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Q & A: magnetic substances

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Q:
What is the difference between a magnet and a magnetic substance? give examples of each
- Jessica (age 15)
SA
A:
When people say 'magnet' they usually mean something that has a big magnetic field. That requires that the little magnetic domains in the material mostly line up with each other. Any permanent magnet (Alnico, samarium cobalt...) can provide an example.
I guess 'magnetic material' usually means some material with magnetic domains but not necessarily lined up. That would include anything you could make a permanent magnet out of (see above) as well as materials that lose their alignment quickly. Plain iron is a good example.

Mike W.

(published on 02/03/09)

Follow-Up #1: iron and other magnets

Q:
Is iron (and its alloys) the only materials that are attracted to/by magnets? I keep reading about the domains in the iron that act like tiny magnets--are the domains the same thing as the molecules of iron? Do other metals not have these domains? And how are the domains different from polar molecules (like water molecules?)
- Gloria (age 43)
Springfield MO, USA
A:

We've addressed parts of this question before: http://van.physics.illinois.edu/qa/listing.php?id=435http://van.physics.illinois.edu/qa/listing.php?id=8686. Lots of other materials are magnetic, and many iron compounds aren't. Some simple elemental metals- e.g. cobalt, nickel, and neodynium-do have magnetic domains at room temperature, but most don't.

Magnetic domains are generally not the same as molecules. In fact, most of the crystals we're talking about here, for example pure iron, have no molecules in them. They are just a bunch of atoms stacked up in a regular pattern.

The polar molecules you're thinking of have more positive electrical charge on one side and more negative electrical charge on the other. That makes them electrical dipoles. Unless the electrical dipole moments on many neighboring atoms or molecules line up with each other, there are no domains. If they do line up, they form ferroelectric domains, not ferromagnetic domains.

The magnetic domains we're talking about typically have no electrical dipole moment. They have most of their electrons spinning the same way. That gives them a magnetic dipole moment. 

Mike W.


(published on 09/15/13)

Follow-up on this answer.