Van de Graff Demos

The Van de Graff is stored in a large cardboard box. There you will also find a passive metal sphere on a plastic stand, a wire connecting the passive sphere to the base of the van de Graff, a plastic milk crate, a coiled bare wire on top the v.d.g., and a mirror in the packing crate for the v.d.g.

Here are some ideas for demos:

  1. Remove the coiled wire from the top of the v.d.g. Turn it on and let it charge up (20 seconds or so). Bring the passive sphere close until it sparks. Note the length of the spark. The breakdown field in dry air is about 30 kV/cm, so you can get a rough idea of the potential on the v.d.g.
  2. Repeat (1.) with the coil of wire sitting on top. You should only be able to get a short spark. You can talk about discharge from smooth surfaces, sharp point, lightning rods, etc.
  3. Select a volunteer with long fine hair. Swedes tend to work best. With the v.d.g. off and discharged, have the volunteer stand on the milk crate and put his/her hand on the globe of the v.d.g. Turn it on. Watch the hair stand on end. Talk about electrostatic repulsion of like charges. Use mirror to show the victi...volunteer the effect. Turn off the v.d.g. and discharge it with the passive sphere. NOTE: the volunteer should neither step off of the milk crate nor remove his/her hand from the v.d.g. before it is discharged. He/she will probably feel a small shock when the v.d.g. is discharged, but the shock will be much worse if he/she makes either of these mistakes.
  4. You will probably not be able to turn off the v.d.g. without shocking yourself to some degree. It is harmless and it amuses the class. Shocks to the professor's head or eye glasses are even better and will result in higher evaluation scores at the end of the quarter.
When the 1223/1723 people are done with the v.d.g, please pack it back in its crate. The effectiveness of the v.d.g. is largely a function of the quality of the surface of the globe, so please treat it with care and try not to scratch, dent or soil the globe. This v.d.g. belongs to John Swain (at least until we can figure out a way to reimburse him) so please be gentle with it.