Teaching Physics with the Physics Suite

Edward F. Redish

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Magnets and charge*

A bar magnet is hung from a string through its center as shown in the figure. A charged rod is slowly brought up slowly into the position shown. In what direction will the magnet tend to rotate? Suppose the charged rod were replaced by a bar magnet with the north pole on top. In what direction will the magnet tend to rotate? Is there a difference between what happens to the hanging magnet in the two situations? Explain why you either do or do not think so.

* From the University of Washington Physics Education Group

Note to the instructor: Although electric charge is often used as an analogy to lead into the study of magnetic poles (dipoles) this can be problematic. A significant fraction of students (a majority in my calculus-based physics class for engineers) confuse electric and magnetic poles and think that they are somehow the same thing, despite warnings to the contrary. This problem neatly reveals the confusion.

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Page last modified October 19, 2002: MG01