Give the principle of working of a Van de Graaff generator. With the help of a labelled diagram, describe its construction and working. How is the leakage of charge minimized from the generator?
Solution
Principle: if a charged conductor is brought into internal contact with a hollow conductor, all its charge passes to the outer surface of the hollow conductor, irrespective of the latter's potential. Construction and working: a large metal sphere is mounted on an insulating column; an endless insulating belt runs over two pulleys. A sharp comb (at a high positive potential) sprays charge onto the belt at the bottom; the belt carries it up, where a second comb connected to the inside of the sphere collects it and it spreads over the outer surface. Repetition builds the sphere to a very high potential, used to accelerate charged particles. Leakage of charge is minimised by surrounding the sphere with a vessel filled with gas at high pressure (which raises the breakdown field) and by keeping all surfaces smooth and rounded to avoid corona discharge.
