Editing Van de Graaff generator (section)

== Description ==
[[File:Van de Graaff Generator.svg|thumb|267x267px|right|Van de Graaff generator diagram]]

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. A comb-shaped metal [[electrode]] with sharp points (2 and 7 in the diagram), is positioned near each roller. The upper comb (2) is connected to the sphere, and the lower one (7) to ground.  When a motor is used to drive the belt, the [[triboelectric effect]] causes the transfer of electrons from the dissimilar materials of the belt and the two rollers. In the example shown, the 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.<ref name="maglab">{{cite web |title=Van de Graaff Generator – MagLab |url=https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/van-de-graaff-generator |website=nationalmaglab.org |publisher=National High Magnetic Field Laboratory |access-date=10 May 2022}}</ref>

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 of the comb, the field becomes strong enough to [[ionization|ionize]] air molecules. The electrons from the air molecules are attracted to the outside of the belt, while the positive ions go to the comb.  At the comb they are neutralized by electrons from the metal, thus leaving the comb and the attached outer shell (1) with fewer net electrons and a net positive charge. By [[Gauss's law]] (as illustrated in the [[Faraday ice pail experiment]]), the excess positive charge is accumulated on the outer surface of the outer shell, leaving no [[electric field]] inside the shell. Continuing to drive the belt causes further electrostatic induction, which can build up large amounts of charge on the shell. Charge will continue to accumulate until the rate of charge leaving the sphere (through leakage and [[corona discharge]]) equals the rate at which new charge is being carried into the sphere by the belt.{{r|maglab}} 

Outside the terminal sphere, a high electric field results from the high voltage on the sphere, which would prevent the addition of further charge from the outside. However, since electrically charged conductors do not have any electric field inside, charges can be added continuously from the inside without needing to overcome the full potential of the outer shell. 
[[File:Spark by Van de Graaff generator.jpg|thumb|250px|right|Spark made by the Van de Graaff generator at [[Museum of Science (Boston)|The Museum of Science in Boston]], [[Massachusetts]]]]

The larger the sphere and the farther it is from ground, the higher its peak potential. The sign of the charge (positive or negative) can be controlled by the selection of materials for the belt and rollers. Higher potentials on the sphere can also be achieved by using a voltage source to charge the belt directly, rather than relying solely on the triboelectric effect.

A Van de Graaff generator terminal does not 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 gas]], surrounding it. 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''<sub>max</sub> at which corona discharges begin to form within the surrounding gas. For air at standard temperature and pressure ([[Standard temperature and pressure|STP]]) the [[Electrical breakdown|breakdown field]] is about {{val|30|u=kV/cm}}. Therefore, a polished spherical electrode {{convert|30|cm|in}} in diameter could be expected to develop a maximal voltage {{nowrap|1=''V''<sub>max</sub> = ''R''·''E''<sub>max</sub>}} of about {{val|450|u=kV}}. This explains why Van de Graaff generators are often made with the largest possible diameter.<ref name="hinterberger">{{cite web |last1=Hinterberger |first1=F |title=Electrostatic Accelerators |url=https://cds.cern.ch/record/1005042/files/p95.pdf |website=[[CERN]] |access-date=10 May 2022}}</ref>

{{multiple image
| align = center
| direction = horizontal
| header   =
| image1   = Van De Graaff gen 03.jpg
| caption1 = Van de Graaff generator for educational use in schools
| image2   = Van De Graaff gen 04.jpg
| caption2 = With sausage-shaped top terminal removed
| image3   = Van De Graaff gen 06.jpg
| caption3 = Comb electrode at bottom that deposits charge onto belt
| image4   = Van De Graaff gen 05.jpg
| caption4 = Comb electrode at top that removes charge from belt
| width    = 240
| footer   =
}}