Van de Graaff generator

Assume the metal dome of the van de Graaf generator is charged +vely. It is because of induction that -ve charge is induced at the near side of the sphere and +ve charge on the farther side. Because the -ve induced charge is nearer to the generator dome than the +ve induced charge, a net attractive force is acting on the sphere to pull it towards the dome.

On touching the dome, -ve charge on the sphere is neutralized by the +ve charge on the dome. Leaving only +ve charge on the sphere. Repulsion occurs (like charges repel) and the sphere swings away.

After leaving the dome, two processes take place. The +ve charge on the sphere is neutralzed by electrons in the air (these electrons usually stick onto dusts suspended in air), and induction occurs on the sphere by the charged dome. As a result, the sphere acquired -ve charge on its near side to the dome and +ve charge on the far side, and the process repeats itself.

A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. Applications for these high voltage generators include driving X-ray tubes, accelerating electrons to sterilize food and process materials, and accelerating protons for nuclear physics experiments. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.