Van de Graaff Details

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Van de Graaff Details

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Question:

I gather I am limited to 25 lines. I am seeking information on details of the operation of a Van De Graaf generator, I posted a Usenet on sci.physics which has more detail. How does a belt type van de graaff develop the charge anisotrpy it does? The top winds up + relative to the base. My machine has combs top and bottom which do not touch the belt. Does the fact that it stands vertically play a role? (earth's E field) or is this a nit? If the material constants of the belt all that matter then if I could get a different material could I reverse the polarity (in principle It would be nice to have these answers in my back pocket when I run through some electricity demos for a demanding audience of 5th graders. Thank you for your time, in advance.

Replies:

There is a good description of van de Graaf generators in "The Encyclopedia of Physics" edited by Lerner and Trigg. The charging is determined by a discharge circuit at the "bottom", which charges up the belt. Then, when the belt is inside the metal sphere at the top, it loses its charge to the sphere (through those combs). The polarity is determined by the discharge circuit at the "bottom". Gravity does not play any role.

Arthur Smith

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