[Video Link] To explain the perplexing phenomenon of why a spherical magnet falls so slowly through an aluminum tube, I must refer to the right-handed-corkscrew rule.
When I first encountered this rule at the age of 16, I found it very hard to believe. My physics teacher, Mr. Sills, explained it. He was a charming but eccentric vegan who fascinated his students by violating the norms of everyday dress. Because he eschewed all use of leather, he wore tennis shoes, which were mildly scandalous in middle-class British society in 1961. Worse still, because vinyl belts did not yet exist, he held up his trousers with old pieces of string.
But getting back to the corkscrew rule -- he asked me to imagine that I was turning a corkscrew. If the linear travel of the corkscrew into the cork was compared with an electric current flowing through a straight wire, the current would create a magnetic force circling around the wire in the same direction as the turning of the screw. This is shown above (from my book Make: Electronics), where electricity is flowing away from you through the wire.
The right-handedness of this rule depends on the convention that electricity flows from positive to negative. But whichever way you imagine the flow, the rule suggests that the universe must be asymmetrical. I found that very hard to accept. Why should it be a right-handed corkscrew? Why not a left-handed corkscrew? Who ever decided that electricity should behave this way?
Mr. Sills believed in a deity. He said this was the only reasonable explanation for the behavior of water as it freezes. When ice forms in lakes, its expansion causes it to be lighter than unfrozen water, making it rise to the surface, which enables fish to continue living beneath. Hence, life was able to evolve, and because water is the only compound that behaves this way, Mr. Sills speculated that some entity must have rigged the system.
However, he ducked the issue of cosmic symmetry, preferring to demonstrate the truth of the right-handed-corkscrew rule experimentally. This is done by bending the wire into a loop, creating a net magnetic force through the center, as shown above. If you add more loops to build a coil, you have an electromagnet.
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.
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.
[Video Link] 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.
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.
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.
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.
I like to think that if my physics teacher were alive today, he would be doing the demo -- and I might be a little more willing to believe the right-handed-corkscrew rule.
Published 5:30 am Tue, May 13, 2014