Explain the principle on which Van-de-Graaff generator operates. Draw a labeled schematic sketch and write briefly its working. A Van-de- Graaff type generator is capable of building up potential difference of 15×10⁶ V. The dielectric strength of the gas surrounding the electrode is 5×10⁷ Vm-1. What is the minimum radius of the spherical shell required?

Van de Graff generator is a device used for building up high potential differences of the order of a few million volts.

Such high potential differences are used to accelerate charged particles such as electrons, protons, ions, etc.

Principle It is based on the principle that charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.

Construction:

It consists of a large spherical conducting shell (S) supported over the insulating pillars. A long narrow belt of insulating material is wound around two pulleys P₁ and P₂. B₁ and B₂ are two sharply pointed metal combs. B₁ is called the spray comb and B₂ is called the collecting comb.

Working The spray comb is given a positive potential by high tension source. The positive charge gets sprayed on the belt.

As the belt moves and reaches the sphere, a negative charge is induced on the sharp ends of collecting comb B₂ and an equal positive charge is induced on the farther end of B₂.

This positive charge shifts immediately to the outer surface of S. Due to discharging action of sharp points of B₂, the positive charge on the belt is neutralized. The uncharged belt returns down and collects the positive charge from B₁, which in turn is collected by B₂. This is repeated. Thus, the positive charge on S goes on accumulating. In this way, voltage differences of as much as 6 or 8 million volts (with respect to the ground) can be built up.