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Re: Van de Graaf demo



Perhaps the corona discharging is responsible for it.

"Carl E. Mungan" 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, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/