VAN DE GRAFF GENERATOR

In this article, we are going to discuss about, what is van de graff generator? What is it’s construction, principle, working and uses?  In detail, so stay tuned as till end.

WHAT IS VAN DE GRAFF GENERATOR?

Van de graff generator is a electrostatic generator or device, which uses a moving belts to accumulate electric charge on a hollow spherical metallic body. Which is put on the top of a insulating columns.

It creates very high electric potential producing very high voltage direct current (DC) electricity at low current levels. This generator was invented by American physicist Robert J. Van de Graaff in 1929.

The modern van de graff generator can produce potential of as much as 5 megavolts and a table top version can produce potential on the order of 100,000 volts and can store enough energy to  produce a visible sparks.

CONSTRUCTION

In the basic design of Van de graff generator. There consists of a large hollow metallic sphere (S), mounted on two insulating supporting columns C1 and C2.

A long narrow belts of insulating material is wound around the two pulleys P1 and P2. The P2 pulleys is located at the centre of the metallic sphere and the P1 pulley is near to ground level. The pulley P1 is connected to the rotating device like motor.

There is two sharp combs fixed near the pulleys P1 and P2 respectively. These two sharps combs fixed near these two pulleys such that it just touches the belts. The comb B1 is called spray comb and the comb B2 is called collecting comb.

A discharging tube D is used in which acceleration of the ions is done. The point where ions is originated is present at the head end of the metallic sphere and the other end is earthned into the ground. The whole apparatus is placed into a steel compartment. This compartment is filled with nitrogen and methane at high pressure.

PRINCIPLE

It is based in the principle that charge given to a hollow spherical conductor is transferred to the outer surface and is redistributed uniformly over it.

It consists of a large spherical conducting shell S supported over the insulating columns. A long narrow belt of insulating material is wound around two pulleys P1 and P2. B1 and B2 are two sharply pointed metal combs or brushes.

WORKING

The spray combs (B1) 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, the negative charge is induced on the sharp ends of the collecting comb (B2).

This positive charge shifts immediately to the outer surface of S. Due to discharging action of sharp point B2. The positive charge on the belt is neutralized.

The neutralized belt returns down and collects the positive charge from B1, which in turns collected by the B2. This action is repeated.

Thus, the positive charge on S goes on accumulating. In this way, potential differences as much as 6 or 8 millions of volts. This potential difference with respect to the ground.[latexpage]

ACTION OF A SHARP POINT

When a spherical conductor of radius r carries a charge q, then the surface charged density is given by –
\[\sigma=\frac{\text{charge}}{\text{area}}=\frac{q}{4\pi\epsilon_0}\]
For pointed end, radius is very small therefore $\sigma$ is very small.

The particles of air when strike to the point end then it gets similarly charged so it repelled. In this away, a electric wind is set up which takes away the electric charge continuously. This process of spraying of charge is called corona discharge. Thats why conductor used for storing charge is always sphere of large radius.

USES

There a many applications of Van de graff generator, some of important application are listed below:

1). The Van de Graaff generator was developed as a particle accelerator for physics research. Its high potential is used to accelerate subatomic particles to great speeds in an evacuated tube.

2). Van de graff generator is often used in tandem accelerators.

3). It is used as physics model to teach electrostatic. So it is often kept in science mesuems.

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