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Q & A: magnetic iron filings

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Most recent answer: 10/22/2007
Q:
Why do iron filings line up in a magnetic field? would copper filing be equally satisfactory? why is it desirable to tap the glass plate? why is it desirable to use very few filings? what is the method for mapping a magnetic field by the use of a small compass?
- lyka (age 19)
cebu
A:
Iron is one of the ferromagnetic elements. Each iron filing consists of numerous magnetic domains. It turns out these domains can lower their energy by lining up with their fields along the skinny direction of the filing. Then the filing as a whole is a bar magnet, kind of like a compass needle, which can lower its energy by lining up with an external field. The effect should be enhanced by a tendency of the filings to line up end-to-end, influenced by each others' fields. If the filings are sitting on a glass plate friction can be too strong to let them rotate. Tapping reduces the friction temporarily. If you use too many filings they more or less form a uniformly magnetic sheet, losing any tendency to line up end-to-end, and also making something too thick to visualize clearly.

A compass needle is just a good example of a magnet that will line up in a field. If you had a lot of little compasses, they would be like your iron filings. If you have just one compass, you can just move it around to map out the field.
Copper is very weakly paramagnetic, which means there is a tiny effect of the same sign as in iron. I strongly doubt that you will ever notice even the slightest tendency of copper filing to line up in a magnetic field.

(published on 10/22/2007)

Follow-Up #1: some types of magnetism

Q:
what are feromagnetic, paramagnetic and diamagnetic substances
- Tsega (age 18)
Ethiopia
A:
Ferromagnetic substances have interactions between the electrons which tend to make their magnetic moments line up with their neighbors. When those interactions are strong enough and the material is cool enough, the moments line up tigether to form magnetized domains.

In paramagnetic materials the magnetic moments don't interact strongly enough to line up in any regular pattern. In the simplest case, each electron's moment acts completely independently of its neighbors. The moments do line up a little in a magnetic field and hence are pulled into a magnetic field but the effect is not nearly as strong as for ferromagnets.

In diamagnetic substances nearly all the electrons are paired up so that their magnetic moments cancel. However, for peculiar quantum mechanical reasons there's a little moment opposed to the field and pointing the same way for each electron, Therefore diamagnets are repelled by magnetic fields. Usually this effect is very small but for most superconductors the diamagnetism is enormous and the effect is large.

Mike W.
Lee H

(published on 10/22/2007)

Follow-up on this answer.