Re: Van de Graaf demo

[Brian Whatcott <inet@INTELLISYS.NET>]

Hmmm...I asked for an observation of the rotational direction with the
geomagnetic field reversed, to test the notion that the ball is obeying
the motor rule. The pair of Helmholtz coils and power supply are not
difficult to arrange. The question is naturally moot if both directions
of rotation are observed.
 Faraday observed something of the kind, with a wire danglng in a
mercury bowl, I recall.

Brian W


At 12:22 PM 2/8/02, you wrote:
> Gee, Brian, try reversing the Earth's magnetic field and see what happens.
>
> Jon
>
> > ----------
> > From:         Brian Whatcott
> > Reply To:     phys-l@lists.nau.edu: Forum for Physics Educators
> > Sent:         Friday, February 8, 2002 1:13 PM
> > To:   PHYS-L@lists.nau.edu
> > Subject:      Re: Van de Graaf demo
> >
> > At 10:44 AM 2/8/02, you wrote:
> > >Perhaps you have seen the following demo. A ping pong ball sits in
> > >the rim of a metal saucer. A metal plate is above it, touching
> > >neither the ball nor saucer. You connect the saucer to one terminal
> > >and the plate to the other terminal of the VdG and observe the ball
> > >to start rolling around the rim of the saucer. Why?
> > >
> > >My guess is that some small amount of charge is picked up by contact
> > >at the bottom of the ball. Some asymmetry causes it to begin rolling
> > >one way or the other. When the charged part of the ball rolls around
> > >180 degrees so it is closest to the top, the charge drains off via an
> > >imperceptible arc from the nearby plate.
> > >
> > >Is this the correct explanation? Does this demo teach some
> > >interesting detail that makes it worthwhile to present as part of a
> > >sequence of VdG demos? Carl
> > >--
> > >Carl E. Mungan
> >
> >
> > Is the direction of rotation constant? Does it reverse if the Earth's
> >   magnetic field is reversed?
> >
> >
> >
> > Brian Whatcott
> >    Altus OK                      Eureka!
> >
> >

Brian Whatcott
  Altus OK                      Eureka!