Van De Graaff Generator

The Van De Graaff Generator is basically an electrostatic machine that can generate high voltages. A typical Van De Graaff Generator consists of an insulating belt that transports electrical charge to a terminal. The charges that are sent on the belt are generated through a high voltage DC supply. These charges are collected in the inside of the terminal and transferred to its external surface.

A Van De Graaff generator can be used to generator high amounts of potential difference to the order of about 5 Mega Volts. Generally used for scientific experiments, the generated charges are used to speed particles such as ions. Let us now take a detailed look into the history, construction and working of Van De Graaff Generators.

Construction

Inside the Van de Graaff Generator, the Moving Belt is wound on a Plastic Pulley. When the Motor drives the Pulley, the Belt rubs the Plastic Pulley and gives it Negative Charges. As the Motor continues driving the Pulley, the Negative Charges in the Pulley accumulate and they induce Positive Charges in the sharply pointed Metal Brush. The Electric Field between the Pulley and the Brush increases, so the air around the Brush becomes Ionized. The Positive Charges of the air molecules are repelled from the Brush and attach on the surface of the Belt. Those Positive Charges are then carried up into the hollow Metal Sphere, which is called Dome, and transferred to the Dome via air Ionization and a sharply pointed Metal Brush. This leaves a large amount of Positive Charges on the surface of the Dome and the potential of the Dome rises.

How does a Van De Graff Generator Work?

The Van De Graaff generator works simply on the principle of static electricity. All matter, as we know is made up of atoms which further constituted of electrons, neutrons and protons. Electrons carry negative charge whereas protons are considered to be positively charged. When the number of electrons and protons remain the same, the matter is considered to be neutral in charge. A negatively charged matter has more number of electrons than protons while the opposite holds true for a positively charged matter. Electrons can flow from one matter to another.

When two materials are rubbed together, a flow of electrons can take place depending on the triboelectric properties. When such a transfer occurs, the material that lost electrons will become positively charged and the one that gained electrons becomes negatively charged. This basically how static electricity is generated.

A Van de Graaff generator creates static electricity. The current generated by a Van De Graaff generator remains the same, while the voltage changes according to the applied load. A very simple Van De Graaff generator is made of the following:

· A motor

· Rollers, two in number

· Insulated belt

· Brush assemblies, two in number

· Metal sphere as the output terminal

The motor is required to turn the belt at a constant speed around the two rollers. The lower roller is built of a material that has a stronger triboelectric property. Now when the motor starts turning the belt around the lower roller, electrons are captured from the insulated belt onto the lower roller. Slowly more and more charge becomes concentrated on the roller. This phenomenon of concentration of charge results in repelling the electrons from the tips of the brush assembly. It also starts to attract electrons from the air molecules between the lower roller and brush assembly. Due to this phenomenon, the positively charged air molecules get carried on the belt away from the negatively charged roller. The belt therefore gets charged positively and moves towards the upper rollers.

The upper roller is made from or coated with a material that is higher up in the triboelectric series such as nylon due to which it tries to repel the positive charge on the belt. The upper brush is directly connected to the inside of the output terminal or sphere at one end and almost touches the upper roller and belt at the other. The electrons in the brush become attracted to the positive charges on the belt. The air particles break down too and the free electrons move towards the belt. The sphere takes up all of the charge and the excess charge gets spread to the outside of the terminal output or sphere.

It is this simple electrostatic effect that allows the Van De Graaff generator to output very high voltages continuously.
Uses of Van De Graff Generators

In modern times, the application of Van De Graff generators is largely limited to academic purposes to demonstrate the practical aspects and concepts of electrostatic behavior of particles. Primarily designed as a particle accelerator, the Van De Graaff generators are used in laboratories for demonstration purposes only. However, it must be noted that Van de Graaff generators were one of the first methods used to study nuclear physics before the advent of better methods to accelerate particles. Though the use of Van De Graaff generators are limited in today's world, they mark a very important milestone in the study of particles in the history of nuclear physics.