How does a (static) magnetic field accelerate objects (like an iron nail) if we are taught that the magnetic force does always zero work?
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Who taught you that?? Electrical motors run on magnetic forces. – Feb 15 '18 at 18:02
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4There'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. – Michael Seifert Feb 15 '18 at 19:17
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@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 – Buzz Feb 15 '18 at 20:27