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Homework Help: Magnetic Field inside and external to a wire

  1. Apr 3, 2008 #1
    I again bring a question: If a wire of radius R carries a current density J = br (r is radius and b is a constant), in which J = I/A (A is area)...then derive an expression for the magnetic field at r1 (r1 being a radial distance less than R) and at r2 (r2 being a radial distance greater than R).

    My answer at r1 is B = (u*b*r1^2)/2 and at r2 is B = (u*b*R^3)/(2*r2), in which u is the permeability of free space.
    The actual answer at r1 is B = (u*b*r1^2)/3 and for r2 is B = (u*b*R^3)/(3*r2)...which seem to match my answers...just instead of halving each one...its divided by 3...how did they get that?
  2. jcsd
  3. Apr 3, 2008 #2
    Actually...nevermind...I found out why...I apparently had to integrate for increments of I.
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