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Re: [Phys-l] work by magnetic fields



Here are more workable URLs for my previous posts --->

https://carnot.physics.buffalo.edu/archives/2006/03_2006/msg00201.html

https://carnot.physics.buffalo.edu/archives/2006/03_2006/msg00205.html

From: Philip Keller
Sent: Thursday, April 28, 2011 1:13 PM
To: 'Forum for Physics Educators'
Subject: [Phys-l] work by magnetic fields
I know I have asked this in the past, but I still don't understand it and my
AP students have asked again so...

This is about what some books call a linear motor: a long, u-shaped
conducting rail with a conducting bar resting on the rails, free to slide
without friction, completing the circuit. There is a battery in the circuit
and a magnetic field into the plane of the setup. The battery causes a
current to flow and the magnetic field exerts a force on the bar which
causes the bar to accelerate (at first -- the induced back emf then causes
the current to drop and the bar attains a terminal velocity, but that is not
the issue at hand).

The question is: while the bar is accelerating, what force is doing the
work that increases the bar's kinetic energy?

Possible answer #1: The magnetic force, F=ILB, that is exerted by the field
on the current-carrying bar. That force is in the same direction as the
bar's displacement. There is no rule that says magnetic fields can never do
work.

Possible answer #2: The magnetic force is not the one that does the work.
That force acts on the individual charge carriers in the bar and it is
always perpendicular to their motion so it can never do work. But the
individual charge carriers are not free to follow the circular orbits that
the magnetic field would otherwise cause. They are held by electrostatic
forces exerted by the kernels of the lattice atoms. It is that
electrostatic force that ultimately does the work.

Any thoughts?


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Bob Sciamanda
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