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Re: [Phys-l] Force on a charged particle from a magnetic field



Hi Bob-
I don't think you've been directly answered, so here's my take.
You point out that a certain field is purely magnetic in a certain frame. Call it frame M. In a frame moveing with respect to frame M, you point out, the field is a mixture of magnetic and electric. You ask, how do I explain this fact to an introductory class?
My answer is that you are describing a relativistic effect, as you have noted. I would not want to get into relativistic effects in an introductory class. So, unless there is a pressing need to do so that you have not told us about, I would not bring up this particular effect.
Note, by the way, that there is no frame where the field is purely electric. It is characterized by the fact that there is a particular frame where it is purely magnetic. This is analogous to a timelike vector; there is one frame where it has no spacelike component and no frame where it has no timelike comonent.
Regards,
Jack

On Tue, 28 Nov 2006, Bob LaMontagne wrote:



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Tuesday, November 28, 2006 2:01 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Force on a charged particle from a magnetic field


But there is time varying flux in the frame of the proton. The only
way to avoid that involves using an infinitely large magnet.


That's why I carefully defined the magnetic field to be uniform, so there is
no change in flux with time (classically). Specifically, how do you get the
time varying flux that ultimately produces an electric field to move the
proton? (Again, without an appeal to relativity.)


Maxwell's equations have relativity built into them so they are, in
principle, fully capable of answering the question.


I thought that the Lorentz transformations came about through Einstein's
attempt to make Maxwell's equations apply to any inertial frame of
reference.

Anyway, I agree with most of the responder's claims that relativity is
required - and, as Skip said, that was what Einstein was trying to do a
hundred years ago. I am just uneasy with the way various textbook author's
present this example (or a variation of it) before relativity is introduced,
and then expect students to be comfortable with the answer. And yes, it does
seem to provide a great segway into relativity.

Thanks to all who took the time to respond.

Bob at PC

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