VAN DE GRAAFF GENERATOR This is a machine that can build up high voltages of the order of a few million volts. The resulting large electric fields are used to accelerate charged particles (electrons, protons, ions) to high energies needed for experiments to probe the small scale structure of matter. Principle 1. Skin effect 2. Electrostatic induction 3. Action of points Construction A hollow metallic sphere A is mounted on insulating pillars as shown, and a pulley B is mounted at the centre of the sphere and another pulley C is mounted near the bottom.
A belt made of silk moves over the pulleys, and the pulley C is driven continuously by an electric motor. Two comb-shaped conductors D and E having number of needles are mounted near the pulleys. The comb D is maintained at a positive potential on the order of 104 volts by a power supply. The upper comb E is connected to the inner side of the hollow metal sphere. Working The high electric field near the comb D, ionizes the air due to action of points, the negative charges in air move towards the needles and positive charges are repelled on towards the belt.
These positive charges stick to the belt, moves up and reaches near the comb E. As a result of electrostatic induction, the comb acquires negative charge and the sphere acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere. The high electric field at the comb E ionizes the air. Hence, negative charges are repelled to the belt, neutralizes the positive charge on the belt before the belt passes over the pulley. Hence the descending belt will be left uncharged.
