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Electron in circular motion in B field problem check

  1. Nov 10, 2009 #1
    1. The problem statement, all variables and given/known data

    An electron of mass Me, with charge -e, is in a circular orbit in the xy-plane. There is a uniform magnetic field B in the positive z direction. It is moving at constant velocity V.

    Working in CGS units...

    Find R in terms of Me, -e, v, and B

    Find the angular frequency [tex]\omega[/tex].

    Find the current I with the given parameters.

    2. Relevant equations

    F= (2IqV)/(rc2)

    B= (2I)/(rc)

    And i believe F = mv2/r = mr[tex]\omega[/tex] ?

    3. The attempt at a solution

    My thought was to solve B= (2I)/(rc) for I, and substitute that quantity for the I in F= (2IqV)/(rc2) to get F = (B(-e)v)/c, and set that equal to the centripetal force mv2/r.

    Through that subsitution i found r = Mevc / B(-e)

    For the angular frequency i solved mv2/r = mr[tex]\omega[/tex] for r=v[tex]\omega[/tex], substituded that into the equation for radius, and solved for [tex]\omega[/tex] = B(-e) / Mec

    For the current, i used I = Brc/2, and substituted the value i found for r into this equation, for I = Mevc2 / -e

    I am somewhat confused, because i thought both the force on the moving electron and the centripetal force pointed in. Any guidance and/or clarification would be greatly appreciated.
  2. jcsd
  3. Nov 10, 2009 #2


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    Centripetal force mv^2/r = mω^2/r
  4. Nov 10, 2009 #3
    Whoops. Actually its F = mv2/r = mrw^2, though r = vw is still correct.
  5. Nov 10, 2009 #4


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    And there is only one force is acting on the election. The same force can be expressed in two different ways.
  6. Nov 10, 2009 #5
    So the magnetic field is the cause of the centripetal force?
  7. Nov 10, 2009 #6


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    Yes, it is.
  8. Nov 10, 2009 #7
    Great, thanks! So my analysis is correct?
  9. Nov 10, 2009 #8


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    Maybe I'm being dense today, but I don't understand the analysis. Where did you get F= (2IqV)/(rc^2) and B= (2I)/(rc)? What does the "c" mean? The "standard" equation for F is qvB. This, along with F=mv^2/r and mw^2r, should be enough to answer the first and second questions. As for the third question, what current is it asking for? Is it the current produced by the lone electron going around in circles?
  10. Nov 10, 2009 #9
    The F equation i used is for a test charge, in CGS units. "c" stands for the speed of light in this case. The B equation is the magnetic field this moving test charge produces.

    So far as i can understand, u0, the permeability of free space, = (4*pi) / c, if that helps. The books im following tends to work in CGS units, and not SI.

    The test charge, which is an electron in this case, would be moving in the +x direction, and would experience the F i mentioned, in the -y direction, with the B i also mentioned, in the +z direction. This is straight from the textbook "Electricity and Magnetism" by Edward M Purcell.

    Im not 100% certain on which current they are asking for, though i am just about 99% certain its the current produced by the lone electron going in circles, since a charge needs to be moving to create a current.
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