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From: bph@buengc.BU.EDU (Blair P. Houghton)
Newsgroups: sci.physics,sci.electronics
Subject: Re: Homopolar generators/conservation of angular momentum -- how?
Message-ID: <5662@buengc.BU.EDU>
Date: 13 Apr 90 19:03:12 GMT
References: <1990Apr12.043832.6000@ns.network.com> <911@gold.GVG.TEK.COM>
Reply-To: bph@buengc.bu.edu (Blair P. Houghton)
Followup-To: sci.physics
Organization: Boston Univ. Col. of Eng.
Lines: 28

In article <911@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Gregory Ebert) writes:
>In article <1990Apr12.043832.6000@ns.network.com> logajan@ns.network.com (John Logajan) writes:
>>Okay, so a homopolar generator stops in 1/3rd rotation.  Where did the
>>angular momentum go?  Surely it didn't exit via the axle.  Surely it
>>didn't exit (mechanically) via the (as I understand it) diametrically
>>opposing brushes.

The last one.  Because they're diametrically opposed it means that
they produce a pure torque (a "couple"), rather than a torque with
an offset linear force.

>>Signed -- confused in Minnesota.
>
>     Think of it as a flywheel driving a conventional generator. As electrical
>     energy is consumed (by a projectile ?) the beast coasts to a stop.

He was asking about momentum, not energy.

I think you'll find that if you don't have your homopolar generator
bolted securely to something REALLY massive, then when you stop the
rotor the housing will bounce and roll out of the room.

If you do bolt it down, the angular momentum goes back where it
came from: the planet.

				--Blair
				  "If Earth stopped spinning tomorrow,
				   such things would be outlawed."