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Homework Help: Magnetic Fields and insulated wires

  1. Jul 2, 2010 #1
    1. The problem statement, all variables and given/known data

    Two insulated wires cross at a 30 degree angle but do not make electrical contact. Each wire carries a 5.0 A current to the left through their respective wires. There are two points. One point lies between the bottom 150 degree angle, and the other lies between the right 30 degree angle. Each point is 4.0 cm from the intersection and equally distant from both wires. What are the magnitude and direction of the magnetic fields at points 1 and 2?

    2. Relevant equations

    B = [(mu)I]/2(pi)r

    3. The attempt at a solution

    I1 = I2 = 5.0 A
    r1 = r2 = 0.040msin75degrees = 0.039m
    r3 = r4 = 0.040msin15degrees = 0.010m

    Bwire1onp1 = Bwire2onp1 = [(Mu)I]/[2(pi)r]
    =[(1.257 x 10^-6 T*m/A)(5.0 A)]/[2(pi)(0.039 m)]
    =2.6 x 10^-5 T

    Bwire1onp2 = Bwire2onp2 = [(1.257 x 10^-6 T*m/A)(5.0 A)]/[2(pi)(0.010 m)]
    =1.0 x 10^-4 T

    Since Bw1onp1 and Bw2onp1 are both out of the page, then they add to give 5.2 x 10^-5 T out of the page

    Since Bw1onp2 comes out of the page, and Bw2onp2 goes into the page, then they cancel to give 0

    This doesn't seem right to me, can anybody tell me if I'm doing something wrong please?
  2. jcsd
  3. Jul 3, 2010 #2


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    Homework Helper

    Could you please show a picture? It is not clear where those points are.

  4. Jul 3, 2010 #3
  5. Jul 4, 2010 #4
    You are right.
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