Contents
By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
What are the Types of Electrostatic Machines? And How does the Van de Graaff generator work?
Classification of Electrostatic Machines
Electrostatic machines can generate large quantities of electric charges rapidly. These machines are also used to set up high potential difference. Electrostatic machines are of two types
1. frictional machine and
2. induction machine. Frictional machines were not much effective. So after the invention of induction machines, frictional machines became obsolete. The principle of action of induction machines depends on the prin-ciple of electrostatic induction. In this chapter we shall discuss a familiar electrostatic machine known as Van de Graaff genera-tor.
Van de Graaff Generator
In 1931, Van de Graaff invented this machine. With the help of this machine very high potential difference (up to a few million volts) can be produced. The principle of action of this machine is based on the discharging action of points and on the prop-erty of collection of charges of a hollow conductor. This machine is very useful at atomic research centres. At present, many changes and modifications of this machine have been introduced.
Description:
The sketch of a Van de Graaff generator is shown in the Fig. A and B are two hollow spherical
conductors. These are placed on two big insulating stands (X, X). P1 and P2, form two pairs of pulleys. The pulleys are situated at the centres of the spherical conductors and are connected with two electric motors. Silk or rubber belt moves in the path shown by arrows along the body of each pair of pulleys. The belt enters the spherical conductor through the hole S1 and comes out through the hole S2. C, D, F, G are four pointed conductors.
The pointed ends are directed towards the belt. Positive charge is supplied to the small sphere (N) placed in front of the pointed conductor D and negative charge to the small sphere (M) placed in front of the pointed conductor C with the help of a dc generator. The pointed conductors F and G are connected with the spheres A and B respectively.
Working principle:
Positive charge on the small sphere (N) in front of the pointed conductor D induces negative charge on D and the induced positive charge, being free, moves to the earth. The pointed conductor D discharges the negative charge to the belt in front of it. The belt, being a non-conductor, does not distribute the charge all over its body; it remains confined to one place.
The belt carries the charge upwards and when the charge comes near the pointed conductor G, it induces positive charge on G and negative charge on the sphere B. Very soon the positive charge on G gets neutralised by the negative charge on the belt. So the sphere B is charged with negative electricity.
Similarly, due to the negative charge on the small sphere (M) in front of the pointed conductor C, the sphere A is charged with positive electricity.
Discharge of electricity and its remedy: Due to the continuous movement of the belt by the electric motor, a large quantity of charge accumulates on the two spheres A and B and the potential difference between them increases quickly. Due to continuous increase in the potential difference between the two spheres, an electric discharge may start in the neighbouring air, as air cannot bear high potential difference under normal pressure. To avoid pointed end discharge, the spheres and the belts are made very smooth.
The whole instrument is installed in a large metallic case connected to the earth and air inside the case is pumped out with the help of an exhaust pump. Next, the entire case is filled up with nitrogen or freon gas under high pressure. Because, even under high potential difference, tendency of nitrogen or freon molecules to be ionised, is low.
Uses:
- Production of high energy charged particles, in nuclear research.
- Production of hard X-rays.
In Science City, Kolkata there is a Van de Graaff generator for display.