Question

Explain the principle, construction and working of Van de Graff generator.

Answer 1

Robert J. Van de Graff designed an electrostatic machine which produces large electrostatic potential difference of the order of ${10}^{7}V$.
The working of Van de Graff generator is based on the principle of electrostatic induction and action of points. A hollow metallic sphere $A$ is mounted on insulating pillars as shown in the figure. A pulley $B$ is mounted at the centre of the sphere and another pulley $C$ is mounted near the bottom. A belt made of silk moves over the pulleys. The pulley $C$ is driven continuously by an electric motor. Two comb-shaped conductors $D$ and $E$ having number of needles, are mounted near the pulleys. The comb $D$ is maintained at a positive potential of the order of ${10}^4$ volt by a power supply. The upper comb $E$ is connected to the inner side of the hollow metal sphere.
Because of the high electric field near the comb $D$, the air gets ionized due to action of points, the negative charges in air move towards the needles and positive charges are repelled on towards the belt. These positive charges stick to the belt, moves up and reaches near the comb $E$.
As a result of electrostatic induction, the comb $E$ acquires negative charge and the sphere acquires positive charge. The acquires positive charge is distributed on the outer surface of the sphere. The high electric field at the comb $E$ ionises the air. Hence, negative charges are repelled to the belt, neutralises the positive charge on the belt before the belt passes over the pulley. Hence, the descending belt will be left uncharged.
Thus the machine, continuously transfers the positive charge to the sphere. As a result, the potential of the sphere keeps increasing till it attains a limiting value (maximum). After this stage no more charge can be placed on the sphere, it starts leaking to the surrounding due to ionisation of the air.
The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel chamber at a very high pressure.
The high voltage produced in this generator can be used to accelerate positive ions (proton deuterons) for the purpose of nuclear disintegration.