Field Arrangement of Disk Magnets
We are studying magnets and recently my students asked
where the poles where on a circle magnet and if a circle magnet even has
poles. I have not been able to find the answer to this question anywhere.
A magnet must have poles. A permanent magnet is made by lining up the
domains predominately in one direction. A domain is a group of molecules
linked together with their magnetization pointing mainly in the same
direction. When the material is not magnetized, the domains point in random
directions and cancel the fields each other. The domains are lined up by
imposing a strong external magnetic field on the material. Then when the
field is reduced to zero, the domains tend to remain lined up, although not
as much as when the external field was applied. This is called the remnant
field. Note that the remnant field will point in whatever direction the
imposed external field was pointing.
So the disk magnet will have poles, but they can be pointing in any
direction, depending on how the magnet was magnetized.
This may be clearer if you think of a sphere, which can be magnetized in any
direction. You can think of it as a bar magnet inside the sphere. A good
example is the earth's magnetic field, where the south pole is near the
earth's geographic north pole (so the north pole of a compass points at it).
The earth's field is due to currents flowing deep inside the earth. It's
interesting to note that the direction of the earth's field could be in any
direction and, in fact, has changed over the eons. It is, of course,
closely related to the rotation of the earth, which is why the magnetic
poles are fairly close to the geographic poles.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Sounds like a great opportunity for an experiment. Sprinkle a small
quantity of iron filings on some stiff white paper and then hold magnets of
various shapes under the paper. In some orientations you should see the
filings organized into arcs. These arcs run from pole to pole
I believe most circular magnets have the poles at the "ends". That is,
think of the circular magnet as a cylinder. The poles will be at opposite
ends of the cylinder.
Although there is more than one possibility, a common disk magnet uses the
opposing faces. One face is north and one face is south. A way to check
this is with a compass. Orient the disk vertically, rather than lying flat.
Use a compass to verify whether the opposite faces have opposite poles.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012