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Inducing Dipoles with a Van de Graaff Generator

First, a dipole consisting of two conducting balls connected by an insulating rod swings at the end of a long plexiglass rod. The two balls are charged oppositely by touching it to a conducting rod in the field of a Van de Graaff generator. The generator is negatively charged, which induces a positive charge on the closer silver ball, and a negative charge on the further gray ball. Going around the generator, the dipole always aligns itself with the electric field, i.e. the silver ball always stays closest to the generator.

Next, the Van de Graaff generator induces a positive charge on the closer side of a helium-filled conducting balloon, attracting it towards the generator. When it touches the generator, it picks up a net negative charge and is repelled away. When it hits the demonstrator (who can be thought of as "ground"), the net negative charge is removed and it is once again attracted to the generator.

The last clip combines these two ideas. The two outer bells are connected by a conducting rod and form a dipole in the field of the Van de Graaff generator, while the inner bell is grounded. The small balls are attracted to the outer bells the same way that the balloon was attracted to the generator. Once the balls hit the outer bells, they pick up a net charge and are repelled away. When the balls hit the grounded bell they lose their net charge and are attracted back to the outer bells. As you can see, the chimes do not work when the Van de Graaff generator is grounded.

A Van de Graaff generator is a machine that produces a lot of charge (in our case negative) on the outside of a conducting sphere. It was developed by Robert J. Van de Graaff, an MIT professor.

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January 16, 2009 16:40
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