Van de Graaff Generator Pendulum
Requirements:
- Parallel plate capacitor
- Metal sphere hung from a nonconducting string
- Van de Graaff generator
Description:
One of the plates of the capacitor is connected to the dome of the generator, while the other one is grounded. The pendulum bob is centered between the two plates and allowed to move freely. When the generator is turned on, the plate connected to the dome will become negatively charged. This will cause the metal sphere to become polarized such that a net positive charge accumulates on the surface facing the charged plate. The sphere will thus become attracted to the charged plate; upon contact, negative charge will flow from the plate to the sphere, giving the sphere a net negative charge. The sphere will thus feel a repulsive force due to the like charge of the plate and swing towards the grounded plate. Upon touching the grounded plate, it will discharge and again become neutral. The process of charge and discharge will repeat as the sphere swings back and forth between the charged and grounded plates.
Note: use a lighter metal sphere (aluminum, not iron), so that it can acquire larger accelerations and make a louder noise upon hitting the plates.
Another cool demo. Instead of grounding the second plate, short it to the dome as well. The force between the plates will be repulsive, and, assuming proper alignment, the electric forces on the sphere will cancel out. If the sphere is charged by shorting it to one of the plates, now the sphere and the plates will be negatively charged. Any oscillation perpendicular to the plates introduced by moving the sphere will be dampened rather quickly.
Safety precautions:
See the attached document on safety precautions when using a van de Graaff generator.