This demonstration uses the Van de Graaff generator to show how like charges repel.

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Other Demos of Interest:
 Van de Graaff Peanut Fountain Van de Graaff Sparks Ion Wind from Van de Graaff

Teachable Topics:

• Van de Graaff Generator
• Coulomb Force
• Electrostatics

Theory:

The Van de Graaff generator is based on the principle of static friction. It is designed so that the plastic rollers inside deposit electrons onto the rotating rubber belt through friction and carry this excess charge up to the hollow metal sphere. The electrons will then spread themselves around the surface of the sphere since like charges repel. To view this concept directly, place a stack of aluminum pie plates upside down onto the Van de Graaff’s metal sphere before turning it on. The charge deposited onto the sphere from the belt will then also flow into the pie plates as the sphere looks for a way to discharge itself. Each of the pie plates will become negatively charged with electrons and therefore try to repel each other. This repulsion is strong enough to lift the top pie plate from the stack and appear to float off the sphere! Since all of the plates will be trying to repel each other, they will all float above the machine and fly off of the sphere.

Figure 1: How charge travels from the generator to conducting materials touching it

Apparatus:

• Van de Graaff Generator and grounding rod
• 5 or more aluminum pie plates (the smaller, 10cm diameter ones work best)

Procedure:

• Place the Van de Graaff generator in a place where there is room above it for the stack pie plates to float. Plug it in and plug in the grounding rod.
• Place the stack of pie plates upside down onto the top of the generator’s sphere.
• For safety reasons, use the grounding rod to turn on the generator and stand a fair distance away from it while the plates become charged and float off.
• When all of the pie plates have left the surface of the sphere, turn off the Van de Graaff generator with the grounding rod and discharge the sphere by touching it with the end of the grounding rod.

SAFTEY WARNINGS:

• Follow all safety precuations when using the Van de Graaff Generator. Use the grounding rod to turn the generator on and off and always discharge the sphere after use with the grounding rod as well.