Courses: Astr4, P1, P4, P14, P16, P19
We have a somewhat unusual Van de Graaff generator, in that it can be switched to push negative charge toward the dome or away from the dome. For reasons that are very unclear to me, one gets larger sparks when the dome is set to positive. I believe it works better for hair standing on end if one sets it to dome negative.
To generate sparks attach a metal sphere to the ground terminal of the VDG, and hold it near the bottom of the dome -- sparks drawn from further up are shorter in length.
If one applies a pin to the dome, holding it in place with some sticky wax, one will no longer be able to draw sparks, or very small sparks at best. The pin is creating such a significant discharge that the dome is not able to reach high enough potential for significant sparks. This is how a lightning rod prevents lightning strikes.
5B10.10: To charge a person up and have their hair stand on end they must stand on the glass-legged stool. Lacking the insulation from the glass legs one discharges as fast as the generator can charge you up! First ground the generator and have it off. Then have the person put a hand on the dome, and then turn the VDG on. If you turn the generator off and they remove their hand, they will stay charged for a significant amount of time. If they step off the stool they will discharge quickly.
There are several other demos that can be done with the VDG:
- Place a bundle of string, tied together in the center, on the dome. It will slowly stand up and eventually fly off the dome.
- 5B10.25: Place a metal ring and metal pie plate on the dome and fill with styrofoam peanuts. They will fly all over and make quite a mess... But why do they fly off? They're insulators, so do they polarize? For that matter, why do they appear to first stick together, and only later fly off?
- 5B10.15: Tape strings to the dome and energize to illustrate field lines.
- 5A40.24: Blow a stream of soap bubbles at the VDG. First they are attracted to it, but as soon as one touches it and pops they are all repelled.
