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Homework Help: B field intensity of 2 parallel wires

  1. Jun 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Two long parallel wires are a center-to-center distance of 4.60 cm apart and carry equal anti-parallel currents of 4.40 A. Find the magnetic field intensity at the point P which is equidistant from the wires. (R = 4.00 cm).

    2. Relevant equations
    magnetic field for a long straight wire = μi/2*pi*r

    3. The attempt at a solution
    at first i thought it was just 0 T since the current is in opposite directions but that was wrong. so i worked it out and saw that the y components cancel out and you're left with the 2 x directions components. so i thought B = [itex]\frac{2*μ*i*cos(theta)}{2*pi*r}[/itex]

    where cos(theta) = R/r
    where r = sqrt(R2+(d/2)2)

    this didnt work out for me. my answer was 7.1665340225x10^(-4) T and it said it was wrong

    B= [itex]\frac{2*4*pi*10^-7*4.4*.04}{2*pi*(R^2+(d/2)^2)^(3/2)}[/itex]

    anything i did wrong, plugged in wrong numbers or what?

    Attached Files:

  2. jcsd
  3. Jun 10, 2012 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    I see two problems:

    1. I don't see where the "3/2" exponent comes from in your denominator, the units do not work out properly with that exponent there.

    2. It appears you made an arithmetic mistake in evaluating your expression -- I get a different number for the expression you wrote, but as I indicated I don't think the 3/2 exponent there is correct anyway.
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