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Question regarding magnetic fields being unable to do work?

  1. Feb 5, 2014 #1
    I'm currently taking Physics II and had a question. I'd ask my professor, but he's out of town until Monday and this is driving me crazy. Anyway, we recently started learning about magnetic fields. So far, I'm grasping most of it no problem. One thing that is throwing me off though is how they keep emphasizing that a magnetic force can't do work, it can only change direction. This seems counter-intuitive to me since if I let go of a magnet close to my refrigerator and it has zero initial velocity, it has a very rapid acceleration towards the refrigerator (until collision of course). If someone can help make sense of this to me, I'd appreciate it, because if that acceleration isn't coming from the magnetic force, then I'm not sure what it is that is causing it?
  2. jcsd
  3. Feb 5, 2014 #2
    You should do a search. This is an oldie, but goodie. After your search you might have some more specific questions.

    (When I was an undergrad I pressed multiple professors for a satisfactory answer to this and they all responded differently and in some ways, contrary to each other.)

    edit - Maybe this will be interesting->
    Some of the math might be hard to follow, but thats a good paper to read if you are interested in this. If you can glean something from the introduction, post it, because I haven't read it in a few years. :)
    Last edited: Feb 5, 2014
  4. Feb 5, 2014 #3

    Simon Bridge

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    Last edited: Feb 5, 2014
  5. Feb 5, 2014 #4
    Thanks for the information from both of you. I really appreciate it. I always like to make sure I have my mind fully wrapped around these things, so hopefully after going through all that both of you posted, it will.
  6. Feb 6, 2014 #5
  7. Feb 10, 2014 #6

    Simon Bridge

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    The discussions here tend to follow something of a meandering path and contain their own links to other discussions that are similarly meandering. Those who don't want to wade through the interminable links may benifit from something of an executive summary.

    for a particle in a magnetic field, we have to consider two cases:
    1. the particle has no intrinsic magnetic moment
    2. the particle has an intrinsic magnetic moment

    For these situations:
    for 1. the B field does no work since the force is always perpendicular to the motion - but, recalling Newton's laws - it is the net force that does the work. The B-force can contribute to the net force in a way similar to the "normal force" in sliding-block problems.

    for 2. the B field can do work as it interacts with the magnetic dipole. This is the principle behind the Stern-Gerlach experiment for eg.

    Much of the debate revolves around what is "really" happening in #2. So lets be clear:

    #2 is a quantum mechanical effect.
    It is possible to model it as if there were an uneven charge distribution that acts as some sort of current and, thereby, obtain some hand-wavy link to pre-existing understandings of classical electromagnetism. That's not really a useful way to think about it, however, it is common in introductory e-mag courses.

    Better to think of QM as a superset of the classical - seeking to understand QM in terms of classical physics will only get you tangled up.

    Introductory physics texts are usually only concerned about #1 so they tend to leave off the qualification that the statement only applies to charged particles in a magnetic field. You'll notice that the same texts tend to follow the statement with examples involving only uniform magnetic fields - where there is no magnetic dipole interaction.

    Students from early on are taught two different models of magnetism - in one they are encouraged to think of materials as consisting of lots of tiny magnets which are all lined up in permanent magnets and in the other they are encouraged to think in terms of tiny current loops. We get this sort of question as students struggle to find links between the models.

    Further reading:
    ... most of the discussions are summed up in this lecture: slide 17, in particular.

    ... specifically posts 4 and 18.
  8. Feb 10, 2014 #7
    Thanks again Simon for some great information. I skimmed through a bit tonight and will be sure to take a better look at it this week when I have time.
  9. Feb 10, 2014 #8

    Simon Bridge

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    It's written to be the last word so, of course, it won't be :)
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