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Re: motional emf



One last comment on motional emf. Some textbooks further confuse
matters by claiming that "a changing magnetic flux produces an
electric field." (For example, that's the title of Sec. 29.7 of
Giancoli, Physics for Scientists & Engineers, 3rd ed.) This statement
would be correct if it said "magnetic field" not "magnetic flux".

The erroneous chain of logic is:
-time derivative of magnetic flux = emf = line integral of electric field

There is no induced (nonconservative) electric field in the case of
motional emf (in the lab frame). (Of course, if the moving bar does
not contact the slide rail circuit, then charges will pile up at the
two ends of the bar and produce an *opposing and conservative*
electric field, but that's not what's being referred to above.)

I credit Mosca and Tankersley for clearly writing this up but
unfortunately their article was not accepted for publication by AJP.
Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/