Every school which teaches about static electricity should have a Van De Graff generator. It forms the basis for several demonstrations using additional equipment as mentioned below.
The moving belt carries a positive charge to the top comb (usually a stretched wire) and then transferred to the large dome. A huge charge can be built on this dome by allowing the unit to run for a while, this charge will remain until it is discharged, either through air or by conduction to an earthed object ie: a metal sphere. A charge of up to 200,000 Volts can be achieved on a good day depending upon the type of generator used. Some models are powered by a small electric motor, some have hand wheels but all produce charge using the same principle.
To ensure effective operation, the unit should be free of grime so a polish every now and again is recommended. Some older machines have belts that have been worn or have become too slack, replace belts with newer rubber-looking versions – they work much better than older fabric types. Check the top comb is just touching the belt- it will slow the belt down if it is pressing too much, likewise the bottom comb need to just be touching. On damp days especially, the belt and dome may have to be dried and warmed using a hair drier, this ensures easier transference of charge. If possible, the unit should be stored somewhere warm and dry prior to use. Most generators sold by lab suppliers now come with a box of ‘experiments’ and no doubt some of you who have used this equipment will have accumulated items such as bubble blowers, hair strands and polystyrene balls which are put out with the VDG.
Popular demonstrations include...
Stand on a plastic tray (to insulate you from the earth) and place both hands on the top of the dome, after a short time, your hair should become charged and stand on end. Because every hair is being charged with the same potential, they repel each other. To safely let go without getting a shock, take your hands off the dome quickly then step off the tray. The charge you have built in your body will safely discharge through your feet.
Flying Pie Dishes
Place a stack of several aluminium pie dished upside down on the dome and leave to charge, they will fly off one at a time with the same principle above – the dishes repel each other after charging with the same potential.
Hold a small low voltage fluorescent tube near the dome and with a bit of trial and error, it should light up. Depending on the size of the charge, it may require the blinds to be closed to see the effect fully. Small 12v neon lamps also work well when the metal parts come into close contact with the dome.
Tape a metal pie dish to the top of the dome and fill with a handful of polystyrene packing chips. After a short charge they will fly out of the dish like popcorn!
Be aware that discharging near electrostatic sensitive devices may result in damage; USB memory sticks, mobile phones, computers, some watches, MP3 players, televisions, VCRs etc may contain or are connected to sensitive chips.
There are many more Van De Graff demos and attachments being used in schools, feel free to email us your school’s specialities and we’ll include the best on this page.