Question: why such kind of charge is produced at the bottom of the van de graff

title: Van de Graaff generator
text: 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 during 1929 . The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts . A tabletop version can produce on the order of 100,000 volts 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 .