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Radius of charged particle moving in magnetic field

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data
    So, I got the problem from a friend who told me he copied it down in haste, so it's possible that my confusion stems from a missing variable, but I just want to be certain as my exam is in a few days. So the problem states that there's a +2 helium atom that was accelerated by 1700 V. What will be the radius of curvature if it moves in a plane perpendicular to a uniform magnetic field of .5T ?

    2. Relevant equations
    R = mv/qB
    F = Eq
    v = at

    3. The attempt at a solution
    So, the first thing I notice is that the only variable I'm not given that is needed to solve for the radius is the velocity of the particle. I'm assuming the particle started at rest and was then accelerated, so then the velocity is equal to just the acceleration times time. The acceleration could be expressed as force over mass, so then I just needed an expression for the force.

    F = Eq
    a = Eq/m
    v = Eqt/m
    R = (m/qB)(Eqt/m)

    The masses cancel, the charges cancel, and I'm left with the voltage times time divided by the magnetic field.

    R = Et/B

    My problem is I'm not given time anywhere in the problem, but it's obviously essential to know how long the particle was accelerating. Am I missing something here?
  2. jcsd
  3. Mar 29, 2016 #2
    accidental post, sorry. disregard
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