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Homework Help: Wire & Magnetic Flux

  1. Jul 31, 2007 #1
    1. The problem statement, all variables and given/known data
    A long straight wire carries a current 36 A. A rectangular loop with two sides parallel to the straight wire has sides 7.5 cm and 15 cm with its near side a distance 3 cm from the straight wire as shown in the figure. The permeability of free space is 4π e -7 T*m/A. Find the magnetic flux through the rectangular loop. Answer in units of Wb.

    http://img112.imageshack.us/img112/9541/mblf4.jpg [Broken]
    Shot at 2007-07-31

    2. Relevant equations

    Magnetic flux = BA
    B = u * I / (2*pi*r)

    3. The attempt at a solution

    I found the magnetic field

    B = u * I / (2*pi*r) = (4*pi*10^-7 * 36 A) / (2*pi * 0.03 m) = 0.00024 T
    Then I found the area of the rectangle which A = 0.075 * 0.15 = 0.01125

    I multipled BA to get the magnetic flux. My final answer was 2.70e-6 Wb but this answer is wrong. Could someone please tell me what I'm doing wrong? Thank you.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jul 31, 2007 #2
    the magnetic field is not constant throughout the area. therefore you have to integrate B*dA, the flux, from r=3 cm to r=3+7.5= 10.5 cm
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