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From: gjk@talcott.UUCP (Greg J Kuperberg)
Newsgroups: net.physics,net.puzzle
Subject: A puzzle in physics
Message-ID: <145@talcott.UUCP>
Date: Thu, 29-Nov-84 19:14:22 EST
Article-I.D.: talcott.145
Posted: Thu Nov 29 19:14:22 1984
Date-Received: Fri, 30-Nov-84 08:29:07 EST
Distribution: net
Organization: Harvard
Lines: 18
Xref: genrad net.physics:2141 net.puzzle:514

In response to Craig Werner's suggestion, I attempt to change the subject
by presenting a puzzle:

Many of you probably know that momentum in the electromagnetic field is
proportional to E cross B.  Thus we get net momentum for a plane wave of
light, because E and B are perpendicular for light.

So suppose you have a cylindrical coil that produces a net magnetic field
inside of it.  Let the field point upward.  Now put two curved capacitor
plates around the coil.  These capacitor plates will produce a net electric
field that points in some horizontal direction.  Thus E cross B inside the
coil is non-zero, and there is net momentum in the field.  Where did this
momentum come from?
---
			Greg Kuperberg
		     harvard!talcott!gjk

"Eureka!" -Archimedes