# Q & A: magnetic spaceship drive

Q:
Ok in space there is no gravity so a magnet should have unlimited energy. Like when a magnet repales them selves. So with this couldent magnets be used to send probes or what ever we see fit to send hurling through space at high speeds. Would my idea work?
- Steven
Clarmore oklahoma U.S.A.
A:
Well, magnets have the same amount of energy stored in their magnetic fields regardless of whether there's gravity around or not. The amount of work that can be done by two magnetic objects repelling each other is the integral of the force between them over the distance they move apart. It turns out that the repulsive force between two magnets gets weak very rapidly as a function of distance, and it is not a great way to store lots of energy. But there is some energy there.

That energy doesn't come for free however. You have to push the two repelling magnets together before you can release them and get your energy back. That means the energy really came from some other source (your hand or whatever pushed the magnets together), and the net effect is a bit like a spring or a rubber band. The magnets really act just as energy storage mechanisms. If you crack a simple bar magnet in the middle, the two halves will attract each other so there's no way to win here either. If you crack the bar magnet and turn around one of the halves so it repels the other, you have to add energy in order to turn one of the halves around. You'll get (almost) all of it back when you release it and it flies apart.

Magnets can be used in propulsion systems however. One far-out application is called an ion drive. Atoms or molecules can be ionized and accelerated electrostatically to high speeds and ejected from a spacecraft. Conservation of momentum (good old action-reaction pairs of forces) implies that the spacecraft will get a push in the direction opposite to the ejected ions. Magnets can be used to aid in the ionization process and to steer the moving charged particles in the desired directions.

It's kinda funny in this way, though, since the magnetic force on a moving charged particle is always perpendicular to the velocity of the moving charged particle. So the magnet cannot do any work on the charged particle -- all the work has to be done electrostatically. But magnets can play important focusing and steering roles for moving charged particles.

Tom

(published on 10/22/2007)