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Name: Heather
Status: educator
Grade: 6-8
Location: IL
Country: N/A
Date: 1/9/2006


Question:
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.


Replies:
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


Heather,

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.

Greg Bradburn


Heather,

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
Physics Instructor
Illinois Central College



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