Purpose:

To understand how the Van de Graaff Generator works.

Discussion:

The Van de Graaff generator is an impressive electrostatic generator that is capable of producing enormously large static electric potentials. In fact, giant Van de Graaff generators can produce millions of volts leading to awesome displays of corona and lightning. More modest "class room" sized Van de Graaff generators typically produce 100,000 V to 500,000 V.

The Van de Graaff generator is named after Dr. Robert J. Van de Graaff who patented his electrostatic generator in 1935. Interestingly, he developed this generator for studying the acceleration of charged particles to explore the atom. Originally referred to as his "tin can generator", the generator was built using a pure silk ribbon driven by a small motor. The charges on the ribbon were collected on a terminal that actually was a tin can. His early devices were capable of developing 80,000 volts. Of course, this device was limited by the edge effects of the tin can. However, his theory lead to Van de Graaff generators that have produced upwards to 10 million volts. Interestingly, Van de Graaff may not have been the first to develop a belt driven electrostatic generator. In 1893, Von Busch presented a device having two pulleys and a horizontal belt with a charge collector comb and an insulated sphere. Even earlier than this, Rouland invented an electrostatic generator in 1785 using a continuous silk ribbon running between two horizontal pulleys with a collector tube at the center.

Regardless of its "true" inventor, Van de Graaff generators have been instrumental in a number of fields, including particle acceleration, electrostatics, ESD, as well as providing an awesome display of the power of electricity.

Simplified Theoretical Description of the Van de Graaff:

Figure 1 illustrates a simple construction of the Van de Graaff generator. A small 12-Vdc permanent magnetic motor placed at the base end drives a plastic pulley. A rubber belt is drven by the pulley. As the pulley rotates, it acquires a negative charge from the belt by triboelectrification. Thus, the inside surface of the rubber belt becomes negatively charged. By induction, the outside of the belt becomes positively charged. A conducting brush, referred to as the charge-spray-comb, at the base of the pulley drains the negative charges on the outside of the belt to ground. At the top of a belt is a metal pulley and a charge-spray comb which is connected to the "collector," which is the outer metal sphere. The collection of positive charge continues while the belt is being driven. It is also noted that one could reverse the charge of the sphere by simply reversing the two pulleys.

Due to the geometry of the outer sphere the free charge will be uniformly distributed about it's surface. Thus, the electrostatic potential will roughly be Q/(4 Pi epsilon R), where Q is the total charge on the sphere. As the generator continues to charge, a potential difference between the sphere and the grounded base of the Van de Graaff can reach nearly one-half of a million volts. In fact, the sphere will continue to build up charge until a voltage break down occurs in the air, resulting in an arc between the sphere and the base, or some other grounded object in close proximity (such as an unsuspecting finger!). Prior to the breakdown, the air around the sphere becomes ionized. This ionization is sometimes refered to as an electric wind since one can feel the movement of the air.

Results:

A Van de Graaff Generator	A spark was generated between the two metal balls when it was operating.

Other Van de Graaff Links

Van de Graaff Generator Experiment

Any comments?

Instructor: Stephen Gedney
Department: University of Kentucky, EE Department
Created by: Tang, wee-hua and S. Gedney