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Magnetic forces/fields don't do work?

  1. May 18, 2013 #1
    Now, I'm not sure if I misunderstood the text book but I read that magnetic fields and forces don't actually do any work. How is this the case? Consider a bar magnetic and some iron filings. If you use the magnet to attract the filings towards it wouldn't some sort of work be done? Thanks.
  2. jcsd
  3. May 18, 2013 #2


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    Staff: Mentor

    That is a simplified description for static magnetic fields, with free charges moving in them. Your system does not have that.
  4. May 18, 2013 #3
    Well the magnetic force term definitely drops out when we calculate the work done on a small displacement, but that does not mean the magnetic field does not take part.
  5. May 18, 2013 #4
    Thanks for this.
  6. May 18, 2013 #5
    That's true for classical electrodynamics but not always the case in certain quantum mechanical situations. E.g. if you were to place a charged particle at rest in a non-uniform magnetic field then the magnetic field would do work on the charged particle because it has a magnetic moment and thus acts as a microscopic bar magnet.

    Work is being done, just not by the magnetic field. It's the electric field of the charges which make up the iron fillings which are doing the work. This is all explained in the following two articles

    Work done on a charged particles in magnetic fields, Charles A. Coombes, Am. J. Phys. 47(10), October 1979

    Magnetic Forces doing work?, Eugene P. Mosca, Am. J. Phys.42, 295 (1974).

    If you want to read these articles I can look into creating an e-mail account in order to send them to you with.
  7. May 18, 2013 #6
    Great stuff. Thanks for that. I'm not sure that I'll be doing much further reading on the subject at the moment. This is great for now.
  8. May 19, 2013 #7
    You're most welcome. I love the American Journal of Physics. It's my favorite physics journal because I love thinking about the foundations of physics.

    I've gathered a considerable number of articles over the years and have scanned them all into my computer. I lost a lot of them due to a system crash so I'll have to start over and scan a lot of them in again. If you ever want to read something on a particular subject that is in that journal then let me know. I can obtain a copy and forward it to you. I wrote to the editor of that journal and was given permission to do so, so don't worry about copyrights. They said it was okay to disseminate an article for educational purposes.
  9. May 19, 2013 #8
    Brilliant. Thanks for that and I'll be sure to ask for those if I ever feel the need to read on.
  10. May 19, 2013 #9
    You're most welcome. I finally realized that you can upload files into posts that can be read by anyone reasding the post. I greatly enjoy helping people learn physics and math in anyway that I'm able to. It just feels so good to do so. Ever notice that yourself? :tongue:
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