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Why are there 2 definitions of Magnetic Moment?

  1. Feb 10, 2009 #1
    Hello everyone, this is my first post at Physics Forums!

    I am trying to understand electromagnetism, and when it comes to the magnetic moment, when reading around I see [tex]\mu = IA[/tex] where A is area of a loop, which is an expression I can sort of understand. But when reading about magnetic drifts (I am a little confused for the moment) I see [tex]\mu = \frac{m v^2}{2 B}[/tex]. I am sure both refer to the same value but I cannot visualise how.

    Would someone be kind enough to explain this to me?

    Thank you!

    Last edited: Feb 10, 2009
  2. jcsd
  3. Feb 10, 2009 #2


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    Staff: Mentor

    I don't remember ever seeing your second equation. If you found it on the Web, can you give us a link to it, or if in a book, which book it was and what the context of the equation was? (What was the book discussing at that point?)
  4. Feb 10, 2009 #3

    The context of the second equation is in space plasmas, describing the ratio between perpendicular particle energy and magnetic field.
    I found the equation in a book called "Basic Space Plasma Physics" by Baumjohann W. and Treumann R. A., Imperial College Press 2004.
    What I don't get is how do they get to this expression, and how does it "fit" with the other one (=IA).

    Thank you for your help!
  5. Feb 10, 2009 #4


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    Staff: Mentor

    Unfortunately, I don't know squat about space plasma physics. :uhh: Maybe someone who does know something about it will see this.
  6. Feb 11, 2009 #5


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    Gold Member

    I'd say that your second equation is a derived equation appropriate to a particular problem. Note that (mv^2)/2 is kinetic energy and mu B has units of energy, so this equation relates the energy of a moment in a field to some situation where particles are moving. It is not a fundamental definition of moment like the first equation.
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