Re: mag. force on wire?

[Maurice Barnhill <mvb@UDEL.EDU>]

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