Why does a spherical magnet fall so slowly through an aluminum tube?
Charles Platt presents a science demonstration involving magnets that will interest juggalos and non-juggalos alike.
[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.
Quarterly.co is launching a brand new Maker Box subscription. This new Maker box features DIY kits and hands-on projects perfect for makers of all ages. You’ll receive kits to build your own gadgets, electronics, quirky tools, and more. Each quarter will feature a new curator, new ideas and new projects. The first curator is Boing […]
Brainiac75 made a pair of wooden tools that he uses to separate and stack large neodymium magnet discs. I’ve pinched my fingers quite a few times with tiny neodymium magnet discs and have learned to respect them. These big ones are very dangerous. You could easily lose a finger if these magnets were to smack […]
I don’t think I’ll make string from a plastic bottle, but I enjoyed watching how this guy made his PET bottle-cutting tool with some screws, washers, and a pencil sharpener blade.
Although there will never be a consensus about the best way to make coffee, any coffee connoisseur will agree that controlling the grind of your beans and balancing water temperature are the keys to a tasty cup. Since your plastic coffee pot doesn’t really allow for that kind of customization, going back to the French […]
Not all hackers are malicious information thieves—white-hat ethical hackers work with technology companies to ensure the security of their computer systems and user data. With all of today’s high-profile data breaches, ethical hackers are in considerable demand. To learn these critical skills and break into the high-paying cyber security field, try taking the courses in this […]
Making people aware of goods and services in the digital age requires an array of new strategies from social media and email to number-crunching tools like Google Analytics. To get a handle on the techniques used to capture attention and convert traffic into dollars in a crowded online environment, the Full-Stack Marketer Bundle offers 22 hours of training to get […]