[Physics] the speed of magnetism

electromagnetismforcesmagnetic fields

I'm not a physicist (just have a few basic notions) and this is a question out of curiosity, so as I'll probably word it the simplest I can, please take that into account when answering.

tl;dr

Let's take an electromagnet, power it off and hang it over a table. Put a small metallic something below it, on the table. Power on the magnet and suppose for the sake of simplicity that the magnetic field form instantaneously. After how much time the metallic something will be attracted to the magnet? And at what speed it will move toward the magnet?

In the end, what is the speed of magnetism?

Long version

This is been a curiosity I've had for years about at which speed happens the interaction in magnetics fields. I know that $F=ma$, so I suppose that if a magnet is given with a certain F, the a of an attracted objects will depends by its mass (well, and the distance too decreasing F)

Then I saw years ago an experiment about it on Mythbusters, and then, yesterday, I remembered about Physics.se

In Mythbusters, they were trying to divert the trajectory of a bullet by shooting it over a long line of very powerful magnets. Well, the bullet was not impressed that much and never diverted a bit (visibly, at least). I can somehow realize that there are a lot of things at play, but still I fail to understand how a simple bullet exerciting a momentum of few kg can avoid the strength of a bunch of magnets capable of lifting 150kg. The only thing that came to my mind is that the bullet travel too fast for the magnetic field to interact with it, thus the question about the "speed of magnetism," but still this seems the wrong explanation to me, even.

Best Answer

There are several time delays I can think of.

  1. When you "turn on" an electromagnet, the current doesn't instantly go to its final value. The coil is an inductor, and inductors generate EMF that opposes changes in current. This doesn't affect the Mythbusters experiment because their magnets were already on.
  2. The magnetic field itself forms at light speed following Maxwell's equations.
  3. Magnetic forces on macroscopic objects depend on reactions of many electrons in the materials. Eddy currents (for a "drag" force) don't form instantly. Magnetic domains (for an attractive force) don't organize instantly.
  4. The bullet's path changes if its momentum changes. Impulse is the change in momentum, and it is force times time. The bullet passes by the magnets quickly so the force doesn't have much time to act.
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