I started asking this question after one of my students had trouble with a problem asking for the net force on a current loop in a homogeneous magnetic field (on my first day of TAing, no less!). A current loop in a magnetic field will experience a torque, but not a net force. I always assumed that this was because magnetic fields do no work, until I noticed that a magnetic dipole in a non-homogeneous magnetic field will accelerate. Bar magnets are ab example of this. So here's my question. David Griffiths says, in his textbook, that magnetic fields never do any work. My E&M professor, however, told me today that magnetic fields are able to do work. Which is it?