Build Your Own Baloney Detector

A tool-kit for avoiding being fooled

Sunday, March 7, 2010

Magnetic Fields

Magnets are probably one of the most amazing things we ever get to play with. Watch a kid play with some standard magnets sometime. You’ll remember how wonderful magnets really are. How do they attract and repel each other? How do they disrupt your TV screen? (Assuming you have one of those rapidly disappearing CRTs.) How does a compass know which way north is all the time? Even Einstein as a child was delighted and perplexed by this force. (Of course, he went on to create Special and General Relativity. I went on… to play with magnets well into adulthood. Look, we can’t all be geniuses, OK?)

Unfortunately, the very wonder and mystery of magnets also makes them a popular tool for charlatans. It also causes no end of the confuse among well-meaning people who don’t have good intuition for magnetic fields. (And why would anyone? They’re not particularly part of our natural everyday experience — we’ll see why in a minute — so our ancestors never needed to know it. Evolution, you work too slowly!) So I figured I’d spend a little time talking about what magnetic fields do and don’t do.

First of all, what is a magnetic field?

No, I refuse to answer that even remotely fully. We could spend an entire course on that topic and just barely get into it. So let’s do the abbreviated version.

A magnetic field is a field that interacts with charged matter. Magnetic fields tend to deflect charges, altering their paths. (A time-varying magnetic field can also generate an electric potential, driving current down a wire. This is the principle behind an electric dynamo.) The fact that magnetic fields only act on charged matter in important. Most matter in the universe isn’t very charged, as it turns out. Sure, matter is made up of those electrons (negative charge), protons (positive charge) and neutrons (little buggers have no charge and are aloof regarding magnetic fields; snobs). But as it happens, positive and negative charges respond precisely the opposite way to a given field. So if you bind a proton and an electron together (making a hydrogen atom, in this case), they behave as if there is no charge there at all and proceed merrily on their way1.

So you and I and this desk and even our planet are mostly uncharged because almost all of our electrons have a matching proton to live with. OK, it’s true you can give yourself a net charge by scuffing you feet on the carpet. Go ahead and try it. (We’ll all wait while you find a friend or, better, a relative to zap. Just, please, don’t zap a pet. They can’t zap you back and fair’s fair.) But as it turns out, that charge is small. “Small compared to what?” you ask. Well, first of all, small compared to the number of protons and electrons in your body. You can show that even a minuscule charge imbalance, say one extra electron in about 10 billion, would cause you to explode violently thanks to the electrostatic repulsion of like-charged particles. In fact, any time you do carry a net charge, you’re basically encouraging electrons to either join you or leave you to make you neutral again.

Another good comparison is the ratio of your charge to your mass. Electrons, being small and charged have a high charge-to-mass ratio. They react strongly to magnetic fields. Protons, being nearly 2000 times more massive (and equally charged) also have a pretty high charge-to-mass ratio, but smaller than the electron. They react well to magnetic fields, but they are more sluggish and don’t react as well as electrons. An electron moving through a magnetic field make a circular orbit around the field lines. (If the field is uniform and the velocity is perpendicular to the field anyway. That’s the simplest case, so let’s run with it.) A proton will also gyrate around the field line (albeit in the other direction thanks to its opposite charge), but with a much larger radius. Two thousand times larger, in fact.

So you or I, who have only a small number of extra electrons or protons, barely have any physical response to magnetic fields at all. In fact, if you or I had one extra eletron for every 10 billion in our bodies and walked along at a 1 m/s stroll (about 2 miles/hr), the force of Earth’s magnetic field on one of us would be about 100,000 times weaker than the air pressure from walking.

The conclusion here is that most macroscopic matter (that is, stuff you can see, starting from dinner and yourself up to and including planets and stars) are basically invisible to magnetic fields and do no react to them. Humans are not magnetic creatures. Intuitively, this is kind of obvious, I think. Humans invented the compass precisely because we aren’t magnetic and we need help finding magnetic north.

Given all of this, how effective would you expect magnetic fields to be at curing diseases? Or causing cancers?

Not very.

1 — In the interest of full disclosure, the hydrogen atom doesn’t behave exactly as if there were no charges. The energies that the electron can have as it “orbits” the proton are changed, for example. But that’s a minor effect and one that we can reasonably ignore. It certainly doesn’t alter the atom’s bulk movements, for example.

posted by John Weiss at 12:28  

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