What Causes Magnetism?

Imagine a rowboat at rest in the middle of a perfectly still lake. (Perhaps someone has taken a break from rowing to read some Wordsworth and enjoy their own natural piety.) As long as the boat remains in one spot, the water around it stays perfectly calm. But suppose the rower realizes that the rustic lunch they were going to eat was left on the shore, and begins rowing back. Now, as the boat moves across the surface of the water, what do you see? Ripples range out from the sides of the boat, especially in a direction perpendicular to the boat’s path.

You can understand magnetism in much the same way as these ripples. The electric field around a charged particle is calm until the particle begins to move; but when it courses forward through space, it creates a disturbance to the left and right, like the ripples around a boat. This disturbance is a magnetic field, and it forms a circle around the charged particle’s path.

A quick way to say this is that an electric current produces a magnetic field around its path.

That is in fact, the way it is said in most physics classes. Your textbook probably has a sentence such as the one above.

(Like a lot of things in physics, however, this explanation turns out to be not quite true when you understand special relativity. But that’s a mathematical mind-melt you probably don’t need when you’re first studying electromagnetism. For most purposes, it’s enough just to say, “An electric current produces a magnetic field.”)

Current? What Current?

One thing you might wonder about on seeing that explanation of magnetism is how, say, a refrigerator magnet works. You don’t have to plug it in. There’s no battery. So there’s no current, is there? What’s creating the magnetic field around this everyday item?

You’re right, there is no “current” in the normal sense of free electrons being passed from one atom to another along a chain. But we’re using “electric current” here as shorthand for any changing electric field. So even if only one electron is changing its position, that should be enough to create a magnetic field.

And if you look closely enough at your refrigerator magnet, or any physical object really, that’s what you’ll find. In every atom of that thing, electrons are constantly spinning and whizzing around. When you consider all the electrons together, each atom creates a tiny little magnetic dipole moment (that is, a field with a north and south pole). None of these is very strong on its own, but if you add up all the magnetic moments in the refrigerator magnet, it adds up to plenty.

So Why Isn’t Everything Magnetic?

If you’re curious about the nuances of electromagnetism and special relativity, a simple place to start would