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WHAT FOLLOWS is a multi-part exposition of science (you’ll excuse the word) based on charge, force, and magnetic field, better known among students of physics as Charley F. Murphy. My exposition arises from several recent YouTubes posted by talented people, two of whom have even devised a Kickstarter effort for what they call the Littleville Kit, the world’s smallest maglev train.
Eduardo Carrizosa is a student at the University of California Berkeley. He wrote me about the Littleville Kit, a 1/1000th-scale train that operates on the same principle as Shanghai’s Maglev Train (and also the Electronic Aircraft Launch System catapult of our most modern aircraft carrier, the U.S.S. Gerald R. Ford).
The proposed kit is described in detail at littlevillerailroad.wordpress.com. Its oval track contains copper coils that are the “primary” of its linear motor. Its engine and passenger cars contain magnets that act as the “secondary” of the motor. When the coils are electrified, the secondary glides atop the primary at an all but imperceptible gap.
The Littleville Kickstarter effort has only seven days to go. Alas, it’s pretty far from its goal for a Kickstarter “all or nothing” project, but I wish Carrizosa and Gutman well in this interesting endeavor.
So where does Charley F. Murphy fit in?
Physics 201, as taught back in the 1960s (when there wasn’t as much physics as today), made use of Charley F. Murphy and human hands to describe the relationship between an electrical current and the magnetic field associated to it.
In 1820, Hans Christian Ørsted noticed deflection of a compass needle when a nearby wire carried an electrical current. In fact, Italian lawyer Gian Domenico Romagnosi, 1761–1835, wrote about this relationship in Italian newspapers some two decades before Ørsted’s work. What with one thing and another (twice tried for treason because of Francophile tendencies didn’t help), Romagnosi got little credit for his observations. By contrast, Ørsted got the unit of magnetic field strength, the orsted, named for him.
There’s a lesson here, but perhaps not a satisfying one.
As I remember it, if an electrical current, or charge or Charley, travels through a wire in the direction of my right thumb, then the resulting magnetic field, or Murphy, curls around in the direction of my right fingers. The force, his middle name, concerns which way the wire is forced, but don’t trust me on this part. I believe I was setting up a rally for the Worcester Tech Auto Club during that lecture.
To learn more about how Charley F. Murphy propels tiny maglev trains, there’s a well-executed video by Wayne Schmidt at “How to Build the Simplest Electric Train.” He makes the point that all this wouldn’t be possible were it not for the discovery in the early 1980s of super magnets composed of rare-earth neodymium, iron, and boron.
Schmidt also advises that not just any neodymium magnets will do. These are rated by their magnetic flux output per unit volume, shorthanded to N35 through N52. Settle for no less than N48. Schmidt gives excellent safety tips on handling these super magnets. What’s more, he offers a slick means of fabricating the 18 gauge bare copper wire into the necessary coil. (See 11:50 of his video.)
For a full-blown analysis of these tidy little electric trains, there’s “World’s Simplest Electric Train,” by C. Criado, Departamento de Fisica Aplicada I, Universidad de Malaga, Spain, published in the American Journal of Physics 84, 21 (2016). A free abstract is available.
To know which way the train travels, just ask Charley F. Murphy. ds
© Dennis Simanaitis, SimanaitisSays.com, 2017