What causes the attraction: The constraint forces that are causing the
electrons to maintain a particular orientation of their orbits relative to
the bulk material of the magnet.
Consider a charged ball that is tied to a string and forced to roll about a
fixed central point on a smooth table with no electrical or magnetic
properties. Now apply a magnetic filed along the plane of rotation of the
ball and have the direction of rotation of the ball to be such that the net
force is downward toward the table. If the table is on springy legs, it will
be forced downward. This is because of the contact force of the ball with
the table, not directly because of the interaction of the ball and the
magnetic field. The spiraling motion of the ball reduces its rotational KE
allowing for vertical KE of the ball and table.
Going back to your question, the table is equivalent to one of the magnets
and the ball to the orbiting electrons within the magnet.
Bob at PC
(this is a slapped together response between classes that I probably will
regret later)
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Rauber, Joel
Sent: Wednesday, March 22, 2006 5:24 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Magnetic force and work
This doesn't seem to be getting through, so I'm trying again. Apologies
if its a repeat
______________
I'm curious as to how some of the experienced veterans on the list would
respond. I just finished deriving the fact that the magnetic force law
on a charge implies that magnetic forces do no work.
After class a thinking student came up and started fiddling with the two
bar magnets I had and asked me (I'm changing the question a bit)
If I take the two magnets up into outer space (no other influences) and
align them co-linearly so the South pole of one is near the north pole
of the other and let go, they'll accelerate toward each other. Does
this contradict the idea that magnetic forces do no work.
Since magnetic forces can do no work, what force IS doing the work when
a bar magnet causes a paper clip to jump off a table and stick to the
magnet?
Asked by: Steven Leduc
Answer:
The original assumption that a magnetic field can do no work is
incorrect. A magnetic field has an energy density that is equal to the
magnetic induction (B) squared divided by twice the permeability (mu sub
zero). If you were to sum (integrate) this energy of the magnet over all
of its field before it picked up the paper clip and compared it to the
same sum after you picked up the paper clip, you would discover that
there was a loss of field energy. The paper clip has in effect 'shorted
out some lines of magnetic flux'.
How much energy was lost? If you took hold of the paper clip and pulled
it out to such a distance that the magnetic pull was insignificant, the
work you did in this process would exactly equal the amount of energy
lost when the clip was on the face of the magnet. When you picked up the
clip with the magnet the clip was accelerated toward the magnet
acquiring kinetic energy. This kinetic energy will equal, ignoring air
drag, the loss of magnetic energy in the field. This kinetic energy will
be dissipated in the form of heat on impact of the clip with the magnet.
For further understanding of the energy in a magnetic field, you may
want to study magnetic fields in solenoids. See the Reference below.
Answered by: Robert Gardner, M.S., Retired Physicist
________________________
I mention this as a precaution against arguing from "authority"; also as
a "caveat emptor" regarding getting answers off of the web
Joel Rauber
Department of Physics - SDSU
Joel.Rauber@sdstate.edu
605-688-4293
________________________
Joel Rauber
Department of Physics - SDSU