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

[Jack Uretsky <jlu@hep.anl.gov>]

IHi all-
	I think I understand the source of John's discomfort with the 
Lorentz force equation F=q(E+vxB).  It makes E and B look like two very 
different kinds of animals which, to my mind they are.
	I learned from a text that started the discussion of magnetic 
fields, after a chapter on steady currents, with Ampere's experiment 
showing the existence of a force between current elements. The B-field is 
contrasted with the E-field by calling it a "solenoidal" field.  The 
discussion of B-fields also occasions the introduction of the 
cross-product of vectors.
	This approach is consistent with that of Feynman in I-12 of the 
Lectures.  Note that he did not try to burden the students with the force 
between tso moving charges, calling it "much too complicated."
						Regards,
							Jack


On Wed, 29 Nov 2006, John Mallinckrodt wrote:

> Despite John Denker's willingness to consider agreeing with the
> spirit of my comments about Maxwell's equations being, in principle,
> adequate to answer Bob LaMontagne's original question because they
> have relativity "built into them," I think he's giving me too much
> credit.  Although it is true that the Lorentz transformations are in
> some sense mandated by Maxwell's equations, that was not what I had
> in mind.  I have learned something from reading the responses that
> remark generated and stand corrected.
>
> But with that mea culpa out of the way, I would like to register my
> disagreement with Jack's and Bob's suggestion that we stay away from
> this example in an introductory course.  In my opinion, the moment we
> introduce the Lorentz force, we become almost obliged to point out
> the seeming absurdity of a force that depends on velocity--not
> *relative* velocity, just velocity--and that vanishes in the frame of
> the moving particle.  It is not asking too much of students to ask
> them to appreciate that an immediate implication of that fact is that
> the electromagnetic field is frame-dependent.
>
> By all means, leave the details to a later course.  But why not plant
> the seeds of understanding early.  We all know how much more
> effective it is to return to a topic than to see it for the first time.
>
> John Mallinckrodt
>
> Professor of Physics, Cal Poly Pomona
> <http://www.csupomona.edu/~ajm>
>
> and
>
> Lead Guitarist, Out-Laws of Physics
> <http://outlawsofphysics.com>
>
>
>
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