Van de graaff generator is used to generate high voltages of the order of a few million volts. This results in generation of large electric fields for experimental purposes. Youtube - Hindi.
Principle The inner sphere has a higher potential than outer if the charge q is positive. If the two spheres are connected through a wire, the charge will flow from high to low potential. This accumulates close to millions of volts. A long narrow endless belt insulating material, like rubber or silk, is wound around two pulleys — one at ground level, one at the center of the shell.
This belt is kept continuously moving by a motor driving the lower pulley.
It continuously carries positive charge, sprayed on to it by a brush at ground level, to the top. There it transfers its positive charge to another conducting brush connected to the large shell.
Thus positive charge is transferred to the shell, where it spreads out uniformly on the outer surface and a voltage difference of as much as 6 or 8 million volts with respect to ground can be built up. Class 12 Physics Capacitance.