Science In Real Life

Magnetism is like electricity’s slightly retarded younger brother. The latter is straightforward and gets right down to the business of making your body’s nervous system look for employment elsewhere, while the former has a habit of going in circles. I mean that last part literally – magnetism is produced by electric charges going around in circles. There’s some other stuff to it that doesn’t involve making electrons dizzy, but it all comes down to changing electricity.

It is, unfortunately, impossible to discuss electricity and magnetism without introducing the concept of a field. Physics is the science of understanding everything we can observe about the Universe. At times during the pursuit of this course it becomes 1) necessary or 2) exceedingly convenient to work with ideas that, strictly speaking, have either an ill-defined physical nature or none at all. Electric fields and magnetic fields are two such objects. Physicists are still debating whether it is the electric and magnetic fields that are more ‘real’, or if that distinction belongs to related entities called the potentials. (This argument hinges on a very complex notion called ‘gauge invariance’, and it is well beyond the scope of these essays.)

For our purposes, the following suffices: Imagine I have a proton, a positively charged particle. I know it will affect other charged particles without touching them because I can see like charges repel and opposite charges attract. So we imagine electric ‘field lines’ radiating outwards in all directions from the proton and any other positive charge like light rays out of a star. For a negative charge, we imagine the field radiating inward from all directions like media attention on a movie star. The lines per se are not really there, but any Joe Q. Electron that happens to wander by acts just like they are.

Magnetic fields are more mysterious (you can tell by the appropriate amounts of alliteration). Our day-to-day experience of magnets is of the refrigerator variety, where somehow one piece of metal sticks to another. What’s really happening there? Let’s go back to our proton. It’s just sitting there radiating electric field lines like a hobo radiates body odor. But now I grab the proton and move it in a straight line. As soon as it starts moving, a magnetic field arises. Only instead of coming out of the proton, it goes around the proton in an unbroken loop. What happened? The electric field changed: it’s now coming from a source in a different place, so a kink travels down the field lines as they update.

Remember the analogy about the slightly retarded younger brother? When electricity gives his sibling a shove, he spins around. If electricity keeps pushing, i.e. charges keep changing their positions, the magnetic field keeps whirling around. So if I have a wire carrying a current (a power line), the magnetic field lines circle it all along its length. You can see an image of it here.

What happens if I bend the wire around so that it forms a loop? Now I have charges going in a circle, and a donut of magnetic field lines. Only this donut is dense and concentrated in the middle but light and sparse on the outside. This has to do with the way vectors add. Inside the loop all those magnetic field line circles are going the same way. Outside they’re trying to go in two opposing directions at once, and so they go nowhere. The result looks like this:

(Image credit)

If we were to zoom out from that enough, it would look like there’s a single straight arrow, impaling the loop through its hole, that represents the magnetic field. The direction of that arrow is part of what physicists call the magnetic moment. This is what your ordinary everyday magnet is made of. Zoom in again: a magnet, like anything else, is made of atoms. The atoms have electrons whizzing around the outside. The actual nature of what the electron is doing inside an atom is more complicated and arcane than whatever Willy Wonka is doing inside his chocolate factory, but it IS moving, and since the nucleus isn’t letting it escape, it effectively has to move in circles. Presto, a tiny magnet. If the molecular properties of a substance are just right, all those tiny little magnets line up in the same direction until you get one big magnet. Iron does this very well, which is why these materials are called ferromagnets.

So what happens when I stick one to my fridge? The door has a layer of metal in it, usually aluminum. Aluminum is not a ferromagnet, but its atoms have little magnetic arrows too. When a ferromagnet gets close, they all align in the same direction as the ferromagnet’s arrows. The actual magnetic attraction, therefore, is like a twisted version of electric charges attracting, only this time it’s because of the charges moving in the same way. Just like … (shudder) line dancing, all the electrons doing the same dance congregate together. (Sorry about that one.) This, by the way, is how credit cards work. The black strips have little magnets aligned in patterns that computers can read as numbers. Up-down-up-up-down translates very well to binary: 10110. This is also why a strong enough magnet can wipe the card clean. Get it close enough to Mr. Magnet over there and he’ll force all the lil’ guys in your card to be the same direction. Congratulations, your credit card number is now 0000-0000-0000-0000.

Now, here’s where it really gets interesting. Electricity can push his little brother magnetism, and magnetism can push back to make electricity. Spin a magnet around a coil of wire and a current will flow (this is called an electric generator and it’s what produces all of our electricity). It’s road trip time, and the two brothers are both in the back seat of the car. If one of them pokes the other, the other pokes right back, ad nauseum. It becomes a self-sustaining cycle. The same thing can happen with electric and magnetic fields. An electric fluctuation gives rise to a magnetic fluctuation, and so on. You no longer need any charges to keep it going, it flies off on its own. It flies off, in fact, at the speed of light, because that’s exactly what it is. All light is electromagnetic radiation, piggybacking off itself into eternity.

In fact, this division I’ve been maintaining all along between electricity and magnetism is an illusion. I’m not going to get into how, but Einstein’s Special Relativity requires that physics look the same no matter how the observer is moving relative to the thing observed, which you can use to turn electricity into magnetism and vice versa. The solution is that they are in fact one and the same. There is only one field, and it can have either electric behavior or magnetic behavior or both. It is at this point that we must abandon the analogy of two brothers in favor of a single person who probably needs psychiatric counseling. But when he wants to, he can really shine!