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Magnetc force from current-carrying wires - help

  1. Oct 20, 2004 #1
    In the figure below, a long straight wire carries a current of 8.5 Amps.
    Find the magnitude of the net force (in N) on a rigid square loop of wire of side, L, equal to 5.35 cm. The center of the loop is d = 10.40 cm from the wire and the loop carries a current of 10.0 Amperes.

    I don't really know where to start on this one. I realize that the segments perpendicular to the length of wire with current I1 can be discarded as not contributing any force right? I don't know where to proceed from there. Your help is greatly appreciated.

    Well I sort of figured out how to do it but I think I'm screwing the sign up somewhere:

    [tex]F = iL \cross B[/tex]
    [tex]B = \frac{\mu I_1}{2 \pi r}[/tex]
    And sum up the force on the top and bottom of the square from the long wire? Do I add the force being exerted on the top of the loop from the bottom of the loop and vice versa?

    Edit: Here is my work (let subscript t be top and b be bottom)

    [tex]F = \frac {u_0I_1I_t L_t}{2 \pi r_t} + \frac {u_0I_1I_bL_b}{2 \pi r_b}[/tex]

    [tex]F = \frac {2 \times 10^{-7} \times 8.5 \times -10 \times 0.0535}{0.07725} + \frac {2 \times 10^{-7} \times 8.5 \times 10 \times 0.0535}{0.13075}

    = about 9 \times 10^-6N[/tex]

    which is wrong

    Attached Files:

    Last edited by a moderator: Oct 20, 2004
  2. jcsd
  3. Oct 20, 2004 #2
    Nevermind I got it. I have been plugging in the wrong numbers for hours :\
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