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Homework Help: Period of an Electron Around a Magnetic Field

  1. Mar 2, 2010 #1
    1. The problem statement, all variables and given/known data

    An electron enters a uniform magnetic field of 0.152 teslas such that the electron follows a circular path. What is the period of the electron's motion? Electrons have a mass of about 9.11 x 10-31 kilograms each. Hint: Since the radius of the electron's path is not given, it must cancel out of the equations.

    2. Relevant equations

    Fm = Q * V * B

    F = M * A

    A = V^2/r

    V= 2(pi)r/T

    3. The attempt at a solution

    Q * V * B = M * A
    Q * V * B = M * V^2/r
    Q * 2(pi)r/T * B = M* V^2/r
    Q * 2(pi)/T * B = M * V^2
    (1.6E-19) * 2(pi)/T = (9.11E-31) * (3.00E8)^2
    Solve for T
    T = 1.86E-6 s
    But it is wrong...
  2. jcsd
  3. Mar 2, 2010 #2


    User Avatar
    Homework Helper

    How can you use v=3.00E8? Electrons can't possibly travel at the speed of light, and even if they could, the question doesn't say that they do.

    Try substituting 2(pi)r/T in the v on the right side as well. Better yet, cancel out one v in Q * V * B = M * V^2/r, then substitute 2pi*r/T into the remaining v.
  4. Mar 2, 2010 #3
    Cool got it... here is what I did if somebody else would like to see...

    Q * V * B = m * v^2/r

    Q * B = M * V / R

    Q* B = M * (2(pi)r/T)/r

    Q * B = M * (2pi)/T

    T = 2(pi) * M / Q * B

    I got 2.35E-10 seconds... worked great... thanks
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