Action of Points and Van de Graaff Generator

Action of Points

For uniform shaped body, distribution of charge is uniform but for other shaped body the distribution of charge is inversely proportional to the radii of curvature of the surfaces. So surface charge density is maximum at the pointed ends of a conductor. When air molecules come in contact to the pointed ends of a charged conductor, they get similar charged and are repelled. Other uncharged molecules come to take place of previous molecules and get similarly repelled after getting the charge by conduction. This process takes place continuously.

As a result, a strong electric wind is set up near the pointed ends. The wind may made to blow a candle flame aside as shown in the figure. Each particle moves away, carries with it a small amount of charge from the conductor. The action of losing charged conductor due to its pointed ends is called action of points.

Applications of action of points

1. The action of points is used in lightning in conductors. Lightning conductors with sharp pointed ends are fitted at the top of a building in order to have safe discharge of electricity generated in the atmosphere.
2. The electric poles supporting the high voltage transmission lines are fitted with pointed conductors at the top. If there is any leakage of charge from the transmission lines to the poles, it is discharged through the pointed ends.

Van de Graaff Generator

Van de Graaff generator is an electrostatic machine designed by Robert J. Van de Graaff in 1931.Van de Graaff generator are continuously used to accelerate streams of charged particles. This type of generator is used in high voltage X-ray tube and for splitting experiments.

Principle

It is based on the following electrostatic phenomena

1. The action of sharp points
2. A charge given to a hollow conductor is transferred to the outer surface and spread uniformly over it.
3. It is based on the principle of electrostatic induction.

Construction

It consists of a large hollow metallic spheres, supported on two insulated columns Q, Q. P₁ and P₂

are two pulleys over which a belt BB of insulting materials eg. Silk passes through it. The belt can be the desired high speed by the help of an electric motor. C₁ and C₂ are two combs having number of sphere needles pointed towards the belt. The upper comb C₁ is connected to the hollow metallic sphere and C₂ is connected to a positive end of high tension supply of which negative end is earthed. The comb C₁ is called collecting comb and C₂ is called spray comb.

Working

The high tension is switched on to obtain the high potential and the electric motor is set in operation. Since P₂ is driven by an electric motor, the belt moves up and down between two pulleys P₁ and P_2. The comb C₂ is connected to the positive end of the high tension battery and gets positively charged. The comb C₂sprays positive charges on the conveyer belt due to the action of points and by the help of this belt charge carries upwards. The negative charge is induced on C₁ when the positive charge reaches close to the comb C₁. So negative change induced on C₁ is neutralised by the positive charge. Now, an electrically neutral belt moves down on the other side of pulley P₂. As this process is continuous, more and more positive changes collect on the outer surface of the metallic sphere whose potential rises continuously and its value may reach high up to 10⁶ volts.

Precautions

• In this generator, sharp corners and edges must be avoided otherwise the leakage of charge may take place due to the action of points.
• To prevent leakage and sparking, the outer of the metallic sphere is made very smooth and highly polished.