How does a (static) magnetic field accelerate objects (like an iron nail) if we are taught that the magnetic force does always zero work?


marked as duplicate by Emilio Pisanty, Chris, Kyle Kanos, stafusa, Pieter Feb 16 '18 at 18:50

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  • $\begingroup$ Who taught you that?? Electrical motors run on magnetic forces. $\endgroup$ – Pieter Feb 15 '18 at 18:02
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    $\begingroup$ There's a good discussion of this in Chapter 8 of the most recent edition of Griffiths's Introduction to Electrodynamics; but unfortunately I don't have it in front of me to summarize. I'll try to remember to do so next time I have the book in front of me and some free time. $\endgroup$ – Michael Seifert Feb 15 '18 at 19:17
  • $\begingroup$ @MichaelSeifert The discussion in Griffiths is very good, and it's not wrong; however, it's one of several instances where Griffiths avoids mentioning some additional subtleties. Magnetic fields acting on current do no work, but magnetic fields also interact with intrinsic spins; it then becomes a matter of precisely how you define "work" when such spins are involved whether the magnetic forces are doing work or not. -Brett $\endgroup$ – Buzz Feb 15 '18 at 20:27