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Homework Help: Finding the B-field at a point outside ring of current IN Plane of ring

  1. Sep 4, 2008 #1
    1. The problem statement, all variables and given/known data

    Determine the B field at a point P with distance d from centre of current ring radius r (d>r ie outside ring of current) but IN the Plane of ring (i.e off the axis)

    2. Relevant equations

    Biot savart: db=(u.I/4pi.r^2).dl.sinx

    u=mu, I=current, x=angle made with current vector element dl and vector r connecting P to dl.

    3. The attempt at a solution

    the angle x will vary at all points along ring with vector joining point P, as will r.

    I will integrate from 0 to 2pi my expression for db but with r^2=a^2+d^2-2adcosy

    where y is angle between radial vector and d.

    I am stuck because cannot determine how to express y in terms of x. and thus cannot integrate!

    many thanks!
  2. jcsd
  3. Sep 5, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi maxmax1! :smile:

    Hint: P is at (d,0).

    The tangent at Q = (r cosy, r siny) is … ?

    So the angle betweent he tangent and PQ is … ? :smile:
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