Sunday, March 1, 2009

Magnetism: What is attracted to a magnet?

My son's school is having its science fair this week (okay, two weeks ago now; I'm late). My son is in kindergarten. His teacher made participation in the fair a required activity (have I mentioned how much I like her?).

So, DS wanted to play with magnets for his project.

His question: What sticks to a magnet.

His methodology: Get a bunch of different kinds of things and test whether they stick to a magnet by trying to pick them up with a magnet. He tested plastic objects, metal objects, coins, paper, and some other things. If an object responded at all to the pull of the magnet, he considered it to have stuck.

His results: Metal sticks to a magnet. However, not all metals do; coins, for example, do not stick to magnets. No non-metal objects stuck to the magnet.

He asked how do magnets work. His mom answered, "magic." Seriously. She thinks magnets are magical. /sigh Two steps forward, 1.5 steps back. Okay, maybe she doesn't seriously think that, but come on...

So. How do magnets work?

Actually, it's a little difficult, so my DW has a good excuse for explaining it away with magic.

Before I go any further, I need to confess that I slept through most of my magnetohydrodynamics class in graduate school; I only earned a B. The fundamentals of magnetism is a difficult subject for me so most of what I tell you is just scratching the surface and may not be entirely accurate.

First, a magnetic field is essentially an area of influence caused by an electric current. An electric current can travel through wires, such as your computer, or microscopically when individual electrons move in their orbit around an atom's nucleus. Now, it's not just the flow of electrons that creates a magnetic field. Electrons "spin" in a particular direction. In normal materials, the electron spin is pretty much randomly distributed, but in a magnet, the spin is aligned such that the flowing electrons all have the same spin.

This flow of spin-aligned electrons causes magnet(ic field)s to be dipolar. That is, a magnet has a "north" and a "south" pole. (dipolar, not bipolar; magnets are not manic-depressive.). By convention, electrons flow from the north pole to the south pole. You can see this by putting a bar magnet under a piece of paper with a bunch of iron fileings on the top; they'll align along the field lines.

When spin-aligned electrons encounter other spin-aligned electrons going in the opposite direction, they're repelled from each other. Thus when you put the north (or south) poles of two magnets together, they'll push each other apart. If you put the north and south poles together, they'll attract each other.

So, why do non-metal objects not display magnetic properties? Because their electrons' spins are randomly distributed. Most metals also have more randomly distributed electron spins. Ferrous iron and a few other metals have highly ordered electrons and are magnetic (or even are magnets). ALL materials will respond to a strong enough magnetic field because every electon with spin (every electron) is basically a little magnet. At some (very large) magnetic field strength, a piece of paper will respond as though it were magnetic because the average spin of its electrons will be slightly greater in one spin direction than another.

Relativity requires that both electricity and magnetism be two expressions of the same thing; if either one is neglected, the other is inconsistent with relativity.

So, it's magic. ;)

Drexel university


deborah said...

I had no idea that paper would "stick" to a magnet if the pull of the magnet was strong enough. So if the pull of the magnet is big or strong enough would we stick? And I don't mean if we had on the right kind of metal belt. I mean just us in the buff? If yes how huge or strong would that magnet have to be?

I am Moses. said...

Yes. This is just conjecture on my part, but I'm pretty sure the magnetic field and associated electric current would kill you before you were actually moved around by it.

deborah said...

O.K. then....I guess I don't want to experience that magnetic pull.