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Re: Bar magnets



Without thinking too much I attempted an experiment
which failed, as described below. Comments and
suggestions will be appreciated.

I wanted to measure the attractive force between the
poles of two bar magnets (each 13 cm long and 1 cm
in diameter. Keeping these magnets by hand I started
to feel the attraction when the distance between the
coaxially positioned bars was 1 cm or so.)

Interesting question. I have observed similar behavior of magnets
when brought close together. I suspect the problem has something to
do with the instability of the system when the force is attractive.
When the distance is large enough, the force is too small to make
much effect. I suspect is has some effect but it would be small due
to the nature of the force--if the distance is large compared to the
length of the dipole, the force is inverse cube. But it looks like
you were staying in the near field region where the force is probably
closer to inverse square. In any event, when you get close enough
that you can see the stretching of the spring, as the spring
stretches, and the magnets get closer together, with the force
between them then increasing, and since the force is basically
inverse square, it will increase faster than the spring force, which
is linear. Thus the system in unstable and as soon as it starts
moving the net force continues to get bigger and the two clamp
together.

Let me suggest an experiment that we used to use to measure the
magnitude of the pole strength. Take two cylindrical bar magnets such
as you described. Anchor one at the base of an inclined plane. Then,
matching the same poles, so the force is repulsive, set the other
magnet on the incline above the fixed one and carefully set it so it
is at rest under the opposing influences of gravity and the magnetic
repulsion between the two bars. Then measure the distance between the
two magnets at rest, and knowing the mass of the upper magnet and the
angle of the incline, the component of gravity parallel to the
incline must match the net magnetic force (for accuracy, include the
effects of both lower poles on each of the upper poles). Using
cylindrical bars minimizes the effect of friction. Use a steep enough
incline so that the magnets come to rest only about 1 cm apart

It would be a good idea to do a fairly careful mapping of the fields
around each magnet individually, since the poles are not always at
the very ends of the bars, and we have noticed an occasional case
where there is a "third" pole in the middle of the bar (meaning, I
think, that the bar has been magnetized as a linear quadrupole). I
would not recommend using on with this anomalous magnetization. So if
you know where the poles on each magnet are, then you can make
careful calculations of the components of the forces between each
pair of poles and come up with an estimated pole strength. It's been
a while since we did this experiment and I don't recall if we endued
up with enough equations to enable us to get the individual pole
strengths or just the average between the two bars. If not, I think
you can generate enough equations if you use three bar magnets and
measure the forces between each of the three possible pairs.
Interchanging the two magnets on the incline may help, too.

Hope this helps.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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