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How the Van de Graaff generator works

Demonstration

The film explains the principle of the Van de Graaff generator, explains how to overcome problems when using it, and shows one simple but effective demonstration.

Apparatus and materials

For the demonstration

Van de Graaff generator

Hairdryer

Isopropyl alcohol (for cleaning)

Metal cake tins

Health & Safety and Technical notes


Note that any student with a pre-existing heart condition should remain at a safe distance from the generator – a metre should be adequate.

More detailed safety guidance can be found here:

Read this comprehensive safety note.

 

Procedure


The film below explains the construction and functioning of a Van de Graaff generator. It also shows how to clean it in order to avoid problems with dirt and humidity.

Van de Graaff generator

a For the simple demonstration, place a stack of metal cake tins, upside down, on top of the uncharged dome of the generator.

b Switch on the generator. As it charges up, the cake tins will one-by-one lift off the top of the stack.

 

Teaching notes


1 The Van de Graaff generator can be used to demonstrate effects involving static electricity in a spectacular fashion. The film explains that it is essentially a device which transfers electrons from its base (i.e. from the earth) to the upper dome, where they accumulate. You can estimate the potential difference (voltage) between the dome and earth knowing that it takes about 30 kV per centimetre to produce a spark in air (this figure depends on humidity etc.). So a 4 cm spark requires 120 kV.
Although these voltages are high, the amount of charge stored is small so the current in any spark will be small, and lasts for a very short time. For this reason, it is unlikely that any fit person will be harmed by a spark – but see the safety notes above.

2 The film explains that charging occurs by a process which is similar to the familiar charging which occurs when one material is rubbed against another. The difference is that, in the Van de Graaff generator, rubbing does not occur. Contact between two materials (the belt and the roller) is sufficient to start charges moving, and the machine is designed to make this a continuous process.

3 Note that some generators are arranged so that electrons are transferred downwards, leaving the dome positively charged. You can test whether the dome is positively or negatively charged using a coulombmeter; you can reverse the direction of charging by swapping over the two rollers.

4 You can use a hairdryer to remove water vapour on the generator. The acrylic uprights may also need cleaning with isopropyl alcohol.

Related guidance


See also our page on how the Van de Graaff works: http://www.practicalphysics.org/van-de-graaff-generator-basics.html
It is useful to know the triboelectric series; this will help you to decide which of two materials will become positively charged when they are rubbed together:

http://en.wikipedia.org/wiki/Triboelectric_effect

Related experiments


A series of experiments using the Van de Graaff generator, together with notes on safety: http://www.practicalphysics.org/van-de-graaff-generator-0.html