With the help of labelled diagram of Van-de-Graaff generator. Explain its principle, construction and working of van-de-Graaff generator.

Explanation

Van-de-Graff generator is a device which is used to produce a very high potential difference `~~ 10^(7)` volt) and accelerate the charged particles.
It was designed by Robert J. Van-de-Graff in 1931.
Principle : 1. The electric discharge takes place in air or gases rapidly at the sharp pointed ends.
2. If a charged conductor is brought into internal contact with a second hollow conductor, then entire charge is transferred. to the hollow conductor. The hollow conductor cbntinues accepting the charge irrespective of the fact that how large its1 potential may grow.
Construction and Working : It consists of a large conducting spherical shell (S) of few .i metre radius supported on an insulated stand of height of many metre. A belt made of insulated-fabric runs over two pulleys `P_(1)`and `P_2`. Pulley `P_(1), P_(2)` and the belt are run by an electric motor. A spray comb `C_(1)` (made of metallic needles) is connected to high voltage rectifier .(E.H.T.) which is earthed. A collecting comb `C_2` (made of metallic needles) facing the belt is connected to the spherical shell S (Figure).

Let spray comb Cj be charged to a high.+ve potential which sprays +ve charge,to the belt which in turn becomes positively charged (sharp teeth of the comb has very high charge density which sets up corona discharging action, thus setting up a sort of wind of positive charges).. Since belt is moving up, so it carries this positive charge upward.
Opposite charge appears on the teeth of collecting comb `C_2` by induction from the belt. As a result of this, positive charge appears on the outer surface of shell S. As the belt is moving continuously, so the charge on the shell S increases continuously. Consequently, the potential of the shell (S) rises to a very high,value. When the charge on the spherical shell is very high, leakage due to ionisation of surrounding air also increases.
This process is repeated again and again. In each cycle some charge is given to globe whose potential keeps on increasing. Theoretically, there is no limit to the potential of the globe. Practically however, there is a problem. When the potential has gone very high, charge starts leaking to the atmosphere in the form of a spark. Following measures can be taken to avoid the leakage of charge:
(i)n Leakage can be reduced by making the surface of globe smooth. A smooth surface has no pointed corners. Therefore, the surface charge density remains reasonably low.
(ii) Leakage can also be reduced by filling a steel chamber (which is properly earthed) with air or methane or nitrogen gas.
By taking these precautions, the potential of globe S can be increased to a very high value.
Now the charged particles at the top of the tube (T) are at very high potential with respect to the lower end of the tube which is earthed. Thus, these particles are accelerated downward and hit the target after emerging from the tube.
The energy acquired by a charge q is = qV, where Kis the potential of the spherical shell S.