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Homework Help: Proton orbit direction

  1. Oct 4, 2011 #1
    [Solved] Proton orbit direction

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

    A solenoid has a magnetic field in the +z direction. A proton in the center of the solenoid moves towards the sides sides of the solenoid. Will this proton move with a clockwise or counterclockwise motion?

    2. Relevant equations

    F = qv x B
    B in a solenoid = (u0)(n)(I)

    3. The attempt at a solution

    I have no idea how to start this problem.
    Last edited: Oct 4, 2011
  2. jcsd
  3. Oct 4, 2011 #2
    Are you aware of the right hand rule?
  4. Oct 4, 2011 #3
    Which rule, the one for the cross product (F=qv*B) or the screw rule (magnetic field direction around a current-carrying wire)?
  5. Oct 4, 2011 #4
    For the cross product
  6. Oct 4, 2011 #5
    Oh, as I move out from the center, that's where I get v from... B is known, so F tells me the direction around the center. Got it, thanks. :smile:
  7. Oct 4, 2011 #6
    Yep :-) What direction did you get, out of interest?
  8. Oct 4, 2011 #7
    Looking from the origin down +z: if v moves left and B moves into the page, F goes down. This is counterclockwise. Alternatively, if v is moving right, the force on it pushes it up, so it's counterclockwise.

    But if I look from somewhere down the +z axis towards the origin, it appears to spin in the opposite direction even though the forces at those points are the same.
  9. Oct 4, 2011 #8
    Usually questions like this are asking if you are looking down onto it (if the proton is at the origin, you are at +z). Also, I thought the question says that the B field is is in a +z direction?
  10. Oct 4, 2011 #9
    If I'm at the origin and I look down the +z axis, B appears to go in to the page. I wrote both answers, one is if I'm at the origin and the proton is further along the z axis. The second is when I'm somewhere down the z axis looking in the opposite direction as the first case, and the proton is at the origin.
  11. Oct 4, 2011 #10
    Now I see! That seems fine.
  12. Oct 4, 2011 #11
    Great, that's done. :smile:
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