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Homework Help: Magnitude of force on electron in magnetic field

  1. Oct 13, 2013 #1
    1. The problem statement, all variables and given/known data
    An electron in a vacuum is first accelerated by a voltage of 41300 V and then enters a region in which there is a uniform magnetic field of .145 T at right angles to the direction of the electron's motion.

    What is the magnitude of the force on the electron due to the magnetic field?
    2. Relevant equations
    3. The attempt at a solution
    At first I though the 41300 V was just a number to throw me off (they do that a lot), but now I think it is important. I know the charge, magnetic field, and sin90=1. So the only thing I'm missing is velocity, but I have no clue how to calculate that from the information I was given. Any help?

  2. jcsd
  3. Oct 13, 2013 #2
    You can find the energy of an electron by using qV = U, where U is the Kinetic Energy of the particle. Then it's just a matter of solving for v from the KE equation.

    EDIT: I should point out that the energy will be in eV, which you can then just use a conversion factor to get it into joules.
  4. Oct 14, 2013 #3

    rude man

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    The electron is moving fast enough to warrant relativistic correction for calculating the velocity.
  5. Oct 14, 2013 #4
    Perfect. Thanks
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