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Homework Help: Calculating electric field given velocity and magnetic field

  1. Jul 3, 2011 #1
    1. The problem statement, all variables and given/known data

    In the figure below, an electron moves at speed v = 93 m/s along an x axis through uniform electric and magnetic fields. The magnetic field B is directed into the page and has magnitude 4.00 T. In unit vector notation, what is the electric field?

    Figure: http://imageshack.us/photo/my-images/233/webassign2.jpg/

    2. Relevant equations


    3. The attempt at a solution

    I know that the answer is 0 for the i and k direction. I thought for the j direction, I just had to use the formula v=E/B, which would give me 372. But that's incorrect. I don't know what else to try.
  2. jcsd
  3. Jul 3, 2011 #2


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    Have you tried -372 ?
  4. Jul 3, 2011 #3
    That did it! Thanks.
  5. Jul 3, 2011 #4


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    Good. Usually when they don't specify a direction for the z-axis, you will need to try both + and - signs.
  6. Jul 4, 2011 #5


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    Note that getting the sign right isn't a matter of guessing. From the right-hand rule, you should be able to see that the magnetic force will point in the +y direction. To cancel it, the electric force must point in the -y direction.
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