The Universe of Discourse

Sun, 29 Jun 2008

Freshman electromagnetism questions: answer 3
Last year I asked a bunch of basic questions about electromagnetism. Many readers wrote in with answers and explanations, which I still hope to write up in detail. In the meantime, however, I figured out the answer to one of the questions by myself.

I had asked:

Any beam of light has a time-varying electric field, perpendicular to the direction that the light is travelling. If I shine a light on an electron, why doesn't the electron vibrate up and down in the varying electric field? Or does it?

And one day a couple of months ago it occurred to me that yes, of course the electron vibrates up and down, because that is how radio antennas work. The EM wave comes travelling by, and the electrons bound in the metal antenna vibrate up and down. When electrons vibrate up and down in a metal wire, it is called an alternating current. Some gizmo at the bottom end of the antenna detects the alternating current and turns it back into the voice of Don Imus.

I thought about it a little more, and I realized that this vibration effect is also how microwave ovens work. The electromagnetic microwave comes travelling by, and it makes the electrons in the burrito vibrate up and down. But these electrons are bound into water molecules, and cannot vibrate freely. Instead, the vibrational energy is dissipated as heat, so the burrito gets warm.

So that's one question out of the way. Probably I have at least three reader responses telling me this exact same thing. And perhaps someday we will all find out together...

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