What is VAN DE GRAAFF ELECTROSTATIC GENERATOR

A van de graaff 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 like electrons, protons, ions etc. needed for various experiments of Nuclear Physics.

It was designed by Van de graaff in the year 1931.

Principle

This generator is based on

(l) The action of sharp points i.e. the pheno­menon of corona discharge.

(ii) The property that charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.

Construction

The essential parts of Van de Graaff generator are shown in Fig. 1(d) .21. S is a large spherical conducting shell of radius equal to a few metres. This is supported at a suitable height (of several metres above the ground) over the insulating pillars P1, P2. A long narrow belt of insulating material like, silk, rubber or rayon is wound around two pulleys P1 and P2. P1 is at the ground level and P2 is at the centre of S. The belt is kept moving continuously over the pulleys with the help of a motor. B1 and B2 are two sharply pointed metal Combs fixed as shown. B1 is called the spray comb and B2 is called the collecting comb.

The positive ions to be accelerated are produced in a discharge tube D. The ion source lies at the head of the tube inside the spherical shell. The other end of the tube carrying the target nucleus is earthed.

The generator is enclosed in a steel chamber c filled with nitrogen or methane at high pressure.

Working

The spray comb is given a positive potential (~ 104 volt) w.r.t. the earth by high tension source H.T Due to discharging action of sharp points, a positively charged electric wind is set up, which sprays positive charge on the belt {corona discharge). 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, a negatively charged electric wind is set up. This neutralises the positive charge on the belt. The uncharged belt returns down, collects the positive charge from B1, which in turn is collected By B23. This is repeated. Thus the positive charge on S goes on accumulating.

Now, the capacity of spherical shell

                  C = 4π ∈₀R,    where R is radius of the shell.

V = Q /C                      

V = Q/ 4π ∈₀R

Hence the potential V of the spherical shell goes on increasing with increase in Q.

The breakdown field of air is about 3 x 10 ki power 6 V/m.