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Homework Help: Proton enters magnetic field in spiral trajectory

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A proton moves in a spiral path when enters a magnetic field of 5 T. If the pitch is 5 cm, find the angle between v and B.

    2. Relevant equations
    F = B q v sin θ
    F = m v2 / r

    3. The attempt at a solution
    I am guessing that I need to break down the velocity into two components but still don't know how to use it. What is pitch actually?

  2. jcsd
  3. Jan 28, 2013 #2
    That is it went 5 cm up or down after one revolution.
  4. Jan 28, 2013 #3
    The pitch of a spiral is the distance moved after one complete revolution.
    The diagram in section 2.5 of this wiki article is fairly clear:-

    The two velocity components would be
    vsinθ perpendicular to B
    vcosθ parallel to B

    vsinθ should be used in both the equations you give not just the first one.
    The two forces can then be equated.
    The pitch of the spiral is equal to vcosθ x (time period for one revolution, T) ie distance=speed x time
    But T also is equal to 2πr/vsinθ.

    Using the above ideas I found an expression for θ that still had one unknown in it. Is there any other information given in the question?
  5. Jan 30, 2013 #4
    That's all the information given. I get your idea. We have to know either the speed or radius to be able to solve for angle.

    Thanks a lot for your help :)
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