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Homework Help: Electric & Magnetic fields, application of Lorenz's Law to -eV electron

  1. Mar 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A velocity selector consists of electric and magnetic fields described by the expressions vector E = E k-hat and vector B = B j-hat, with B = 0.0130 T. Find the value of E such that a 830 -eV electron moving along the negative x axis is undeflected?

    2. Relevant equations

    3. The attempt at a solution

    So I know i want to find E such that an electron with velocity = -v i-hat is undeflected. I also know form Lorenz's force that:

    F = q[E + (v x B)] = 0

    So i really just need to make E equal to the cross product (v x B) but opposite sign. I did this using using the fact that 830 -eV is the kinetic energy, set this equal to 0.5mv^2 and solved for v to be 4.268837e16 m/s.

    Next I computed the cross product to get a value of -5.549e14 k-hat. So E should be equal to this but with opposite sign, not working however for my online submission, any suggestions as to where I am going wrong?
  2. jcsd
  3. Mar 16, 2010 #2


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    4.268837e16 m/s is ridiculous. The speed of light is about 3e8 m/s. I'd check that part of the calculation first.
  4. Mar 16, 2010 #3
    right right, i should have noticed that
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