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Why does an a moving charge spin in a magnetic field

  1. Feb 8, 2013 #1
    I am very much familiar with the Lorentz force. From what I know, that is HOW a moving charge in a magnetic field gains centripetal acceleration. My question is this : Is there any explanation over why a moving charge in a magnetic field moves in a circular motion? I know the magnetic field is putting a force on the moving charge and is causing the charge to gain centripetal acceleration. I would love to know if there was any reason why the external magnetic field made the moving charge gain centripetal acceleration. Thank you guys!(:
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  3. Feb 8, 2013 #2


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    I am not very clear on whether you're asking why a moving charge in a magnetic field fields a force, or if you are asking why the force causes that type of a motion.

    You said you are "familiar" with the Lorentz force. Then you know about the cross product between the B-field and the charge's velocity (notice that if the charge is moving parallel to the B-field, it feels no force at all!).

    If you are asking WHY it feels a charge, then I'll ask you this: are you comfortable with a charge experiencing a force in an electric field? I would guess you are since you didn't ask that question. But what if I tell you that

    1. a charge moving across a B-field detects a time-varying B-field

    2. a time-varying B-field produces an equivalent E-field (or to be exact, the curl of the E field)

    3. and thus, from 1 & 2, the charge should experience a force?

  4. Feb 8, 2013 #3
    If I read correctly, are saying that the reason why the magnetic force on the moving charge is there is because of the electric force? In terms of relativity I am guessing(:
  5. Feb 8, 2013 #4


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    Look at Maxwell equations. There's a term equating the curl of E with dB/dt.

    I'm not invoking any Relativity here.

  6. Feb 8, 2013 #5
    But you are considering this from the charges point of view. Were you originally saying that the electric force between the "time-varying" magnetic field and the "stationary" charge is the reason for the force on the charge? That is what I thought but if this was true then the "time-varying" magnetic field would be doing work on the stationary charge. Is this what happens?
  7. Feb 8, 2013 #6


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    Yes, but this has nothing to do with any relativistic transformation. Who says I can't use a simple Galilean transformation? I wanted to illustrate that a changing B-field is equivalent to the presence of E-field. And since you had no problems with accepting a charge having a force an an E-field, I thought it was something you can relate to.

    Not in the way you think. Remember, the geometry isn't that simple here because dB/dt corresponds to the curl of a field, and this field is no longer conservative because its curl isn't zero!

  8. Feb 8, 2013 #7
    Oh I was referring to Galilean relativity!(: I'm a little confused though. I'm not sure I am understanding what you are trying to say though. I know the geometry is not so simple but is the time varying magnetic field doing work on the charge anyway?
  9. Feb 10, 2013 #8
    Would you say that the magnetic field applies torque to a moving charge?
  10. Feb 10, 2013 #9
    you can say that:

  11. Feb 10, 2013 #10
    I don't think so, not anymore than why a moving charge moves in the SAME direction as the E field. qE is the electric force and qv X B the magnetic force......If there is some
    fundamental reason for these particular orientations, I like to know what it is....
  12. Feb 10, 2013 #11
    Thank you very much Naty1(:
  13. Feb 11, 2013 #12
    It occurred to me after posting that I could have offered some suggestions for further reading. I tried but did not find much.

    If anyone chooses to investigate further, I'd suggest the best possibility for an explanation might be found via quantum electrodynamics...but it may end up being mathematical rather than 'intuitive'....I have no idea. Perhaps reading paramagnetism and diamagnetism could also offer insights.

    For example:

    But this, once again, addresses observed effects rather than causes it seems.


    I wonder how, if at all, string theory deals with the electromagnetic force??

    I also checked my copy of Lisa Randall's WARPED PASSAGES....she's the Harvard physics professor; She discusses spin in a very few pages but not the orientation of resulting forces.

    also, it occurs to me I have never even seen a theory as to why spin exists...fermions, bosons, etc.. as Feynman said regarding another subject "Who ordered THAT??" I suspect we may have some theories why it's quantized, but why does it even exist??
    Last edited: Feb 11, 2013
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