The demonstration of Lenz's law by dropping a powerful magnet down a nonmagnetic metal pipe has become a classic lecture-hall demonstration.1,2 An inexpensive version is packaged as a professional magic trick3 called “Newton's Nightmare.” Combining sleight-of-hand with a demonstration of Lenz's law is a surefire way to heighten student interest. The subsequent student discussion motivated by a desire to understand the magic trick can lead to a memorable physics lesson. This paper will discuss Lenz's law magic and review literature that reveals the subtlety of the physics.
REFERENCES
1.
Examples include the Lenz's Law Demonstrator from PASCO (MG-8600, a 1.5-meter tube) and the Lenz's Law Apparatus (CENCO) available from Sargent-Welch (CP32691-00, a 91.4-cm tube). These are constructed so that an attached spring scale can be used to also demonstrate Newton's third law.
2.
Lenz's Law Apparatus (P8-8400) from Arbor Scientific. This is a small version with a viewing-window slot along most of one side of the pipe. I first learned about this one from my departmental colleague Forest Davenport.
3.
“Newton's Nightmare,” created by William J. Schmeelk of Wellington Enterprises for FUN Inc., 2100 N. Major Ave., Chicago, IL 60639, can be purchased from magic shops (both local and online).
4.
Magician Ricky D. Boone, Magic Central, 175 Weaverville Highway, Suite L, Asheville, NC 28804.
5.
Jhules A. M.
Clack
and Terrence P.
Toepker
, “Magnetic induction experiment
,” Phys. Teach.
28
, 236
–238
(April 1990
).6.
The aluminum pipe has a mass of 72 g and it can be placed vertically on a laboratory electronic balance. The magnet, which has a mass of 12 g, can gently be released without tipping over the pipe. The scale rapidly jumps to as the magnet makes its way down the hollow cylinder.
7.
Joseph
Priest
and Bryant
Wade
, “A Lenz law experiment
,” Phys. Teach.
30
, 106
–108
(February 1992
).8.
W. M.
Saslow
, “Maxwell's theory of eddy currents in thin conducting sheets, and applications to electromagnetic shielding and MAGLEV
,” Am. J. Phys.
60
, 693
–711
(Aug. 1992
).9.
John A.
Pelesko
, Michael
Cesky
, and Sharon
Huertas
, “Lenz's law and dimensional analysis
,” Am. J. Phys.
73
, 37
(Jan. 2005
).10.
Charles A.
Sawicki
, “Lenz's law: Feel the force
,” Phys. Teach.
34
, 38
–39
(January 1996
);Charles A.
Sawicki
, “A Lenz's law experiment revisited
,” Phys. Teach.
38
, 439
–441
(Oct. 2000
).
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© 2006 American Association of Physics Teachers.
2006
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