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Updated June 13, 2007 

Technology Investigation: The Van de Graaff Generator - What is the Secret to its Charge?


Background:

The Van de Graaff generator (VDG) is one of the most entertaining and fascinating electrostatic devices. As shown in the photo to the left, a VDG can literally make your hair stand on end!

Obviously there are electrical charges at work in the function of a VDG, but often there is a great deal of misunderstanding regarding exactly how that electrical charge comes about. Exactly how does a VDG obtain its charge - what is the secret at the heart of the VDG?

The photo to the right shows an early VDG and its talented inventor, Robert J. Van de Graaff. As you can see in the picture, this early VDG consisted of of two spheres, each of which was charged differently. Visit this page to see an excellent diagram and description of the original VDG.

Why would the first VDG have two spheres that were charged differently? The answer to this question is a major clue to solving the secret to the charge of the VDG.

While the VDG is the perfect device for investigating static charge, that was not the original intent of the device. Robert J. Van de Graaff designed his generator to be a particle accelerator, that is, the very high voltage difference between the two spheres had the ability to make charged particles move very fast from one sphere to the other. This was an important advancement in physics, because much insight into the structure of matter can be gained by smashing charged particles at high speed. Visit this page, and you will see amazing photos of a massive VDG meant to generate voltage differences in the millions of volts.

The Secret!

So how does a VDG get its charge? If some VDGs have two spheres that are charged differently, how is this accomplished?

It turns out the answer is as simple as this: contact between two different substances will often result in a transfer of charge. If you have ever rubbed balloon on your hair, you know that the balloon gains a charge, and this can be used to attract the balloon to a wall. In a VDG, this contact occurs at a very fast rate, thanks to a belt travelling over a roller at high speed:

One of the most important parts of the VDG is the belt, which carries charge created by the contact between the belt and the roller to the dome at the top of the VDG.

Whether positive or negative charge is carried upward depends on what substances make up the roller and belt:

VDG 1 - Belt Carries Negative Charge Upward

In this VDG, the roller is made of nylon and the belt is rubber. In this case, electrons move from the roller to the belt, resulting in the belt becoming negatively charged.

VDG 2 - Belt Carries Positive Charge Upward

In this VDG, the roller is made of vinyl and the belt is made of rubber. In this case, electrons move from the belt to the roller, resulting in the belt becoming positively charged.

VDG 1 - Result - Negatively Charged Dome

When the negatively charged side of the belt reaches the top of the VDG, the excess electrons are transferred to the dome.

This build up electrons results in a negatively charged dome.

Electrons repel each other, so negatively charged electrons spread and distribute over the entire dome.

Note that having given up its electrons, the belt now has a postive charge, and it returns to the bottom roller where the cycle repeats.

VDG 2 - Result - Positively Charged Dome

When the positively charged side of the belt reaches the top of the VDG, electrons on the dome move to the belt.

This movement of electrons from the dome to the belt results in a postively charged dome.

Although it is actually only electrons that move, this positive charge is often described as spreading and distributing over the entire dome.

Note that having gained electrons from the dome, the belt is now negatively charged, and it returns to the bottom roller where the cycle repeats.

So now you know the secret to the charging of a VDG, however, it must be said that the what you have read so far is a bit of a simplification. The diagrams and descriptions above represent the most common designs, but VDGs can be built in many different ways. For example, the rollers at the top and the bottom may be built from different materials. The brushes at the top and bottom (one inside the dome and one that touches the bottom roller) may also be of different materials.

Whenever materials are changed, the transfer of charge is affected. Which leaves us with one more mystery - how can you predict how charge will transfer when two different materials touch?

Your Task

The mystery to how charge transfers between different materials can be solved with something called the TRIBOELECTRIC SERIES.

Using the Internet or library resources, research the triboelectric series and write a paragraph that explains what it is. If necessary, include a chart in your answer.

Consider designing a simple experiment that demonstrates how the triboelectric series can be used to predict the transfer of charge, then perform your demonstration for your classmates.

Further Information

Triboelectric Series - who gets charged, and how?

Van de Graaff Generators and Their Uses - how do they work and what you can do with them?

Copyright 2012 - McGraw-Hill Ryerson

 

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