Van de graaff generator

Van de Graaff Generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It wasinvented in 1929 by American physicist Robert J. Van de Graaff. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. The Van de Graaff generator can be thought ofas a constant-current source connected in parallel with a capacitor and a very large electrical resistance.
Description: A simple Van de Graaff-generator consists of a belt of silk, or a similarflexible dielectricmaterial, running over two metal pulleys, one of which is surrounded by a hollow metal sphere.[1] Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, arepositioned respectively near to the bottom of the lower pulley and inside the sphere, over the upper pulley. Comb (2) is connected to the sphere, and comb (7) to the ground. A high DC potential (withrespect to earth) is applied to roller (3); a positive potential in this example.
As the belt passes in front of the lower comb, it receives negative charge that escapes from its points due to theinfluence of the electric field around the lower pulley, which ionizes the air at the points. As the belt touches the upper roller (6), it transfers some electrons, leaving the roller with a negativecharge (if it is insulated from the terminal), which added to the negative charge in the belt generates enough electric field to ionize the air at the points of the upper comb. Electrons then leak fromthe belt to the upper comb and to the terminal, leaving the belt positively charged as it returns down and the terminal negatively charged. The sphere shields the upper roller and comb from theelectric field generated by charges that accumulate at the outer surface of it, causing the discharge and change of polarity of the belt at the upper roller to occur practically as if the terminal were...