A magnet fighting its own fall


A magnet that fights its own fall? In a copper tube that conducts electricity so well, a magnet can do this.

The slow magnet: Dropping a rare-earth magnet in a copper tube: buy or borrow a pure copper tube with 3/4″ inside diameter and about 18″ long (hardware stores, alas, don’t yet do metric!). Also get a strong rare-earth magnet about 1/2″ in diameter.

It’s important to have it as long as or longer than its diameter so that it won’t tumble going down.  Stack a bunch of button magnets, if need be.  Hold the Cu tube over a soft pad (so that the magnets don’t crack hitting the desk or floor). Drop a nonmagnetic piece of similar shape down the tube; it comes out fast (about  0.3 sec.). Drop the magnet down the tube, especially while a student watches from above.  Better yet, let the student do it. The magnet takes several seconds to fall.  As it moves down the tube, its magnetic field “cuts” the copper tube to create an electrical current circulating around the circumference.  That creates it own magnetic field that opposes that of the magnet, slowing down its fall. You might ask if the fall could be stopped completely (with an answer that should be obvious) or several other questions.


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