Re: [Phys-l] magnetic forces & potential energy

["Krishna Chowdary" <chowdark@evergreen.edu>]

Hi Paul and all

Modeling is simplified somewhat if you have permanent (magnetically hard)
magnets, which from your description below, I think you might.  The
situation is further simplified if the magnets are small enough or the
distances are large enough that you can treat the magnets as point dipoles.
It's even further simplified if the fields that you are talking about are
small compared to the reversal fields of the permanent magnets (they most
likely are).

You can look up the magnetic field of a dipole, which will be a function of
the dipole moment and the field vector (along the axis of the dipole, the
field drops off as 1/r^3, for example).  The torque, potential energy, and
force of/on a dipole in an external magnetic field can also be looked up.

Some difficulties might arise from determining the effective magnetic moment
of your permanent magnets and the strengths of your magnetic fields.  I have
some suggestions for those, if this is in line with what you and your
student need.  If you're working with magnetically soft materials (so for
example using a permanent magnet or electromagnet to attract an iron nail),
then as John D. points out this is a much more difficult problem.

-Krishna

On Wed, May 14, 2008 at 8:54 AM, Paul Lulai <plulai@stanthony.k12.mn.us>
wrote:

> Hello.
>
> I have a student that is trying to design an electric motor.  I know
> there are a million different (& easy) electric motor designs available.
> He is trying something a bit unique.
>
> The student has magnets that state they have a 'strength' of 200 pounds.
> I believe that this is the force of attraction btn the magnet and a
> ferromagnetic substance when the two are in contact.  Is there a high
> school method (algebra or calc 1 method) that would make it possible to
> calculate:
> 1- the force of attraction btn two magnets (one electromagnet & one
> permanent magnet) when they are separated by some distance?
> 2- the potential energy of a two magnet system in which the two magnets
> are separated?
>
> Your help is appreciated.  I haven't used much of my e&m since I took
> the course.  It appears I've forgotten quite a bit.  A bit shameful.
>
> Thanks for your input.
>
> Paul Lulai . . . To wonder is to begin to understand
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