🚀 go-pugleaf

Hello to everyone.  I'm new on the list and this is my first question.
With your patience, some background. I teach Honors and AP Physics "C" at
Deerfield (IL) High School.  I have a 12 year old genius (Chris Hirata) in
my AP course who gets perfect scores on the released AP multiple choice
tests and on the free-response questions.  Chris loves physics and math.
Like Feynman, Chris works more than just the assigned problems and we have
yet to find a problem in University Physics (Sears, et.al.) that Chris has
not been able to solve.  Every school day after my AP physics class, Chris
takes some topic that we discussed that day and working at the board, takes
to next level or several level higher.  Only one example for now:
Yesterday, starting with an electric charge (+q) moving at speed (v) and
picking the locus of points at distance (r) and angle (A) in front of the
moving charge (radius of circle = r sinA), Chris derives the equation for
the B-field from the integral form of Ampere's - Maxwell's law (Chris likes
the CURL form better) arriving at the expected  B = (u/4pi)(qv sinA)/r^2.
Since there is no current (Ic = 0), Chris did this by finding the time
derivative of the electric flux through the area bounded by the circle in
front of the moving charge.  Chris has been doing this sort of thing all
year and starting in November, 1994, I've been video-taping these
presentations and now have over 12 hours recorded.

FYI:  On Saturday, April 1,1995, Chris will present a paper at the Chicago
Section of the AAPT on the period of harmonic motion where the magnitude of
the restoring force is given by F = kx^n and he will evaluate the general
equation for n= -2,-1,0,1,2,3   His interest in this was sparked by an AP
free-response question based on F = -kx^3
On April 2, 1995, Chris will be featured in the Tempo Section of the Sunday
Chicago Tribune Newspaper.

Although AP Physics "C" is the highest level physics course taken by Chris,
I deserve no credit for what Chris knows and can do other than being
willing to let him teach me for about one class period per school day.

Now for my question:  Help me please !!

While my AP class was studying the magnetic force on an electric charge
moving through a magnetic field, I made the statement that since the force
was perpendicular to the velocity, the force could do no work on the moving
charge.  It could change the direction of the velocity but not the speed
and kinetic energy.  Chris, who seems to immediately see universal
consequences of my statement, raised his hand and asked, "If magnetic
forces can do no work on moving charges, how can your big horseshoe magnet
pick up a steel plate like you showed us?"  Of course, I had never thought
of such a question and could not immediately answer him and I did my usual
"I'll have to think about that."

My answer currently is (you may laugh but if you do, please send a better,
more satisfying [?] explanation):  As the magnet is brought near the steel
plate, there will be a changing magnetic flux through any chosen area
inside the plate creating an electric field which will do work on the
moving charges.  Within the atom, the electron spins and orbital planes (?)
will be re-oriented and, macroscopically, the magnetic domains will align
in the external B field.  This will result in attraction between the magnet
and the steel plate and an increase in magnetic potential energy.  When the
magnetic attraction is greater than the weight of the plate, the plate will
accelerate up towards the magnet, decreasing magnetic potential energy and
increasing gravitational potential energy.

This satisfies me for now and I have yet to get back to Chris.  I wanted to
get your considered opinion first.  How would you answer Chris's question
in the Feynman introductory lecture style.  Please consider your audience
(me !!).  Idealizations and approximations will be appreciated.

I'm sending this out on both PHYS-LIST and PHYSHARE.  Sorry for the repeat
for those who subscribe to both.

David Thiessen
Deerfield High School                      FUN + PHYSICS = PHUN
Deerfield, IL 60015
Home phone (815) 337-2883                  PHYSICS IS PHUN !!
School phone (708) 405-8488
Voice Mail (708) 374-3811 then *610
School Fax (708) 945-0970