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Homework Help: Magnetic fields and wire

  1. Oct 29, 2008 #1
    Hi everyone,

    Part (a)
    Find the magnetic field above and below an infinite current sheet (infinite straight wires with current I along the positive z-axis). The sheet is on the xz plane. There are N wires per meter counting along the axis.

    Part (b)
    There are two parallel sheets with the same givens as part (a) except that the second sheet is separated by a distance d above the first. The sheet above carries current I along the positive z-axis. Find the magnetic field above, between, and below the sheets.

    Ampere's Law:
    Integral of (gradient cross B) times dA=Integral of B times dl
    with B=magentic field

    3. I'm having more trouble with part (b) than I did with part (a).
    This is how I went about solving part (a). I'm not sure if it's completely right or not though.

    I drew a rectangular loop parallel to the xy plane extended an equal distance above and below the surface. I then used ampere's law...
    Integral (B times dl) = 2Bl
    It is 2Bl because one is from the top segment and one from the bottom.
    2Bl=(mu sub 0)(I enclosed)(l)
    where I is the current and l is the length and mu is the permeability of free space

    Setting 2Bl = 0(I enclosed l)
    B=+ (mu sub 0/2)I in the x direction (when y<0)
    B= - (mu sub 0/2)I in the x direction (when y>0)

    If anybody has any ideas to solve part (b) or any corrections to my part (a) solution, I would really appreciate it! Thanks!!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 1, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to PF.


    This should be
    2BL=(mu0)(I enclosed)

    That is, no "L" at the far right. And I've switched to uppercase L to avoid confusing it with current I.

    Then it's a matter of figuring out (I enclosed):
    Each wire has a current I.
    There are N wires per unit length along x.
    The loop spans a length L along x.
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