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Re: Van de Graaf demo



This what you remember?

http://www.bbc.co.uk/science/scienceshack/backcat/experiments/mafaradaymotor.shtml

I'm looking for the copper sphere motor I've seen somewhere in the very dim
past.

bc

Brian Whatcott wrote:

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!