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Homework Help: Magnetic Field Strength

  1. Aug 7, 2007 #1
    1. The problem statement, all variables and given/known data

    A magnetron is a microwave source in which a "bunch" of electrons travel on a circular orbit in a uniform magnetic field. As the electrons pass the electrodes, a high-frequency alternating voltage difference occurs. Suppose the resonant frequency is f = 2.82×10^10 s-1; that is, the electron period of revolution is T = 3.55×10^-11 s. What is the corresponding strength of the magnetic field?
    The electron charge is -1.6E-19 C and the electron mass is 9.1E-31 kg.

    2. Relevant equations

    f=qv * b
    where q is the charge, v is the velocity, and b is the magnetic field

    3. The attempt at a solution

    I think I need to set a centripetal force equal to the magnetic force. The only equation I know for centripetal force is f=m(v^2 / r) and I'm not sure how to solve that with the period. I'm really just not sure if I'm even going about this right or how I can solve for v and r. Any help would be appreciated.
  2. jcsd
  3. Aug 7, 2007 #2


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    You are only missing one equation (which is quite simple). Consider a particel going in a circle of radius r. If it takes a time T to go around once, what is the speed of the particle?
  4. Aug 7, 2007 #3
    Oh wow, I didn't even think of that. Thanks.
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