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Mechanical force and the electrical force generated by a magnet

  1. Dec 18, 2006 #1

    I want to graph the mechanical force and the electrical force generated by moving a ferromagnetic object into a magnetic field at varying velocities.

    So, the X axis of my graph would be velocity and Y the two forces plotted separately.

    My math sucks and my physics is not a lot better, however, I am a fairly good programmer. If someone could please spell out the formulas involved in a manner it which I could code or stick in a spread sheet, I would really appreciate it.

    Thanks for all help
  2. jcsd
  3. Dec 18, 2006 #2


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    Unfortunately this kind of problem usually requires knowledge of physics so you can analyse your particular situation. In this regard it differs from, say, projectile motion where the equations are simple and can be easily programmed.

    For instance, what do you mean by electrical force? this doesn't make sense as written, so we'll have a hard time helping you out. Magnetization is field dependent, that is, the field in your ferromagnet depends on the field it finds itself in, as well as the history of every field it ever was exposed to. Unless it's a "hard" ferromagnet and your field is weak, in which case you might assume the magnetization is constant.

    In other words, it would help if you learned some E&M for this problem. We can help you along the way.

    Edit: another approach is to use a commercial package (finite difference, etc.) that is made especially for this type of problem.

    Give us a little more background so we know what you're trying to do (and for whom? school? employer?).
  4. Dec 18, 2006 #3
    Thanks for responding Marcusl.

    hmmmm. It is difficult framing the question when I don't know enough about the physics involved.

    This is not for school or an employer, but for my own knowledge advancement and interest.

    With that said, let me see if I can express this better or perhaps make some small baby steps and you can help along the way.

    Also, there are no specific magnet sizes, field sizes etc. so we/I can just pick some arbitary numbers. I am more interested in the shape of the curves rather than specific values. So here goes...

    If I move a ferromagnetic object into a magnetic filed, as I understand it, there are two things at work. 1. the basic magnetic mechanical effect of repulsion or attraction 2. an electric current is induced and according to Lenz's law this causes a causes a magnetic field to be set up such that it opposes the object entering the field.

    When the object is withdrawn from the magnetic field, the current induced magnetic field reverses polarity as required by symmetry and preservation of COE.

    (how am I doing?)

    So as I understand it, there are two fields a work here and as the object is moved into this field at say 1 inch per second, these fields will have a certain strength. If the object is moved at 2 inches per second the individaul fields will have a different strengths and so on. That is what I would like to graph.

    Any better?
  5. Dec 18, 2006 #4


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    Lenz's law and induced currents, you are clearly envisioning producing the magnetic field with a coil. Your description is ok but I'll repeat that you'd be better off with some E&M under your belt. For example your experiment would induce an emf (voltage source) in the coil, and that might or might not result in a current. You seem to have in mind a passive shorted coil (classic Lenz's Law application) but that wouldn't produce a preexisting magnetic field that you say your object moves into. For that real applictions would have a power supply holding the current through the coil constant regardless of induced emfs. If you then talk about hard materials (a magnet, that is) you can dispense with hysteresis curves and other complications, calculate the internal magnetization and equivalent dipole moment. If the moment and external field don't line up, there will be a torque. If they are aligned there is a linear force if the external field has a gradient. There's no force in a uniform field.

    You can see this is not as trivial as grabbing a formula and programming it. And you need a better understanding just to frame your problem. Sorry! :smile:

    Maybe others on this forum know of web sites that describe these concepts and provide formulas.

    As for books, I'd recommend:
    - Purcell, Electricity and Magnetism. 1st year physics book written by Nobel prize winner and known for clarity and readability. Uses older physics units (cgs) instead of SI.
    - Reitz and Milford, Foundations of Electromagnetic Theory. Straight-forward presentation, more applied than theoretical. Good sections on magnetism. Loses stars in most reviews because it often quotes results without full mathematical derivations, but that might be good for your purpose.

    Both are old and available used (cheap) if you don't have university library access and don't want to spend a bundle.

    - Also check out engineering electromagnetics books. I'm not familiar with them but you might find more applications-orientation.
  6. Dec 18, 2006 #5

    Bought both books, so we'll see how it goes ;)
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