Van De Graaff Generator

What is a Van De Graaff generator?

A Van De Graaff Generator is an electrostatic generator and a high electric potential accumulator on a hollow metal globe.

Van de Graaff generators are described as "constant current" electrostatic devices. When you put a load on a Van de Graaff generator, the current (amperage) remains the same whereas the voltage that varies with the load.

A Van de Graaff generator operates by transferring electric charges from a moving belt to a terminal.

A typical Van de Graaff generator (labeled) looks like the following:

(Photo credit : Wikipedia)

Parts of the above generator are as follows:

Hollow metallic sphere (with positive charges)
The brush (ensures contact between the electrode and the belt)
Upper roller (Plexiglass)
Side of the belt with positive charges
Side of the belt with negative charges
Lower roller (metal)
Lower electrode (ground)
Spherical device with negative charges (used to discharge the metal sphere)
Spark (Produced by the difference of potentials)

How does it work?

It is important to know that the concentration of the charge on the roller is greater than in the belt.

Thus the roller's electric field should be greater than any point (specially at contact surfaces) in the belt. Thus the strong negative charge from the roller repels the electrons near the lower electrode (marked 7 above) and begins to strip the surrounding air molecules with these electrons.

(Photo credit : daviddarling.info)

Thus the belt is positively charged and it carries this charge to the upper parts of the generator.

The electrons in the brush move to the tips of the wires because they are attracted to the positively charged belt.

A similar occasion to the lower part, but in contrast, occurs in the upper part. The positive atomic nuclei go into the brush and thus the belt transports negative charges.

The positive charge on the brush gives the metal sphere attached to it a positive charge.

Therefore, the belt is actually the key for the optimum operation of the generator.

Van de Graaff generators can also be created using the friction to static electric method, popularly called the triboelectric effect. Here the friction created between the belt and the rollers charges the two parallel parts of the belt with opposite polarities.

It is always better to use a sphere as the outer surface of the generator as a rounded terminal minimizes the electric field around it.This allows greater potentials to be achieved without ionizing the surrounding air.

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