28. Magnetic Fields and Forces
Summary of Magnetism Problems
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What we said was, if you make one of these solenoids, you can generate a magnetic field. You can in fact like pick up little things, okay? But those things that you're picking up need to have some magnetic properties about them. And the one that I'd like to talk about is called Ferromagnetism, or Ferromagnetic materials. And there's two other magnetic terms that you need to be aware of. One is called paramagnetic and one is called diamagnetic. Paramagnetic means the internal field gets stronger with an applied magnetic field. Diamagnetic means it opposes the applied field. And the reason is in paramagnetic materials, it is the spin of the electron. In diamagnetic materials, it's the orbit of the electron. And so there's two classes of material. Those with even numbers of atoms– of protons and electrons, and those are diamagnetic. And the ones with odd numbers are typically paramagnetic. But ferromagnetic is something differently entirely, okay? It's something entirely different. What is Ferromagnetic? Well, what do you see here in this word? You see ferrous, okay? Iron. If you look at the periodic table, iron is represented by that. Fe, ferrous material. It's ferromagnetic and so iron is ferromagnetic. And what that means is, if I take a big iron bar and I think about the little magnetic dipoles that are in there, what do I see? I see the following: there are domains where all the magnetic dipoles line up. And as I go from one domain to the next, they are scrambled. So these are the magnetic domains. And these are which way those magnetic dipoles are pointing. Magnetic dipoles are inherent in atoms and molecules, and particularly iron. So, all these things are scrambled. This domain is all pointing in one direction but the next one is random. But if I put the whole thing in a B field, then I can get them to in fact line up. So those domains will stay the same. Try to draw it the same. But with a B field pointing to the right, all of those magnetic dipoles are going to align to the right. Okay, these are microscopic domains. You're talking about micron-sized chunks of iron. But if I put it in a B-field, they start to line up. And so, you can align all those dipoles in your magnetic material and Ferromagnetism means when I take off the B field, they stay there. They will stay in that orientation. Now you can scramble it up by applying a B field in a different direction, or you can heat up the bar and that will scramble them up. So there's various ways to de-program your magnet, but it will keep it in that direction in the absence of any scrambling. Okay, so let's see if we can use this to our advantage.
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