia: Van de Graaff generator

A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. Applications for these high voltage generators include driving X-ray tubes, accelerating electrons to sterilize food and process materials, and accelerating protons for nuclear physics experiments. 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: Photograph of a Van de Graaff generator. ... Description: Photograph of a Van de Graaff generator. ... An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ... In electrical engineering High voltage refers to a voltage which is high. ... An X-Ray tube is a vacuum tube designed to produce man made X-Ray photons on demand. ... Properties The electron (also called negatron, commonly represented as e−) is a subatomic particle. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... For alternative meanings see proton (disambiguation). ... Nuclear physics is the branch of physics concerned with the nucleus of the atom. ... Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. ...

Description

Schematic view of a classical Van de Graaff generator.
1) hollow metallic sphere (with positive charges)
2) electrode connected to the sphere, a mesh in close proximity (but not contacting) the electrode and the belt
3) upper roller (for example in plexiglass)
4) side of the belt with positive charges
5) opposite side of the belt with negative charges
6) lower roller (metal)
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. Two electrodes, E1 and E2, in the form of sharply pointed cones, are positioned respectively near to the bottom of the pulley and inside the sphere. E2 is connected to the sphere, and a high DC potential (with respect to earth) is applied to E1; a positive potential in this example. Image File history File links Van_de_graaf_generator. ... Image File history File links Van_de_graaf_generator. ... A dielectric, or electrical insulator, is a substance that is highly resistant to electric current. ... An electrode is an electrical conductor used to make contact with a metallic part of a circuit (e. ...

The high voltage ionizes the air at the tip of E1, repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge induces a negative charge to the electrode E2 and a positive charge to the sphere (to which E2 is connected). The high potential difference ionizes the air inside the sphere, and negative charges are repelled from E2 and onto the belt, discharging it. As a result of the Faraday cage effect, positive charge on E2 migrates to the sphere regardless of the sphere's existing voltage. As the belt continues to move, a constant charging current travels via the belt, and the sphere continues to accumulate positive 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. Electrostatic induction is a method by which an electrically charged object can be used to create an electrical charge in a second object, without contact between the two objects. ... Ionization is the physical process of converting an atom or molecule into an ion by changing the difference between the number of protons and electrons. ... Entrance to a Faraday room A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking (also known as arcing) is not favoured. ...

The other method for building Van de Graaff generators is to use the triboelectric effect. The two rollers for the belt are made of different materials, far from each other on the triboelectric series. When the belt comes into contact with one and is then separated, charge is transferred from the roller to the belt, and the roller becomes charged. When the belt comes into contact with the other roller and is then separated, charge is transferred from the belt to the roller, and that roller develops an opposite charge. 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). The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after coming into contact with another different material, and are then separated. ... It has been suggested that optical field be merged into this article or section. ...

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. It has been suggested that this article or section be merged with Voltage source. ... In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. ... Breakdown Voltage (Insulator) = The minimum voltage that makes an insulator react as a conductor. ...

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) Download high resolution version (2416x1716, 645 KB)Van den Graaff particle accelerator in the basement of the Jussieu Campus in Paris Copyright © 2004 David Monniaux File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Download high resolution version (2416x1716, 645 KB)Van den Graaff particle accelerator in the basement of the Jussieu Campus in Paris Copyright © 2004 David Monniaux File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... For the DC Comics Superhero also called Atom Smasher, see Albert Rothstein. ... (16th century - 17th century - 18th century - more centuries) As a means of recording the passage of time, the 17th century was that century which lasted from 1601-1700. ... An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current. ...

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. ^[1] The first machine used a silk ribbon bought at a five and dime store as the charge transport belt. In 1931 a version capable of producing 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. Friction machines History 1929 (MCMXXIX) was a common year starting on Tuesday (link will take you to calendar). ... Robert Jemison Van de Graaff, (December 20, 1901 -- January 16, 1967) was an American physicist and instrument maker, and professor of physics at Princeton University. ... Princeton University is a private coeducational research university located in Princeton, New Jersey, in the United States of America. ... Five and dime was a common nickname in the United States for five-and-ten-cent stores (also called 5 and 10s), popular in the early to mid-20th century. ... Pyrex is a brand name of borosilicate glass introduced by Corning Glass Works in 1915. ... An electrostatic generator, or electrostatic machine, is a mechanical device that produces static electricity, or electricity at high voltage and low continuous current. ...

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. The Massachusetts Institute of Technology (MIT) is a private, coeducational research university located in Cambridge, Massachusetts. ...

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 Green. 1933 (MCMXXXIII) was a common year starting on Sunday. ... The MIT radio telescope at Round Hill Round Hill is a location in Dartmouth, Massachusetts of historical significance. ... Colonel Greens Mansion at Round Hill with Buzzards Bay in the foreground. ...

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. An electrostatic potential map of the nitrate ion (NO3âˆ’). Areas coloured red are lower in energy than areas colored yellow An ion is an atom or group of atoms which have lost or gained one or more electrons, making them negatively or positively charged. ... Potential difference is a quantity in physics related to the amount of energy that would be required to move an object from one place to another against various types of force. ...

One of Van de Graaff's accelerators used two charged domes of sufficient size that had laboratories inside each dome - 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 spectacular 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 Holyfield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The 1970s decade refers to the years from 1970 to 1979. ... Sulfur hexafluoride (SF6) is a gas that consists of one part sulfur and six parts fluorine. ... A combination of federal, state and private funds is providing $300 million for the construction of 13 facilities on ORNLs new main campus. ...

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 machine. Pelletron is an electrostatic particle accelerator similar to a Van de Graaff generator, which is able to reach several million electron-volts (MeV) of kinetic energy. ...

A common misspelling of the name is Van der Graaf (with an R and a single F). See also Van der Graaf Generator, a rock group. This article is about the band. ...

Van de Graaff generators on display

One of the largest Van de Graaff generators in the world, built by Dr. Van de Graaff himself, is now on permanent display at Boston's Museum of Science. With two conjoined 15 foot aluminum spheres standing on columns many feet tall, this generator can often reach 2 million volts. Shows using the Van de Graaff generator and several Tesla coils are conducted several times each day. Outside the Museum of Science, August 2005 The Museum of Science (MoS) is a Boston, Massachusetts landmark, located in Science Park, a plot of land spanning the Charles River. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ... Josephson junction array chip developed by NIST as a standard volt. ... Tesla Coil at Questacon, the Australian National Science Centre museum A Tesla coil is a type of resonant transformer, named after its inventor, Nikola Tesla. ...

Comparison with other high voltage generators

Other classical electrostatic machines like a triplex Wimshurst or a Bonetti machine can easily produce more current, but the less insulated structures result in smaller voltages.

Patents

References

Contents

Description

History

Van de Graaff generators on display

Comparison with other high voltage generators

Patents

References

See also

External links

Contents

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History

Van de Graaff generators on display

Comparison with other high voltage generators

Patents

References

See also

External links

Description