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Homework Help: Direction of a magnetic field

  1. Mar 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A positively charged particle moving in the direction of the positive x axis feels no magnetic force when it passes through point P. When moving in the direction of the positive z axis, the particle experiences a magnetic force in the positive y direction when it passes through point P. Thus the magnetic field B at point P is directed along:.

    2. Relevant equations

    F=q(v x b)

    right hand rule

    3. The attempt at a solution
    Since the particle feels no force when traveling in the direction of positive x, (v x b) must be 0 and the only way this happens is if the angle between the is the angle between the velocity and B field is 0 or 180. So the B field must be parallel to the x axis. Now the question states when the particle moves along the + z axis, it feels a force in the positive y direction. Due to the right hand rule, I believe that this must mean the b field is in the -x direction. Fingers in the direction of the velocity (+z), thumb in the direction of the force (+y), curl fingers towards the B field, which then must be the -x direction.

    However, the answer is not the -x axis. Could you please show me where I went wrong in my reasoning and help me find the correct answer?
  2. jcsd
  3. Mar 24, 2009 #2


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  4. Mar 24, 2009 #3


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    Your description of how you solved this is correct, but for some reason you came up with -x instead of +x.

    I am just guessing, either you used your left hand by mistake, or you are picturing the x,y,z axes incorrectly.

    If the axes are set up properly, then +x × +y = +z

    LowlyPion beat me to it.
  5. Mar 24, 2009 #4
    Yup, after reviewing that, it appears that I definitely used a left hand coordinate system. Thanks for your help.
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