Most of us have seen the device, known as a Van de Graaff generator, that makes your hair stand on end. The device looks like a big aluminum ball mounted on. Van de Graaff Electrostatic Generator Page. VAN DE GRAAFF GENERATOR HINTS & CLASSROOM DEMO NSTRATIONS · VAN DE GRAAFF QUESTIONS. Many a visitor to science museums has encountered a Van de Graaff generator. These contraptions are staples of hands-on demonstrations in labs and at.
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A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current DC electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in A tabletop version can produce on the order ofvolts and can store enough energy to produce a visible spark.
Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics ; larger ones are displayed in some science museums. The Van de Graaff generator was developed as a particle accelerator for physics research; its high potential is used to accelerate subatomic particles to great speeds in an evacuated tube.
It was the most powerful type of accelerator of the s until the cyclotron was developed. Van de Graaff generators are still used as accelerators to generate energetic particle and X-ray beams for nuclear research and nuclear medicine.
Particle-beam Van de Graaff accelerators are often used in a ” tandem ” configuration: When the particles reach the terminal, they are stripped of some electrons to make them positively charged and are subsequently accelerated by repulsive forces away from the terminal.
This configuration results in two accelerations for the cost of one Van de Graaff generator, and has the added advantage of leaving the complicated ion source instrumentation accessible near ground potential.
The voltage produced by an open-air Van de Graaff machine is limited by arcing and corona discharge to about 5 megavolts. Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials of as much as about 25 megavolts. A simple Van de Graaff generator consists of a belt of rubber or a similar flexible dielectric material moving over two rollers of differing material, one of which is surrounded by a hollow metal sphere. Comb 2 is connected to the sphere, and comb 7 to ground.
The method of charging is based on the triboelectric effectsuch that simple contact of dissimilar materials causes the transfer of some electrons from one material to the other. For example see the diagramthe rubber of the belt will become negatively charged while the acrylic glass of the upper roller will become positively charged.
The belt carries away negative charge on its inner surface while the upper roller accumulates positive charge. Next, the strong electric field surrounding the positive upper roller 3 induces a very high electric field near the points of the nearby comb 2.
At the points, the field becomes strong enough to ionize air molecules, and the electrons are attracted to the outside of the belt while positive ions go to the comb. At the comb 2 they are neutralized by electrons that were on the comb, thus leaving the comb and the attached outer shell 1 with fewer net electrons. By the principle illustrated in the Faraday ice pail experimenti. Electrostatic induction by this method continues, building up very large amounts of charge on the shell.
In the example, the lower roller 6 is metal, which picks negative charge off the inner surface of the belt. The lower comb 7 develops a high electric geraador at its points that also becomes large enough to ionize air molecules. In this case, the electrons are attracted to the comb and positive air ions neutralize negative charge on the outer surface of the belt, or become attached to the belt.
The exact balance of charges on the up-going versus down-going sides of the belt will depend on the combination of the materials used. In the example, the upward-moving belt must be more positive than the downward-moving belt. 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 peak potential. In the example, the wand with metal sphere 8 is connected to ground, as is the lower comb 7 ; electrons are geradir up from ground due to the attraction by the positive sphere, and when the electric field is great enough see below the air breaks in the form of an electrical discharge spark 9.
Since the material of the belt and rollers can be selected, the accumulated charge on the hollow metal sphere can either be made positive electron deficient or negative excess electrons. The friction type of generator described above vraaff easier to build for science fair or homemade projects, since it geraador not require a high-voltage source. A Van de Graaff generator terminal does gerafor need to be sphere-shaped to work, and in fact, the optimum shape is a sphere with an inward curve around the hole where the belt enters.
A rounded terminal minimizes the electric field around it, allowing greater potentials to be achieved without ionization of the air, or other dielectric gassurrounding. Outside the sphere, the electric field becomes very strong and applying charges directly from the graaff would soon be prevented by the field.
Since electrically charged conductors do not have any electric field inside, charges can be added continuously from the inside without increasing them to the full potential of the outer shell. 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 maximal achievable potential is roughly equal to the sphere radius R multiplied by the electric field E max at which corona discharges begin to form within the surrounding gas. This explains why Van de Graaff generators are often made with the largest possible diameter. The concept of an electrostatic generator in which charge is mechanically transported in vzn amounts into the interior of a high-voltage electrode originated with the Kelvin water dropperinvented during by William Thomson Lord Kelvin in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge.
Kelvin himself first suggested using a belt to carry the charge instead of water. The first electrostatic machine that used an endless belt to transport charge was constructed during by Augusto Righi. The charge was applied to the belt from the grounded lower roller by electrostatic induction using a charged plate. John Gray also invented a belt machine about Swann was developing during the s in which charge was transported to an electrode by falling metal balls, thus returning to the principle of the Kelvin water dropper.
The reason that the charge extracted from the belt moves to the outside of the sphere electrode, though it already has a high charge of the same polarity, is explained by the Faraday ice pail experiment. The Van de Graaff generator was developed, startingby physicist Robert J. The first model was demonstrated during October He did get the money, with some difficulty. Byhe could report achieving 1. An ordinary lamp socket provides the only power needed.
Van de Graaff applied for a second patent during Decemberwhich was assigned to Massachusetts Institute of Technology in exchange for a share of net income. The patent was later granted. 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 was from generators that ran off the belt, and several sessions came to a rather gruesome end when a pigeon attempted to fly between the two domes, causing them to discharge. The accelerator was set in an airplane hangar. It marked the beginning of nuclear research for civilian applications.
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. By the s, as much as 14 million volts could be achieved at the terminal of a tandem that used a tank of high-pressure sulfur hexafluoride SF 6 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 greatest potential sustained by a Van de Graaff accelerator is A further development is the pelletronwhere 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 greater velocity than a belt, and both the voltage and currents attainable are much greater than with a conventional Van de Graaff generator.
The Nuclear Structure Facility NSF  at Terador Laboratory was proposed during the s, commissioned duringand opened for experiments during It consisted of a tandem Van de Graaff generator operating routinely at 20 MV, housed in a distinctive building 70 m 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 vqn rare isotopic and radioactive beams. Perhaps the most important discovery made using 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 The largest air-insulated Van de Graaff generator in the world, built by Dr.
Van de Graaff during the s, is now displayed permanently at Boston’s Museum of Science. With two conjoined 4. Shows using the Van de Graaff generator and several Tesla coils are conducted two to three times a day.
Many science museums, such as the American Museum of Science and Energyhave small-scale Van de Graaff generators on display, and exploit their static-producing qualities to create “lightning” or make people’s hair stand up. Van de Graaff generators are also used in schools and science shows. Other electrostatic machines like the Wimshurst machine or Bonetti machine  work similarly to the Van De Graaff; charge is transported by moving plates, disks, or cylinders to a high voltage electrode.
For these generators, however, corona discharge from exposed metal parts at high potentials and poorer insulation result in smaller voltages. In an electrostatic generator, the rate of charge transported current to the high-voltage electrode is very small. After the machine is started, the voltage on the terminal electrode increases until the leakage current from the electrode equals the rate of charge transport.
Therefore, leakage from the terminal determines the maximum voltage attainable. In the Van de Graaff generator, the belt allows the transport of charge into the interior of a large hollow spherical electrode. This is the ideal shape to minimize leakage and corona discharge, so the Van de Graaff generator can produce the greatest voltage. This is why the Van de Graaff design has been used for all electrostatic particle accelerators.
In general, the larger the diameter and the smoother the sphere is, the higher the voltage that can be achieved.
From Wikipedia, the free encyclopedia. This article is about the machine used to accumulate electrical charge on a metal globe. For the progressive rock band, see Van der Graaf Generator.
Comb egrador at bottom that deposits charge onto belt. An educational program at the Theater of Electricity, Boston Museum of Science demonstrates the world’s largest air-insulated Van de Graaff generator, built by Van de Graaff in the s. Retrieved August 31, Retrieved September 1, Burboa Static electric machinefiled: August 13,granted: Journal of the Franklin Institute.
University Physics, 13th Ed.