Physics Q & A - Van de Graaff generator
Shik Ho-yin
(Translation by Tong Shiu-sing and Wong Wing-hung)

Operation of a Van de Graaff generator
Fig. 1  Operation of a Van de Graaff generator.
 
A neutral small metal ball is attracted to the generator
Fig. 2  A neutral small metal ball is attracted to the generator. Do you know why?
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How does a Van de Graaff generator operate?

Van de Graaff generator was invented by the American scientist Robert J. Van de Graaff (1901- 1967) in 1931. Based on the principle of charging by friction, the generator can produce a large amount of charge. The figure shows the model commonly used in schools. It contains a rubber belt set into motion by a plastic roller. Static electricity is generated by the friction or by a high voltage at a pointed electrode (Fig. 1). The rolling rubber belt then carries the charge to the inner surface of the spherical metal cover. Due to mutual repulsion, the charge is repelled to the outer surface of the spherical cover, and hence a large amount of charge will accumulate there.

What are the applications of the generator?

Van de Graaff generator can produce a voltage of over 10 million volts on its spherical cover. In nuclear physics, such a high voltage can be used to accelerate various kinds of charged particles, like protons, electrons etc. Moreover, the generator can be used to demonstrate many interesting phenomena of static electricity. For examples, it can make your hair stand upright, attract a metal ball (Fig. 2) or a polystyrene ball, produce an electric spark, and generate electric wind to set a mini-windmill into rotation. Through these phenomena, we can understand more about the nature of static electricity.

Making hair stand upright

We can stand on an insulated chair, and put our hands the spherical metal cover of the generator. Since human body can conduct electricity, charge will be transferred to our body when the generator begins to operate; and because of the mutual repulsion of charge, the hair will stand upright.

Attracting a polystyrene ball

When a polystyrene ball is placed close to the generator, the charges inside the molecules of the ball will be redistributed. In a molecule, the positive and the negative charges will be slightly separated, producing the phenomenon of polarization. In this case the charge on the spherical cover will produce a slight attractive force on the opposite charges in the molecules, and hence the whole polystyrene ball will be attracted.

Producing electric spark

When a small grounded metal ball is placed close to the spherical cover of the generator, the strong electric field will cause the charge to leap towards the ball, producing a large amount of ions and electrons in air. Since the energy of the ions are higher than that of the neutral molecules, they will release their energy spontaneously and produce a spark, which is a discharge in air. Lightning, for example, is a discharge phenomenon in which charge leaps from a cloud to another or to the ground.

Producing electric wind

A sharp point on a charged conductor has a higher surface charge density which will generate a stronger electric field. The strong electric field ionizes the air molecules surrounding the sharp point, and those ions or electrons which have charge opposite to that of the conductor will be attracted towards the sharp point, while those ions or electrons with the same charge will be repelled. This phenomenon is called the point effect. The moving ions drag air molecules into motion, producing an electric wind that can turn a mini-windmill.

Applications of static electricity in daily life

The are many applications of static electricity in daily life, including photocopying, electrostatic precipitator and electrostatic spraying. Besides, knowing more about static electricity can help us to prevent possible hazards. For example, a vehicle carrying inflammable materials has an iron chain attached to its rear; this transfers charge to the ground to prevent fire caused by sparks. For the same reason, since oxygen and inflammable anaesthetic are often used in a hospital, the floor of an operating room is usually anti-static, and all the instruments have to be grounded. This prevents explosion caused by sparks.