Re: [Phys-L] feeler-dealer, third law, et cetera

[John Denker <jsd@av8n.com>]

On 12/12/2013 11:37 AM, Bruce Sherwood wrote:
> "I'm not sure I'm comfortable with the electron/proton example as a
> violation of the third law." I of course agree totally with the fact that
> the right way (and the only feasible way) to calculate magnetic forces is
> to first calculate the field made by other (moving) charges and then
> calculate the cross-product force. But the fact remains that you end up
> with two forces that are not F and -F, which calls for some discussion.

Yes, some discussion ... but it's quick.

> How do you feel about the Coulomb force law? Surely there too one should
> first calculate the field made by other charges and then calculate the
> force (for one thing, retardation effects require this approach).

I wouldn't be so sure of that.  Long experience and/or a 
proper understanding of the Maxwell equations tell us that 
Coulomb's law works just fine for electrostatics.

>  Yet in
> the intro course one rarely requires this of students, and it's probably
> common to say "See? Here's an example of Newton's 3rd law."

Yes, common ... and perfectly reasonable.

>  But then what
> do you say when the two magnetic forces don't match?

I say there is momentum in the fields.

In the electrostatic situation, the amount of momentum was zero,
but in certain other situations it is nonzero.

End of story.  It's not a big deal.

=======================================================

Calling it a 
       "lack of reciprocity for magnetic forces"
is silly several times over.  First of all, if we are talking
about the third law, please let's call it the third law.  If we
are talking about conservation of momentum, please let's call 
it conservation of momentum.  Calling it "reciprocity" is kooky
jargon.

More importantly, it is plain wrong to suggest that the magnetic
example illustrates a violation of conservation of momentum.
Just the opposite, really.  If properly understood, it demonstrates 
the immense power and scope of the conservation law.

See also next message.