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Homework Help: Current produced by a single charge moving in a circlular motion

  1. Mar 25, 2009 #1
    1. The problem statement, all variables and given/known data

    In Niels Bohr's 1913 model of the hydrogen atom an electron circles the proton at a distance "R" with a speed "v". Compute the magnitude of the magnetic field that this motion produces at the location of the proton.

    2. Relevant equations

    Bio Savart

    B = mu*I / 2R = mu * q * (v/2piR) / 2R

    3. The attempt at a solution

    The solution is stated above in the relevant equations section.

    There are some misunderstandings I have as to how they arrived at the equation for the current.

    First the magnetic field given by an infinite wire should be mu*I/2*pi* r but is shown as it is above without the pi in the denominator. Further, I thought that the moving charge should have a current that is equivalent to its velocity, charge, and cross sectional area ( I = nqvA ), but that is not what is correct. I feel as if I'm missing the big pink elephant in the room that is related to the period of the of charge or something. Can someone give me some help?
  2. jcsd
  3. Mar 25, 2009 #2
    I'm sorry the magnetic field in the center of a current loop is B = mu*I / 2*R. I guess I don't understand how the current is being calculated.
  4. Mar 26, 2009 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Current is given by
    I = Q/t
    So current is due to the charge of one electron, moving in a circular loop.
    To get t in the equation, how much time does it take the charge to make a complete orbit around the loop?

    p.s. also, note the formula for a long straight wire does not apply here, since the current is in the form of a circular loop.
  5. Mar 27, 2009 #4
    Geez I don't know why I couldn't figure it out. That makes so much sense and its so simple. I think I just need to go over the material alot more and try to understand the concepts better. Thanks alot I appreciate the help.
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