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Non-Coulomb forces (was: Batteries)



Here's one way to look at it.

The conducting rod, left to itself, will slow down due to its interaction
with the magnetic field (to make a long story short, Lenz's law demands the
appearance of a force that tends to reduce the rod's velocity). The
"missing" bulk kinetic energy of the rod reappears as the energy of moving
electrons.

If an external agent maintains the motion of the conducting rod (say, at
constant velocity), then this agent performs (force x distance) work that,
since it does not increase the rod's bulk KE, also transforms into the
energy of moving electrons.

Another way to look at it: A time-changing magnetic field produces a
non-conservative electric field (Faraday's law) that can do work on
electric charges. Otherwise the magnetic fields in an AC generator would
not be of much use.

In the first paragraph above, it is not clear to me whether the Lenz's law
force would be properly characterized as a purely "magnetic" force
(although it is certainly "electromagnetic"). Perhaps the semanticists on
the list could address this, as I have reached the point where my thinking
is no more clear than my writing. :-)

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

At 14:42 2002/02/13, Joel Rauber wrote:

page 559 paragraph towards the bottom and I quote: they are discussing a
traditional motional emf problem of a conducting rod sliding on rails
immersed in a uniform magnetic field.

"The magnetic force evB is a non-Coulomb force (references to page 220 and
350). The work done by this force in moving an electron from one end of the
bar to the other is FNC L = evBL. . . ."

FNC = non-coulomb force.
evB is magnitude of (e v_vec cross B_vec)

IMO this is a blunder of a statement. magnetic forces of the ev X B variety
do no mechanical work! Do others read this the same way I do? Have more
recent printings changed the wording? I have first edition first printing.