Date: Thu Feb 15 20:15:33 2007

Author: Michael A Thomason

Subject: Re: Van de Graaffs



Overdriving a Van de Graaff doesn't work. If the comb is the optimum
distance from the belt, the belt gets thrown into the comb, or wire mesh,
literally bending back the teeth of the comb. Less charge is transferred to
or from the comb, cancelling any benefit from the higher speed.
Additionally, the belt is losing contact with the roller, and less charge is
being transferred to or from the roller.

If the teeth of your Winsco or Cenco combs aren't getting bent back, then
you have already set the comb farther away than the optimum distance for a
belt that stays in contact with the roller.

Michael Thomason
Director of Physics Learning Laboratories
University of Colorado Boulder Department of Physics

-----Original Message-----
From: [] On
Behalf Of Bernard Cleyet
Sent: Thursday, February 15, 2007 5:14 PM
Subject: Re: [tap-l] Van de Graaffs

I think this is analogous to incandescent lamp use. To obtain max.
illumination over voltage them, for long life, under.

Sam's, mine, et al's experience is discharge length is a + function of
the speed.

bc, thinks the way to go is w/ a speed control, and max. for short periods.

Michael A Thomason wrote:

>I disagree. It has nothing to do with equilibrium charge. Sam just likes
>things that spin fast. I like a fast cadence too when I'm cycling, but the
>commercial Van de Graaffs spin far faster than is needed to maintain the
>highest equilibrium charge.
>Even when we do the leakiest Van de Graaff demonstrations with pointy
>objects arcing to or from the dome, my 3 large Science First Van de Graaffs
>spin more than fast enough to provide enough current.
>A major reason we are constantly replacing belts and adjusting the combs on
>Winscos and Cencos is the belt moves so fast it loses contact with the
>roller due to its momentum and gets thrown into the comb. I've never once
>needed to replace a belt or adjust a comb on any of my 3 Science First
>It's true faster speed can provide higher current, but then you would need
>tighter belt and chunkier mechanical components to maintain tension on the
>belt, which the commercial generators don't have.
>Michael Thomason
>Director of Physics Learning Laboratories
>University of Colorado Boulder Department of Physics
> 303-492-7117
cut (much)