I have enjoyed this discussion very much. Especially the spacetime
approach to magnetic and electric forces has been insightful.
Now I also see why it is possible to say
-- magnetic forces never do work: it is the induced electric field
which does the work)
-- a time-varying magnetic field (which induces the electric field
in the first place) can do work
It seems that both interpretations are physically equally valid.
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There is one reference I'd like to point out: Eugene Mosca (1974) published a paper called "Magnetic forces doing work" in the AJP.
He analyses a conducting rod moving (along conducting rails) with
constant velocity through a uniform magnetic field. This is a classic
example in high school textbooks. Mosca shows that magnetic field
does no work even though some (most?) textbooks quite explicitly
say so. Instead, the force the conductor exerts on electrons does
the work.
Regards,
Antti
Antti Savinainen, Ph.D., B.Ed.
Senior Lecturer in Physics and Mathematics
Kuopio Lyseo High School
Finland
E-mail: <antti.savinainen@kuopio.fi>
Website: <http://kotisivu.mtv3.fi/physics/>
REFERENCE:
Mosca, E. (1974). Magnetic forces doing work? AJP, vol. 42,
pp. 295-297.
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