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Parallel Current-Carrying Wires and Magnetic Force

  1. Aug 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Each current depicted below by an “x” flows into the screen. At which of the following points (A, B, C, or D) is the magnetic force on a charge the largest?


    Note that C is equidistant from both wires.

    2. Relevant equations

    F = q*v*B*sin(theta)
    B = (mu_0*I)/(2*pi*r)

    3. The attempt at a solution

    Direction of magnetic field for wire 1 is to the right (positive) while wire 2’s field direction is to the left (negative)? B = 0 at point C.

    Would the largest force be at point B? The magnetic field will be to the right and its magnitude is greatest since wire 1’s B value is greater than wire 2’s B value at point B as a result of its distance from wire1?

    Thank you.
  2. jcsd
  3. Aug 22, 2009 #2


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    assuming both wires carry the same current, then yes, you are right.

    http://qbx6.ltu.edu/s_schneider/physlets/main/bcompass.shtml" [Broken]
    Last edited by a moderator: May 4, 2017
  4. Aug 23, 2009 #3


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    I agree with the assessment of the relative magnitudes of B, but not with the direction. The magnetic field will point from the bottom of the screen towards the top. Also note there will be no force unless the charge is moving and its velocity has a component perpendicular to the field.
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