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Homework Help: Magnetic field of current loop

  1. Aug 15, 2011 #1
    1. The problem statement, all variables and given/known data
    Find magnetic field along the z-axis of a circular loop, radius R with constant current lying in the z=0 plane.

    2. Relevant equations
    vec(r) = vector of r.
    zhat = unit vector z.

    3. The attempt at a solution
    So starting with the definition, B = (u_o I/4pi) (dl x vec(r))/r^3, where:
    \vec(r) = z(zhat) - R(s hat), thus:
    r = root(z^2 + R^2)

    also dl x vec(r) = vec(r)dl(phi x s) = (dl)(-zhat). Since dl = length around current loop = 2piR:
    dl x vec(r) = -2piR(zhat) vec(r).

    Here's where I get stuck.
    If I continue along this line of thought, zhat (from the cross product) dotted with vec(r) will give me z, and my numerator will end up being -2piRz, but the answer says the numerator should be -2piR^2. Basically, I should have an R instead of z, but I don't know where I'm going wrong. I know other approaches will give me the right answer, but can someone identify along my train of thought what the problem with this approach is?

  2. jcsd
  3. Aug 16, 2011 #2
  4. Aug 16, 2011 #3
    I'm looking at it, but judging from the diagram it looks like dBz should equal cosine theta, not sine theta which is what they're claiming.... Still kind of confused.
  5. Aug 16, 2011 #4
    Draw your own picture. When theta goes to 90 degrees dB = dB_z as required.
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