With the advent of cheap and easy-to-come-by NdFeB magnets, it has become possible to design a number of simple but effective demonstrations of magnetic force. One such demonstration, dubbed the “gauss rifle,” is a type of linear magnetic accelerator. It is relatively easy to assemble and involves a rapid and dramatic increase in kinetic energy of the steel ball bearings used in the demonstration. This makes the demonstration a good attention getter, setting the stage for a discussion of a number of physics topics, including conservation of energy, magnetic energy, and magnetic force. It also has the potential for becoming a laboratory experiment since the materials are relatively cheap, there is some challenge in the arrangement of the magnets, and the performance of the accelerator can be characterized by measuring the initial and final velocities of the bearings. Finally, by using freely available finite element magnetics software, it is even possible to make predictions of the final velocity for different configurations of the magnets.

Topics
Energy conservation, Magnetic energy, Friction, Laboratory procedures, Magnetic materials, Educational aids
1.
David Griffiths, Introduction to Electrodynamics, 1st ed. (Prentice Hall, Englewood Cliffs, NJ, 1981), Vol. I, Chap. 7, pp. 270–271.
2.
David Meeker, Finite Element Method Magnetics Users Manual, version 3.1 (http://femm.berlios.de/, 2002), p. 35.
3.
A.E. Fitzgerald, Charles Kingsley Jr., and Alexander Kusko, Electric Machinery, 3rd ed. (McGraw-Hill, New York, 1971), Vol. I, Chap. 2, p. 93.
4.
See Ref. 2, p. 62.
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© 2003 American Association of Physics Teachers.
2003
American Association of Physics Teachers
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