Learn more physics!
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)
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
(republished on 07/13/06)
Follow-Up #1: some types of magnetism
what are feromagnetic, paramagnetic and diamagnetic substances
- Tsega (age 18)
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.
(published on 03/13/07)
Follow-up on this answer.