A useful aspect of the cause-and-effect relationship between electricity and magnetism is that it's reversible."Useful" is an understatement, as modern civilization totally depends on it. Instead of passing electricity through a coil to create a magnetic force, you can pass a magnet through a coil to generate electro-motive force--that is, electricity flowing through the wire. The moving magnetic field interacts with electrons and pushes them around. This principle is used in the generation of electricity in power stations everywhere in the world.
<p>Suppose we imagine a loop of wire becoming a closed ring of metal. Now imagine that the ring is stretched to become a tube. Moving a magnet through the tube still pushes the electrons around, although now they are just running in circles. These circulations are called eddy currents.
</p><p>For a dramatic demonstration of the consequences, all you have to do is drop a powerful magnet through a tube made of metal that is nonmagnetic but is a good electrical conductor. Copper or aluminum will do the job. The magnet behaves as if it's falling through molasses. Nothing visible is preventing it from falling freely, but its interaction with electrons in the tube requires energy, and the energy is obtained by stealing it from the pull of gravity.
</p><p>Even this is not the whole story. Electrons flowing through a conductor will generate some heat. This is the principle which causes a fuse to blow if too much current flows through it, as the fuse gets hot and melts.
</p><p>Very little heat is created if the conductor has a low resistance--but still, the heat is there. Therefore the work that is done by gravity, pulling a magnet through a tube, is converted partially into heat. Energy, as always, is conserved.
</p><p>In the days when I was learning about electricity and magnetism, I never saw the tube-and-magnet demo. Aluminum was costly back then, and simple iron magnets were not very powerful. A neodymium magnet is necessary to create a significant, dramatic effect. These are the strongest known type of magnets, developed collaboratively by General Motors and Hitachi in the 1980s.
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