Experiments with a Van de Graaff generator

Demonstration

The Van de Graaff generator never fails to inspire.

Apparatus and Materials

• Van de Graaff generator accessories

• Van de Graaff generator

Health & Safety and Technical Notes

It is unwise to operate a computer close to a running Van de Graaff generator, particularly a laptop which is not earthed.

• Read our standard health & safety guidance

• Van de Graaff generator safety

Accessories for the Van de Graaff generator should include a ‘head of hair’ and an insulating handle with a conducting polystyrene sphere suspended from the top.

Procedure

1. Show the Van de Graaff generator, and describe it as a machine transporting charges to its large sphere. Bring up the light, conducting polystyrene sphere, suspended on a long insulating nylon thread from an insulating rod. Let the small sphere touch the large sphere, sharing some of the charge and the repulsion between like charges will be apparent.

Photo courtesy of Mike Vetterlein
2. Fix the insulating rod into the top of the generator. Alternatively, the ‘head of hair’ can be put on the top, again showing repulsion.

Photo courtesy of Mike Vetterlein
3. Allow the sphere to spark to a neighbouring earthed sphere, and then direct to earth by a wire.

Teaching Notes

• The potential difference between the dome and the earth can be 200,000 volts, enough to make a spark jump across a narrow air gap. However, the total charge is so tiny that you only receive a small shock if you touch the dome.
• You might say "near to large concentrations of charge the electric field can be very strong. If a pointed object is attached to the dome then the electric field near to the point is very large indeed. It may be a strong enough field to tear electrons off nearby molecules. Each electron flies away, pulled by an electric field. Soon it smashes into an air molecule. If it has gained enough energy as it accelerated in the intense electric field it can knock an electron off that molecule. There are now two electrons that fly on to make more collisions; a chain reaction. That is a spark."
• You could also demonstrate that the force of repulsion gets larger when the dome holds more charge. Place a fist full of paper punch-outs (from a hole punch) on the dome and turn the machine on. The small pieces of paper get the same charge as the dome and so fly away in all directions, creating a fountain effect. The faster the machine, the greater the force of repulsion and the further away the punch-outs fall. [This suggestion submitted by Dr B S Sidhu from Slough Grammar School.]
• The lower end of the Van de Graaff generator and the base of the neighbouring sphere should be earthed effectively in these demonstrations. Since the discharge will give a sudden pulse, the earth connection (for example, to the earth terminal of a low voltage power supply connected to mains) should be free from sharp bends or kinks.
• By electrostatic induction, you can obtain an opposite charge from that of the Van de Graaff store (the upper, charged, sphere). Bring an uncharged metal ball near to the store so that the store pulls an opposite charge, and pushes a like charge away on the ball. Touch the ball and let that like charge run away to earth; then bring the ball away with the remaining opposite charge on it. See how that attracts another ball with a sample charge direct from the store.

This experiment was safety-tested in July 2007

• A video showing how to use a Van de Graaff generator: