Q.State the principle and explain the construction and working of Van de Graaff generator.

The Van de Graaff generator uses the fact that charge sprayed onto a moving insulating belt is continuously collected and deposited on the outside of a large hollow conducting dome, building up extremely high electrostatic potentials.

Principle

If a charged conductor is brought into contact with the inside of a hollow conductor, all of its charge transfers to the hollow conductor and resides entirely on its outer surface — irrespective of how much charge (and hence potential) the outer conductor already carries. This follows because the electric field inside a conductor in electrostatic equilibrium is zero and all charge on an isolated conductor resides on its outer surface. Consequently, if charge is repeatedly carried inside a hollow sphere and transferred to it, the potential of the sphere can be raised indefinitely in principle, limited in practice only by dielectric breakdown/leakage of the surrounding medium.

Construction

The Van de Graaff generator consists of:

• A large hollow metallic sphere (the dome), typically several metres in diameter, mounted on top of a tall hollow insulating column (support), which is itself mounted on insulating pillars to prevent leakage of charge to the ground.
• An endless belt made of silk, rubber, or a similar flexible insulating material, running over two pulleys — a lower pulley (driven by an electric motor) near the base and an upper pulley inside the dome.
• A spray comb — a comb-shaped conductor with sharp points, placed close to the lower pulley and connected to the positive terminal of a high-voltage DC source (of the order of $10^4\text{ V}$).
• A collecting comb — a similar comb-shaped conductor placed close to the upper pulley, near the inside surface of the dome, connected electrically to the inner (and hence outer) surface of the dome.
• The entire assembly is usually enclosed in a steel tank filled with a high-dielectric-strength gas (such as nitrogen or Freon at high pressure), to suppress leakage and allow a higher maximum potential to be reached.
• The dome is provided with an outlet accelerating tube through which charged particles (protons, deuterons, or other ions) can be accelerated down an evacuated path to a target, using the high potential of the dome.

Working

The high-voltage source maintains a strong electric field at the sharp points of the spray comb, causing ionisation of the surrounding air and a corona discharge ('action of points'). Positive charge sprayed by the comb is deposited on the moving belt as it passes the lower pulley. The motor-driven belt carries this charge upward, inside the insulating column, into the hollow dome.

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