Electricity and Magnetism

Van de Graaff generator

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Van de Graaff generators have a reputation in schools for being “temperamental” in operation. The usual reasons for their failure to perform (assuming the mechanical drive is fine) will be impairment of the belt’s insulation properties and/or drainage of charge to Earth across the surfaces of the pillar support.

Good housekeeping and repairs

Charged surfaces inevitably attract air-borne grime to build up a conduction path to Earth. So it is important to ensure all surfaces are clean and free from moisture before start-up. A convenient and excellent solvent is Swan Lighter fluid – a volatile, light hydrocarbon usable on all plastics likely to be encountered. Dampen a wad of soft tissue and apply lightly to both surfaces of the moving belt, and then wipe the support column thoroughly. (It is also possible to wash the belt in washing up liquid and then dry it thoroughly before use.)

If the cleaned and re-assembled generator does not immediately charge, play hot air from a hair drier on to the moving belt, pillar and dome – maybe for as long as 2 or 3 minutes.

It is not necessary to have the collecting sphere in place to check the onset of charging; use a neon indicator held near the top comb. An alternative is to use the “head of hair” accessory. Once charging starts leave the machine running, playing warm air on the belt for a while; then reassemble with a spark gap of 5-10 mm. Once sparks pass fairly rapidly, increase the gap.

It is also important to keep the outer surface of the collecting sphere clean and unscratched for the production of “fat”, luminous sparks. Otherwise you will get nothing more than corona discharges (characterized by the frying-noise, lack of luminosity and production of detectable amounts of nitrogen oxides) at the sharp points presented by dust particles or surface imperfections.

Keeping a generator in a warm cupboard overnight in preparation for next day’s teaching may be to no avail unless the cleaning has been done first.

In partnership with STEM Learning, the IOP have created this video for teachers to show how to get the best out of a Van de Graaff generator. Michael de Podesta explains how the generator works and gives some tips on getting consistently good results when using the apparatus. The video concludes with a simple but effective demonstration of charge.

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