Question

With the help of a neat diagram, describe the construction and working of van de Graff generator.

Answer 1

Construction:

1. It consists of a large hollow metallic sphere S mounted on two insulating columns M₁ and M₂.
2. There are two pulleys P₁ and P₂. P₂ is mounted at the centre of sphere S while P1 is mounted near the bottom. A long narrow belt made of insulating material passes over the pulleys. The belt is driven by an electric motor, connected to the lower pulley P₁.
3. C₁ and C₂ are two pointed metal combs, having a number of metallic needles, pointing towards the belt. C₁ is called the spray comb and C₂ is called the collecting comb. C₁ is kept at a positive potential of about 10,000 V by using a high tension source (HT) while C₂ is connected to sphere S.
4. D is an evacuated accelerating tube having an electrode I at its upper end. This electrode is connected to sphere S. To prevent the leakage of charge from the spray, the generator is enclosed in a steel chamber filled with nitrogen or methane at high-pressure

Working:

1. When the spray comb is maintained at a high positive potential, it produces charges in its vicinity. The positive charges get sprayed on the belt due to the repulsive action of comb C₁, which is carried upward by the moving belt. A comb C₂, called collecting comb is positioned near the upper end of the belt, such that the pointed ends touch the belt and the other end is in contact with the inner surface of the metallic sphere S.
2. The comb C₂ collects the positive charge and transfers it to the metallic sphere. The charge transferred by the comb C₂ immediately moves onto the outer surface of the hollow sphere.
3. As the belt goes on moving, the accumulation of positive charge on the sphere also keeps on taking place continuously and its potential rises considerably.
4. With the increase of charge on the sphere, its leakage due to ionisation of surrounding air also becomes faster. When the rate of loss of charge due to leakage becomes equal to the rate at which the charge is transferred to the sphere, the potential of the sphere becomes maximum
5. The projectiles such as protons, deuterons etc. are introduced in the upper part of the evacuated accelerator tube. They get accelerated in the downward direction along the length of the tube and acquire very high energy. At the other end, they come out and hit the target with large kinetic energy and bring about nuclear disintegration