Date: Fri, 29 Jun 2001 19:29:37 -0700 (PDT)

Author: William Beaty

Subject: Re: VDG


On Fri, 29 Jun 2001, Jim Krider wrote:

> A professor asked me what the electric field strength of a Van de Graaff
> generator was.
> We have the Winsco N100-V electrostatic generator.
> I have heard that 100,000 volts could build up on the typical classroom VDG.
> Does this seem reasonable?

Does he mean "potential relative to ground?" Field strength is volts per
meter, and if it's higher than 3megavolts per meter (or so), you get
outbreaks of corona which draw a current and pull the voltage back down.
The field strength will be lower if you're up in the mountains.

The total potential relative to ground depends on if there are any tiny
burrs or sharp dust grains on the sphere. If the sphere was perfect, then
the potential depends both on the sphere diameter, and on the distance
between the sphere and the nearest sharp metal points. Those Winsco
generators don't get as high a voltage as they could, since the corners of
the lower case will spit corona and suck down the voltage.

Yeah, the winsco machines are around 100,000V, maybe 200,000 at best, when
the sphere is clean and nobody is anywhere around it. If a person gets
near the sphere or puts their hand on it, the voltage falls drastically.

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