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Homework Help: Magnetic Field Problem!

  1. Nov 19, 2012 #1
    1. The problem statement, all variables and given/known data

    An electron (m= 9.11 x 10^-31 kg) enters a downward magnetic field of 5.00 x 10^-1 T with a velocity of 6.50 x 10^6 m/s West. Calculate the radius of the circular path it will follow when it is travelling within the magnetic field.

    m = 9.11 x 10^-31 kg
    v = 6.50 x 10^6 m/s West
    B = 5.00 x 10^-1 T
    q = -1.60 x 10^-19 C


    2. Relevant equations

    Fm = qvB
    Fc = mv^2/r
    Fm = Fc

    3. The attempt at a solution

    Fm = qvB
    = (1.60 x 10^-19 C)(6.50 x 10^6 m/s)(5.00 x 10^-1 T)
    = 5.2 x 10^-13 N

    Fm = mv^2/r
    r = mv^2/Fm
    = (9.11 x 10^-31 kg)(6.50 x 10^6 m/s)^2 / (5.2 x 10^-13 N)
    = 7.40 x 10^-5 m

    I feel like I may have done this question wrong...
     
  2. jcsd
  3. Nov 19, 2012 #2

    Delphi51

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    Homework Helper

    It looks good!
    May I offer a suggestion?
    It is a little more efficient and quite a bit clearer if you write it up this way:
    The centripetal force is provided by the magnetic force
    Fm = Fc
    qvB = mv²/r
    qB = mv/r
    r = mv/(qB)
    Put in the numbers and run it all through the calculator in one go, so no rounding takes place until you write the final answer.
     
  4. Nov 20, 2012 #3
    Thank you very much!
    I really appreciate your advice as well. :)
     
  5. Nov 20, 2012 #4

    Delphi51

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    Homework Helper

    Most welcome and good luck with Physics.
     
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