Van de Graaff Generator:
Principle- When a charge is given to a conductor, for example, a sphere, the charge gets uniformly distributed over the surface. The surface density of charge i.e., charge per unit area denoted by σ (= Q/A) is equal at all points on the surface of the smooth spherical surface. On the other hand, if the conductor is not smooth or has varying curvature then the surface-density of charge is not uniform, it in fact increases with the curvature of the body. The below figure shows the charge distribution and the electric field over the surface of a sphere and on a pear-shaped charged conductor. Point P of the pear-shaped conductor has a high density of charge as it has high curvature there. A charged conductor with a very sharp point (such as a lightning conductor or combed-shaped conductor) has a very high surface density of charge at the sharp pointed end. Consequently, the strength of the electric field near the point is very great. The intense electric field breaks down the insulation of the air and sends a stream of charged molecules away from the point. The pointed conductor of combed shape is used in electrostatic generators such as Van de Graaff Generator for generating a very high potential difference.
Construction: Van de Graaff Generator has a very large hollow smooth metal sphere S resting on an insulated tall pillar. An endless belt of some insulating material, like silk, move continuously in between two pulley P1 and P2 driven by a motor. The upper pulley P2 is at the center of the large sphere S.
Pointed conductors C1 and C2 face the belt on one side. The pointed conductor C1 is joined to the positive terminal of the dc source whose other terminal is earthed. It provides a potential of the order of 104 V. The pointed conductor C2 is near the upper pulley and it is joined to the sphere S.
Working- When the pointed conductor C1 is joined to the positive terminal of a dc source and the belt is run by the motor, a positive charge is sprayed on the moving belt by the action of points. An earthed conductor is usually placed opposite to the pointed conductor C1. It helped to direct the electric wind on the belt.
The moving belt carries the charge up. When the positive charge approaches the pointed conductor C2, a negative charge is induced on the pointed portion of conductor C2 and a corresponding positive charge is induced on the outside of the sphere S. The negatively charged electric wind from C2 neutralizes the positive charge on the belt. The net result is that the positive charge is collected by the sphere S.
As the charge on the sphere S grows, its potential rises. In order to get extremely high potential, the whole arrangement is enclosed in a casing and the air is pumped out. It is replaced by a gas at a very high pressure to prevent the occurrence of discharge.
Uses- Van de Graaff generators have a wide range of uses in various fields of science, education, and industry. Here are some of the most common applications of Van de Graaff generators:
(1) Education and demonstrations: Van de Graaff generators are commonly used in science demonstrations and exhibits to show the principles of electrostatics, electric fields, and potential differences.
(2) Nuclear physics research: Van de Graaff generators are used to accelerate charged particles to high energies, which are then used in experiments in nuclear physics, particle physics, and other related fields.
(3) Medical treatments: Van de Graaff generators are used in radiotherapy for cancer treatment. High-energy charged particles produced by the generator are used to kill cancerous cells.
(4) Electrostatic painting: Van de Graaff generators are used in electrostatic painting, where a high voltage is applied to the paint to create an electrostatic field that attracts the paint particles to the surface being painted.
(5) Electrostatic cleaning: Van de Graaff generators are also used in electrostatic cleaning, where they create an electrostatic field that attracts dust and other small particles, making it easier to remove them from surfaces.
(6) X-ray generation: Van de Graaff generators can be used to generate X-rays for medical and industrial applications.
(7) Fusion research: Van de Graaff generators are used in fusion research to produce the high temperatures and pressures needed to initiate fusion reactions.