Van de Graff generator uses two principles of electrostatics. The electric discharge takes place in air or gases at pointed conductors readily. This phenomenon is known as corona discharge.
If a hollow conductor is in a contact with another conductor, then the hollow conductor continuously accepts all the charge supplied by the other conductor irrespective of its growth in potential. This charge on the hollow conductor is transferred on its outer surface and is uniformly distributed on it.
Construction of Van De Graff Generator
Van de Graff generator consists of a large hollow metallic sphere S mounted on two insulated supporting columns C1 and C2. An endless belt of an insulating material is mounted on pulleys P1 and P2. The pulley P1 is run by a motor. Two sharp combs B1 and B2 are fixed near the pulleys P1 and P2 respectively touching the belt. Comb B1 is called spray comb, while comb B2 is called collecting comb.
The positive ion to be accelerated are produced in a discharge tube D. The ion source lies at the head of the tube in the sphere. Target is at the other end of the tube is earthed. The whole assembly is enclosed in a chamber filled with nitrogen or methane at high pressure.
The spray comb B1 is given a positive potential of range 104 V with respect to earth using high tension source. Due to the positive charge on the comb B1 electric wind is set and it sprays positive charge on the belt. As the belt moves and reaches the top of pulley P2. the negative charge is induced on sharp ends of collecting comb B2. The equal positive charge is induced on the far end of comb B2. Now, the far end of B2 and the sphere are connected. This positive charge on the far side of B2 moves immediately to the outer surface of the sphere.
Due to discharging action of sharp edges of comb B2 negative electric winds are set and it discharges negative charge on the belt. Thus the positive charge on the belt is neutralised. This is repeated continuously. Thus positive charge gets accumulated on the outer surface of the sphere S.
Now the capacity of the spherical conductor is given by
Hence the potential of the sphere goes on increasing with the increase in the charge.
The breakdown voltage for air is 3 x 106 V/m. beyond which air around the sphere is ionised and leakage takes place. This leakage is minimised by enclosing the assembly in a chamber filled with nitrogen or methane at high pressure.
Projectiles like protons, deuterons etc. can be generated in a discharge tube D and are accelerated in a downward direction along the length of the tube. This accelerated projectile hit the target with large kinetic energy and bring about artificial transmutation.
Uses of Van De Graff Generator:
It is used to create a high potential of a range of few million volts.
It is used to accelerate projectiles like protons, deuterons etc. which hit the target with large kinetic energy and bring about artificial transmutation.
It is used to study collision experiments in physics