Date: Tue, 9 Nov 1999 14:41:50 -0500

Author: (Wolfgang Rueckner)

Subject: Re: momentum stored in EM field


>The professor for our junior-level E&M seminar asked about a demo to
>demonstrate that momentum is stored in electromagnetic fields. The model
>for the demo is an example problem in Griffiths' "Introduction to
>Electrodyamics" based on an article by F.L. Boos in AJP vol 52, p 756
>The set-up is as follows:
>You have a very long solenoid of radius R, N turns per length, with current
>I. There is a cylindrical shell of radius a inside the solenoid with charge
>+Q and a shell of radius b outside the solenoid with charge -Q. So, there
>is a magnetic field B inside the solenoid and an electric field E between
>the charged cylindrical shells.
>In the region a < r < R, where both fields are present, there is stored
>momentum proportional to E X B. If the current in the solenoid is gradually
>reduced, changing B, the cylindrical shells begin to rotate in order to
>conserve angular momentum. Weird!
>So, my question is: has anyone tried this? It's an interesting theoretical
>problem, but is the effect strong enough to be seen easily?
>Mary Ann
>Mary Ann Hickman, Lecturer
>Dept. of Physics and Astronomy
>Swarthmore College
>500 College Ave.
>Swarthmore, PA 19081
>610-328-8384 (office) 610-328-7895 (fax)

If I calculate the effect, it's 10**-11. Not possible to see. Wolfgang