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Homework Help: Simple Kinetic energy problem

  1. Oct 27, 2007 #1
    [SOLVED] Simple Kinetic energy problem

    1. The problem statement, all variables and given/known data

    A proton travelling at 23 degrees with respect to the direction of a magnetic field of strength 2.6mT experiences a magnetic force of 6.5*10^-17 N. Calculate a) the proton's speed and b) its kinetic energy in electron-volts

    2. Relevant equations

    For part a) I simply used F=qvbsin(theta), I got the correct answer of 400,000m/s
    For part b) I used K = 0.5*m*v^2

    3. The attempt at a solution

    For part b) I plugged in the value of 9.1*10^-31 for m and the value of v I found above(400,000m/s). I return 7.28*10^-20 (assume this in in J?). Upon conversion to eV I find a measly 0.45eV. (The answer is 835ev!)
  2. jcsd
  3. Oct 27, 2007 #2
    It's a proton, not an electron.
  4. Oct 27, 2007 #3
    I had a feeling this might be it. I assumed charge of proton and electron is the same, got the right answer, so then I proceeded to assume mass was the same too.

    Now if I plug in 1.67262158 × 10-27 instead, I get 835eV!

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