Van de Graaff generator

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.

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.

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).

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.