Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84; site talcott.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!genrad!wjh12!talcott!gjk 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