Download 5A50.30 Van de Graaff Generator

5A50.30 Van de Graaff Generator
Abstract
The Van de Graaff generator is an electrostatic generator which accumulates charge on the surface of a metal
sphere. The generator uses a moving rubber belt to transfer charge from the roller via the Triboelectric effect.
Once the charge has colleted on the surface of the belt, it is brough up to a brush which is connected to the
inner surface of the sphere. Using this method very large voltages can be created. The Van de Graaff generator
is useful many different demonstrations which show how charge accumulates on, and dissipates from objects.
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Safety Concerns
The Van de Graaff generator produces very high voltages but low current, so shocks from it are harmless.
That being said, these shocks can still be unpleasant so care should be taken to properly discharge the generator
before touching it. This includes not only the metal sphere but the entire generator. People with pacemakers or
heart conditions should not use the Van de Graaff generator.
Equipment
• Van de Graff Generator
• Grounding rod
• Insulating board
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Procedure
Before turning on the Van de Graaff generator, ensure that the grounding rod has been plugged into the
ground outlet at the base of the generator. After it has been turned on, a strong electric field will be created
around the metal sphere. By moving the grounding rod close to the surface of the sphere it can be discharged
through the air.
Another popular demonstration to preform with the Van de Graaff generator is to show how a person can be
charged without being shocked. Have the participant stand on the insulating board so that they are not grounded.
Ask them to place one hand on the metal sphere and then turn on the Van de Graaff generator. The persons
body will become charged and their hair may begin to stand on end. By having the participant point their finger
at another object they can dissipate charge from their body and cause the hair to fall back down.
Theory
The Van de Graaff generator opperates by moving a looped track which is wrapped around two rollers. These
rollers are spun at high speeds using an electric motor. The material that track and rollers are made from are
chosen so that they have different triboelectric properties. The triboelctric series rates how likely certain materials
are to give up electrons when rubber together. The bottom roller in a Van de Graaff will be made from a material
that will strip electrons from the inside of the track. Electrons from the out side of the track are then repeled
negative roller to the bottom brush, which is connected to ground. The now positively charged track move up
to the top of the generator where it comes into contact with the top roller. Electrons are transfer from this top
roller to the track making the roller positively charged.
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Figure 1: Diagram depicting the location of charge
on the Van der Graaf Generator.
At this point the top of the track also comes into contact with a brush which is connected to the metal
sphere. This allows for the surface of the sphere to be positively charged. Despite the strong electric field outside
the sphere, on the inside the charge is nearly zero. This means more charge can continue to accumulate until it
reaches the break down voltage of air.
More inforation can be found in The Video Encyclopedia of Physics Demonstrations. The Van de Graaff
generator video is on disc 17, chapter 41, demo 17-07. A copy of the video can be retrieved on request.
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References
[1] Bill Lee. The TriboElectric Series, AlphaLab Inc. Technical report, 2009.
[2] John Zavisa. How Van de Graaff Generators Work. HowStuffWorks.com, http://science.howstuffworks.com/
transport/engines-equipment/vdg.htm, April 2000. pg 1-8.
[3] Steven Dufresne.
How to build/make a Van
http://rimstar.org/equip/build make van de graaff.htm.
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