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Re: mag. force on wire?

Hi Maurice - good points!
There is no question that the qVxB force on the carriers can only be
transmitted to the lattice by some electromagnetic interaction between the
carriers and the lattice.
The question is whether this interaction is properly modeled as an
electrostatic force caused by the "stationary" separated charges (caused
by the Hall effect), or is it better modeled by collisions of carriers "en
route". In the real case, it might be argued that this is a distinction
without a difference, but . . .

Consider the following gedanken situation:
Suppose I could manually (nanotechnology!) manipulate the electrons in a
piece of wire (no current, no B field) so as to produce a glued down layer
of negative charge on one side, and an equal positive layer glued onto the
opposite surface. Would this wire of itself accelerate because of the
electric force of the separated charges upon the internal lattice.
Certainly not! (This may be what you meant by "Don't carry this model too
far"; I really would not like to carry it at all!) Let's hear other
views.

Note that there is no qVxB force on the (stationary) configuration of
separated charges; they can't "carry the lattice along with them" as they
respond to this force - cuz they don't experience this force.

General aside comment:
This is the general problem of the taxonomy of modeling the LHS of F=mA.
The current carrying wire simply responds to a magnetic field (ie., other
currents). It couldn't care less how we model it. Our problem is to best
describe this phenomenon in terms of the conceptual/mathematical models in
our repetoire. The answer is not always unique (???)

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Maurice Barnhill <mvb@UDEL.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, November 03, 1999 1:36 PM
Subject: Re: mag. force on wire?


There is already a force on the electrons pushing them in the direction
that the wire must accelerate: the external magnetic force itself. The
problem is what moves the ions. Since the electrons will move away from
the center of the wire, their net electrostatic force on the ions will
bring the ions along. The electrostatic force of the ions on the
electrons prevents them from accelerating as much as they would have in
the absence of the ions, which is exactly the effect you want.

Don't carry this model too far. It produces a lot of confusion when
applied to the behavior of electrons and ions in a metal sitting on a
table in a gravitational field.

Bob Sciamanda wrote:

Thanks for the reference, Stanley.
If I read correctly this text is appealing to the charge separation
cause
by the Hall effect, and asserting that the electrostatic field of this
charge separation acts on the lattice ions to move the wire!

I have a real problem - this force is one of a PAIR of forces INTERNAL
TO
THE WIRE. the net force on the wire due to the interaction of the
Hall-separated charges with the lattice ions is zero.

This is my quick reaction. Will re-think and await comments of
others.
I have never read diligently through this book, but have in general
applauded its approach.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Stan Greenspoon <sgreensp@CAPCOLLEGE.BC.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, November 03, 1999 12:51 PM
Subject: Re: mag. force on wire?

For a good account of the charge separation explanation of magnetic
force on
a current-carrying wire, see the relatively recent book "Electric
and
Magnetic Interactions" by Chabay and Sherwood. As a magnetic field
cannot
do any work on a moving charge, the explanation for the work done by
magnetic force (e.g. electric motors) is through the work done by
the
induced electric field due to charge separation on the lattice ions.


Stan

Stanley Greenspoon Tel.: (604) 986-1911 Ext. 2439
Physics Department Fax: (604) 983-7520
Capilano College
2055 Purcell Way
North Vancouver, B.C.
Canada
V7J 3H5


Leon wondered:
Just how is it that the force on moving charges
"contained" in a wire is transferred to the wire
itself? I've problems with the two explanations that
seem pretty standard. That is, by collisions (with
what, exactly, and what happens at the boundary?); and
by electrostatic separation of charges (what happens
if both positive and negative charges are equally
responsible for charge flow?)


I am not familiar with the "electrostatic separation" explanation;
but
the
"collisions with the ion lattice" idea is a useful model.

F=QVxB will produce the same direction of force for a given
current
direction, however that current is divided among positive vs
negative
carriers (both Q and V change sign) That's why the force on a
conductor
can be evaluated in terms only of the current (dF = I dLxB, where
dL
is a
vector in the direction of the current) - with no regard to the
sign(s) of
the carriers.

At the air/conductor boundary, any carriers forced off the surface
will be
called back and will call the conductor body toward them (by N3)
because
of their "image charge" in the conductor.

Hope this may be helpful - don't hesitate to prod further.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716