Built on Facts

Rhett over at Dot Physics is not quite happy about the way magnets are being taught to his young niece:

The other question came from my niece – who is an extremely bright girl (not sure exactly what grade – maybe 5th). She was at home and didn’t have her science textbook, so she asked me the following:

“What happens to the electrons in a metal when it becomes a magnet”

I always worry about magnet questions because magnets are not that simple to understand at a fundamental level. Sure, there are some things you can do with magnets – especially if you want to do some experiments. However, asking questions like this or like “why can iron be a magnet, but not aluminum” to a 5th grader is like asking me how gravity works.

How gravity works might not sound hard either, but at the fundamental general relativistic level it’s exceptionally difficult and at the quantum level nobody has any idea. The problem is this: when discussing things like gravity at the introductory level, it’s possible to give a correct and useful classical description. Air resistance aside – and that too is easily understandable if mathematically difficult – finding trajectories and orbits is easy. It’s one of the first things we teach in into physics classes. The gory relativistic details just aren’t important at any energy scale the students will ever have to deal with. This is true all through the physics of daily life. You don’t need quantum electrodynamics to explain a light bulb, you don’t need the full canonical ensemble treatment to explain hot and cold, so and and so forth.

But magnets? Almost uniquely among everyday phenomena, anything even resembling a remotely accurate explanation is a blisteringly complicated quantum mechanical affair. Even first year textbooks for physics majors tend to gloss over the actual answer to “why are some things magnets?” questions with a few paragraphs of non-mathematical handwaving.

I don’t like this, but nature is what it is. My advice to teachers of 5th graders science is to mostly skip the “why” in the case of magnets and focus on what magnetic materials do. Some things are ferromagnetic, they do this. Some things are diamangetic, they do that. I hate hate hate giving that advice, but I just don’t think it’s possible to simplify magnets to that level without creating additional confusion down the road. Once the students are advanced enough – maybe high school – then start with some non-mathematical simplifications of the quantum origin of magnetism.

Which, come to think of it, might be a good thing to talk about around here…

Comments

#1 Abby Normal
January 11, 2010

Which, come to think of it, might be a good thing to talk about around here…

Oh, yes please. I’d love to learn more about this subject.
#2 Bill
January 11, 2010

But I thought gravity was just God pushing us down. http://www.idrewthis.org/d/20050516.html
#3 D. C. Sessions
January 11, 2010

Once the students are advanced enough – maybe high school – then start with some non-mathematical simplifications of the quantum origin of magnetism.

High school isn’t too early to teach the special relativistic explanation of magnetism, math included.
#4 6EQUJ5
January 11, 2010

A fun thing to do requires a length of copper tubing wide enough for a penny to fit through, a neodymium magnet no wider than a penny, and a penny.

Hold the tubing upright and rop the penny through. It falls as fast as one would expect.

Drop the magnet through and … it falls slowly, even though the copper is nonmagnetic. (It is however an electrical conductor.)
#5 BlackGriffen
January 11, 2010

It’s just a phase transition, right? Just say that the difference between ferromagnets and other materials is like the difference between water and ice – get something cold enough and the little “atomic bag magnets line up.” It’s just that “cold enough” differs between materials.

Granted, it’s probably more akin to the difference between a glass or other amorphous but solid phase and a crystalline phase, but that is, again, probably more than they need.

This also leaves out fun things like spontaneous symmetry breaking, zones, etc, but it gets the basics.

Or am I incorrect in my understanding that just about any homogeneous material, if made cold enough, can become ferromagnetic? Perhaps it’s just the ones with an unpaired electron spin?
#6 Juice
January 11, 2010

People need to give kids some credit. Explain it to the 5th grader the way you would explain it to an undergrad and see what sticks. Answer all questions and eventually, they’ll get it. It just depends on how much time you have.
#7 The Golden Phoenix
January 11, 2010

Usually, I try to explain it by saying that the electrons, which behave as tiny magnets, line up in a magnet, while the electrons in other materials don’t, and when they don’t line up, they cancel each other out, kinda like positive and negative charges cancel each other out. Kids seem to understand that.
#8 Gerry Rising
January 11, 2010

Having no background for this subject, I looked on the web and found . This seems to have some good information, well illustrated, but I am not knowledgeable enough to judge.
#9 Gerry Rising
January 11, 2010

The website I found is:

http://www.coolmagnetman.com/maghow.htm
#10 Britt Torrance
January 11, 2010

I was thinking about this yesterday! In some ways, magnetic materials are the closest we get to the quantum world on a daily basis. It definitely makes exposition a little more difficult than usual. But I believe that the sooner we start discussing funny effects like ‘exchange interactions,’ and other strange quantum effects, the more comfortable people will become with modern physics.
#11 Hamish Johnston
January 12, 2010

The beauty of your niece’s question is that physicists have yet to come up with a precise explanation of why metals such as iron are magnets — and are using fantastic devices like quantum simulators to gain a better understanding. See http://physicsworld.com/blog/2009/09/_by_hamish_johnston_if.html .

Perhaps the best answer is “Nobody knows for certain, but if you become a scientist, you could be the first to answer that question.”
#12 Jeff
January 12, 2010

This is a great blog… a very interesting read!
Thx
#13 Anonymous Coward
January 13, 2010

I like the advice for 5th graders about “mostly skip the “why” in the case of magnets and focus on what magnetic materials do.”

If you wanted to push it a bit further – say high school level – you could talk about how all the electrons are like little magnets, and in some substances they like to cooperate and all point in the same direction as their neighbors (iron) and in others they prefer to point in opposite directions than their neighbors (copper).

If you wanted to get really fancy, you could talk about neighbor-neighbor alignment (like above) vs domains (magnetized iron vs nonmagnetized iron).

When you get to undergrad physics, the door opens a bit:

Once you have intro E&M and know the value of a Bohr magneton you can estimate the maximum magnetic field you can get from a permanent magnet (assuming n unpaired electrons per atom).

Once you have intro E&M and Stat. Mech., you can show that whatever is giving rise to ferromagnetism is NOT the magnetic dipole-dipole interaction of the electrons (the transition temperature would be much lower than what we observe in real life).

Once you have intro E&M and Stat. Mech. and Quantum, you can talk about how it’s the exchange force that causes ferromagnetism/antiferromagnetism.

Once you have an intro solid-state class and Quantum, you can try your hand at understanding why some solids are paramagnetic vs. ferromagnetic.

Once you get into some sort of graduate-level solid state work maybe you can attempt to calculate, for a given lattice, whether the exchange force will give rise to ferromagnetism or antiferromagnetism? But good luck with that.

I have no idea what D.C. Sessions is talking about with regards to how to explain permanent magnets with special relativity.
#14 Still Dumb
January 14, 2010

Regarding special relativity: You can explain magnetic attraction between two current-carrying wires using length contraction between moving charge carriers. I don’t believe it helps with permanent magnets, however.
#15 Tom
January 23, 2010

A simple question to ask, but hard to answer, in an introductory or intermediate class in electromagnetism: since magnetic fields do no work, how is it possible for a magnet to lift a paper clip against the force of gravity? Tricksy, this is…