Re: Induction and moving rod on a U-shaped conductor

[Bob Sciamanda <sciamanda@worldnet.att.net>]

George Panagiotakopoulos wrote:
> When a conducting rod is moving in a uniform magnetic field (rod, magnetic
> field, velocity mutually perpendicular) charge accumulation establish at the
> ends of the rod. If the rod slides along a stationary U-shaped conductor, an
> induced current is established around the circuit.
> My question is about the motion of the elkectrons of the U-shaped conductor
> when these electrons reachthe moving rod: Do the electrons from the U-shaped
> cpnductor go inside the rod?
> An excellent analysis about the motion of electrons has been made by
> Young-Freedman (University Physics- ninth edition pg. 953) But they don' t
> mention what happens if the electrons of U-shaped conductor reach the moving
> rod.
>
> I have another question about the electrons' motion in the U-shaped
> conductor and rod. Suppose that the rod has big length, bigger than the
> opening of the U-shaped conductor so its ends exceed  the U-shaped
> conductor. What happens with the electrons? Do they accumulate at the point
> of the rod, where the rod meets tha U-shaped conductor? If so, why does this
> happen? Don't forget that the whole rod is inside a uniform magnetic
> field....
>
> Thank you
>
> George Panagiotakopoulos -
> Physics teacher
> physiart@ath.forthnet.gr

Answering within a classical electron carrier model, the qvxB forces
will produce a charge separation in the moving rod; the electric field
of these separated charges (and other accumulated charges) will
eventually produce a dynamic equilibrium in which the E field follows
the conducting path.  Charges can accumulate wherever there is a space
variability in conductivity (as happens at resistor terminations); this
includes conductor surfaces (eg., surface charges will appear on the U
surface to provide "centripetal" force) and your "commutator"
interface.  I see no problem with electrons moving across the
"commutator" interface;  remember, the carriers are not "pushing each
other",  they are driven (in the stationary wire) by the electrostatic
field of the separated charge distribution.  The effect of the external
B field on the stationary wire is to produce a Hall effect.

-Bob
-- 

Bob Sciamanda				sciamanda@edinboro.edu
Dept of Physics				sciamanda@worldnet.att.net
Edinboro Univ of PA			http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA				(814)838-7185