In my electromagnetic theory class, I learned that magnetic forces do no work. This is easy to see when observing a cyclotron; the speed never changes, so the kinetic energy never changes and therefore no work is done.

I then tried extending this logic to junkyard magnets, and now I am skeptical. The induced magnetic field appears to lift chunks of metal off the ground, directly opposing gravity and therefore changing the gravitational potential energy of said chunks.

So my question is, does the principle "magnetic forces do no work" fall apart in this case? Or is there something about the design of junkyard electromagnets that permits the principle to hold true?

Thank you in advance for your answers. :)

  • $\begingroup$ The purpose of a cyclotron is to accelerate particles. It is true that the magnetic field is not responsible for that acceleration, but the speed certainly changes. $\endgroup$ – Jon Custer Jan 22 '16 at 13:32
  • $\begingroup$ @JonCuster in a cyclotron the electric field is the accelerating field, while magnetic field is used to turn particles back to the electric field gap again. $\endgroup$ – rmhleo Jan 22 '16 at 13:52
  • 1
    $\begingroup$ I am well aware of how a cyclotron works. The OP perhaps less so... $\endgroup$ – Jon Custer Jan 22 '16 at 14:33
  • $\begingroup$ Hey guys. I'm fine with cyclotrons. Im just using that as an example to show that magnetic forces do no work. My question was this: if magnetic forces do no work in that scenario, what's different about a junkyard magnet? $\endgroup$ – Matt Schembri Jan 23 '16 at 0:49