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Homework Help: Magnetic Force Problem

  1. Dec 19, 2016 #1
    1. The problem statement, all variables and given/known data
    What velocity does an electron need to have in order to travel straight through the crossed E and B fields, given that E = 1.0 x 105 N/C and B = 0.40 T?
    Answer: 2.5 x 105 m/s

    2. Relevant equations

    3. The attempt at a solution
    1.0 x 105N/C / 1.6 x 10-19 = 6.25 x 1023N. I rearranged F=QvB - v=F/QB
    v = 6.25 x 1023N/ (1.6 x 10-19)(0.40T). I'm left with 9.765 x 1042 but I know this isn't my velocity but I'm confused on where to go or what I did wrong.
    Last edited: Dec 19, 2016
  2. jcsd
  3. Dec 19, 2016 #2
    They give you the electric field, not force.
  4. Dec 19, 2016 #3
    I'm still at a loss then. I've switched to the E=KQ/r2 formula and rearranged it to get radius but I don't know how that helps me find velocity.
  5. Dec 19, 2016 #4
    I found the answer but I'm not sure exactly what I did. I found a formula for electromagnetism on wikipedia - F=(v x B) and I took 1.0 x 105/0.40T and got my answer.
  6. Dec 19, 2016 #5
    You were on the right path with F=QvB, and since (E) is given, I used the formula F=E*Q to replace (F) in the first equation.

    Then got EQ=QvB
    v=E/B and got the answer 2.5*10^5 m/s.
    which is the same answer you just found as well.

    Sorry for any confusion, hopefully this helped.
  7. Dec 19, 2016 #6
    Awesome! Thank you! This makes much more sense!
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