Van de Graff generator
A Van de Graaff generator
A Van de Graaff generator is a machine which uses a moving belt to accumulate very high charges on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can be up to 5 megavolts; applications for these exist with high voltage X-ray tubes, sterilization of food, and atom splitting experiments. The generator can be thought of as a constant-current source connected in series with a very large electrical resistance.
Description
A simple Van de Graaff generator consists of a belt of silk 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 E1 is made 10,000 volts positive with respect to earth.
This high voltage ionises the air at that point, repelling positive charges onto the belt and they are carried up 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 ionises the air inside the sphere and negative charges are repelled on to the belt, discharging it. As a result of Faraday's ice pail effect, positive charge on E2 migrates to the sphere regardless of the sphere's existing voltage. As the belt continues to move round, the sphere charges further positive, until the rate of leakage equals the rate at which charge is induced.
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. When it comes into contact with the other roller and is then separated, charge is transferred from the belt to the roller. The charge is picked up by pointed electrodes, and is otherwise the same as the voltage-injecting version above. This form is easier to build for science fair or homemade projects, since it doesn't require a high voltage source, and is not as dangerous. The trade-off is that it cannot build up as high a voltage as the other type, and operation is difficult under humid conditions.
History
The Van de Graaff generator was developed, starting in 1929, by MIT physicist Robert J. Van de Graaff. A common misspelling of the name is Van der Graaf (with an R and a single F). See also Van der Graaf Generator (band).
A recent development is the tandem accelerator, based on tandem generator technology, containing two or more Van de Graaff generators and, in which, heavy negatively charged ions are accelerated through one potential difference before being stripped of more electrons and accelerated again.
A further development is the Kirlian photography
