Draw a labelled diagram of a Van de Graaff generator. State its working principle to show how by introducing a small charged sphere into a larger sphere, a large amount of charge can be transferred to the outer sphere. State the use of this machine and also point out its limitations.
Solution
Working principle: if a small sphere carrying charge q is placed inside a larger hollow sphere and connected to it, the charge flows entirely to the outer surface of the larger sphere, whatever its own potential, because the potential difference between an inner sphere and the shell depends only on q and the radii, not on the shell's charge. Repeating this builds up a large charge and a very high potential on the outer sphere. Use: it accelerates ions/protons to high energies for nuclear physics experiments. Limitation: the potential cannot be increased indefinitely — beyond the breakdown field of the surrounding air/gas, charge leaks away by corona discharge; this is reduced by enclosing the sphere in gas at high pressure.
