Isn't this the sort of question Einstein was asking in 1904?
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-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu]On Behalf Of Bob
LaMontagne
Sent: Tuesday, November 28, 2006 10:53 AM
To: 'Forum for Physics Educators'
Subject: [Phys-l] Force on a charged particle from a magnetic field
I hope I can state this question clearly enough so the discussion doesn't
wander away from what I am interested in.
It is the classic question of whether or not a magnetic field can produce a
force on a stationary charge. I pose the question in the following limited
way. I have a UNIFORM magnetic field that is pointing out of the page (as we
say in textbooks or exams). A proton at a particular instant has a velocity
within the plane of the page pointing to the left. It experiences a force at
that moment toward the top of the page.
Many texts then ask the question, based on relativity, that if we consider
the electron to be at rest and that the uniform magnetic field is produced
by a huge set of pole pieces or a Helmholtz coil that is at rest within a
frame of reference that is approaching the proton from the left side of the
page with a velocity to the right, is there still a force on the proton
toward the top of the page?
At first glance the answer appears to be yes, because of relativity.
However, how does one answer this question on the basis of Maxwell's
equations or a basic E&M approach in an introductory physics course. In that
approach, the proton should only experience a force from an electric field
or when it is moving through a magnetic field.
One usual answer is to appeal to Maxwell's equations and say that a moving
magnet will have a time varying flux at the location of a stationary proton
and therefore an electric field is induced that can move the proton. But
here the field is uniform, so the flux is not changing.
Perhaps the question is ill posed, because the proton is really following a
circular path in the uniform magnetic field and we can't apply special
relativity naively.
As you can see, I'm quite confused about how textbooks can offer this
question in such an offhand manner. Maybe I'm just missing the obvious.