Is It Electric or Magnetic? Depends on Where You Stand

If you took introductory physics, you learned about the “fundamental forces.” It goes something like this: All interactions are the result of one or more of five basic forces: strong nuclear, weak nuclear, gravity, electric, and magnetic. “Doing physics,” then, means identifying the forces in play.

There’s a problem, though, which might not have been mentioned: The forces you observe depend on your vantage point—your “frame of reference.” Look out the window. Trees, houses, lawns—they’re all stationary, right? But look at that same spot from space and it’s moving a thousand miles an hour. It looked still to you before because you were moving with it.

We have this same problem with electric and magnetic fields. Depending on your reference frame, what appears to be a magnetic force from one spot appears to be an electric force from another.

Is your brain melting yet? Just wait—it gets even weirder. To understand what’s going on here, let’s first look at electric and magnetic forces in isolation.

The Electric Force

See all that stuff around you? Everything. It’s all made of just three things: protons, electrons, and neutrons. That’s kind of wild when you think about it. Two of these fundamental particles have an electric charge: the negatively charged electron and the positive proton.

If an object has more electrons than protons, it will have a net negative charge. So, that sock in the dryer that clings to everything? It picked up extra electrons by rubbing against other clothes. If an object loses electrons it has a positive charge.

We can calculate the electrostatic force between two charged objects with Coulomb’s law. This says that the force between them depends on the product of their charges and how far apart they are. To illustrate, I built this high-tech contraption below, which has two little foam blocks hanging on strings. I gave them both a negative charge, which means they’ll repel each other. Check it out: