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Homework Help: Force of Earth's magnetic field acting on a power line?

  1. Apr 12, 2009 #1
    1. The problem statement, all variables and given/known data
    Imagine a power line carrying 1400A through the earth's magnetic field in each of the following directions. Per meter, what is the magnitude of the force acting on the line:
    A. to the North?
    B. to the West?
    C. in a direction that points 30 degrees south of west?

    2. Relevant equations
    and τ= B · I · L(1/2)W · sinθ (magnetic moment)

    3. The attempt at a solution
    First off, I need to say that this is all the information supplied. Looking back in my lesson (Apex online learning AP Physics), I found one mention to the earth's magnetic field as being 4· 10^-5 T, so I have to assume they want me to use this. No dimensions of the line are given, so my idea to find the magnetic moment sounds like it won't work. My other equation above is to calculate the field around the wire, but I would still need "r," the distance to the wire which is not specified. My only other though is that the force will be zero when the line is parallel to the earth's magnetic field. I immediately assumed this would be when the line is running north, but now I am wondering if it could be west (which way does the earth's field actually "run?"). So, I am stuck. The lack of useful equations and information I have been provided with makes it seem like the answer is perhaps easier to find than I believe, but I have no idea. Any thoughts?
  2. jcsd
  3. Apr 12, 2009 #2


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    Homework Helper

    Force on a current carrying wire is given by F = BILsin(theta) where theta is the angle between the current and the field.
    Your answer for A is correct.
    In B the current is perpendicular to the field.
    And in C the angle is 120 degrees.
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