Ask a Teacher

EXPLAIN VANDEGRAPH GENERATOR?

In 1929 Robert .J. Van de Graaff designed and built an electrostatic generator which is used in Nuclear Physics Research. The generator is called Vand de Graaff generator. It is an arrangement for producing a very high potential difference so as to accelerate charged particles like deuterons and protons. Such charged particles is used to trigger nuclear reactions and to break atoms for various experiments in physics.
Principle: 
Van de Graaff generator is based on the following principles :
1.    Action of points: Sharp pointed surfaces have larger charge densities so these can be used to setup charging and discharging action.
2.    Uniform distribution of charges on the surface of a spherical conducting shell. If a hollow conductor is in contact with another charged conductor, then the hollow conductor continues accepting the charge irrespective of its potential.

  Construction and working

Van de Graaff generator consists of a highly polished metal doom mounted over insulating stands. An endless belt made up of good insulating material allowed to run between the pulleys P₁and P₂ controlled by a motor. C₁ and C₂ represents two metal combs, C₁ is the sparyer comb which sprays charges into the blet and C₂ is a collector comb which collects the charges from the belt and transfers to the metal doom. A very high voltage is applied to C₁. D represents the discharge tube which provides the path for the accelerated particle.
Let the spray comb be charged to a high +ve potential which sparys +ve charges to the belt. Since the belt is moving up, so it carries this positive charge upward. The collector comb collects these charges from the belt and transfers it to the metal doom. The process continues till the doom becomes charged to the saturated value. When the charge on the doom becomes charged to a high potential, leakage due to ionization of the surrounding air also increases. To prevent the leakage, the generator is enclosed in an earthed steel tank filled with air or methane under very high pressure.
When the potential inside the doom becomes very high, the particle which is to be accelerated is released inside the tube. The particle gets accelerated downwards due to high potential inside the doom and hit the target after emerging from the tube.
The energy acquired by a charge q is qV, when V is the potential of the spherical doom.

Disadvantages:
1.    It is very dangerous due to the high potential of the doom.
2.    The efficiency of the generator is very small with respect to other type of particle accelerators.
3.    Electrons cannot be accelerated.
4.    The size of the generator is extremely large.
The only advantage of the accelerator is that the same instrument can be used to accelerate both positive and negative charges with out any change in the design of the instrument.