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Re: [Phys-l] Charged particles moving along parallel paths



Anthropomorphism in the pursuit of vivid explanation is no vice, certainly,
but I would rather say, Maxwell's electrodynamic collation can provide a
rather satisfactory description of current.

Brian W


On 12/5/2011 10:59 AM, Robert Cohen wrote:
Would it be incorrect, misleading or otherwise inappropriate to say that
the speed of light is finite and therefore the two charges don't "see"
each other (i.e., the information from one charge cannot "reach" the
other)?


Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of
LaMontagne, Bob
Sent: Saturday, December 03, 2011 5:10 PM
To: Forum for Physics Educators
Subject: [Phys-l] Charged particles moving along parallel paths

I am going to ask a question where I am more concerned with the pedagogy
than the physics itself. I realize that the scenario proposed has been
argued to death in many differeny forums.

Suppose a bright college freshman notices that if two positively charge
masses travel through space along parallel paths, that their mutual
Coulomb repulsion will cause them to accelerate apart (assume that their
velocities are perpendicular to their relative separation vector). The
student also notices that the Biot-Savart Law give a mutual attraction
between the two charges that reduces the rate at which they accelerate
from each other.

The student also notices that in their own center of mass frame the
masses have no relative motion so there is only a mutual acceleration
from the Coulomb force. Therefore the mutual acceleration in their own
frame is larger than in the frame where they are seen to be moving.

How do you respond to the student? Is there a purely classical response
that is satisfactory in a general physics course that will not cover
relativity until the following semester?

Is it necessary to wave off the question by saying that this will become
clearer when relativity is covered? The factor (1-v^2/c^2) naturally
comes up when comparing the forces classically. Is this to be noted as a
stepping stone to relativity and leave it as a paradox in classical
physics?

(If my physics is off here - please feel free to correct it.)

Bob at PC
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