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Homework Help: Magnetic force on a wire problem

  1. Nov 22, 2008 #1
    1. The problem statement, all variables and given/known data
    A packed bundle of 100 long, straight, insulated wires forms a cylinder of radius R = 0.500 cm.
    (a) If each wire carries 2.00 A, what are the magnitude and direction of the magnetic force per unit length acting on a wire located 0.200 cm from the center of the bundle?

    2. Relevant equations
    [tex]F= ILB[/tex]

    [tex]B = \frac{\mu_{0}I_{enc}}{2\pi r}[/tex]

    3. The attempt at a solution

    Here is what I did to get the solution:

    I can find the number of wires in out to where the radius equals 0.002m.
    [tex]\frac{\pi(0.002)^2}{\pi(0.005)^2}*100 = Number of Wires[/tex]

    That gives me 16 wires and since each has 2 amps the current enclosed = 32 A.

    Using ampere's law:

    [tex]B = \frac{(4\pi * 10^{-7})(32)}{2\pi(0.002)}[/tex]

    B = 0.0032

    Knowning B I can find the force per unit length on one wire with 2 amps of current.

    [tex]\frac{F}{L} = (2)(0.0032)[/tex]

    That equals 0.0064 N/m

    That answer is wrong and I can't figure out why. If anyone can help it would be appreciated. Thanks
    Last edited: Nov 22, 2008
  2. jcsd
  3. Nov 22, 2008 #2
    Do you mean .0064N/m
  4. Nov 22, 2008 #3
    yes I did, sorry
  5. Nov 22, 2008 #4
    I can't figure out what could be wrong with this.
  6. Nov 22, 2008 #5
    Looks okay to me too..
  7. Nov 24, 2008 #6
    The instructions request the direction as well as the magnitude...
    You have the magnitude: 6.34mN/m
    I'd say you need to articulate the direction ("inward"), ,otherwise you are absolutely correct.
  8. Nov 24, 2008 #7
    Well I got an answer of 6.4mN/m which is not the right answer. 3 options. Something is wrong in my logic. Something is wrong in my calculations. Or the books is wrong.
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