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March 27, 2017

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Consider a long wire where the current density is not uniform but instead increases as you approach the center of the wire, so that at a distance r from the center the current density is J(r)= I/ 2PiRr. Find the magnetic field strength both inside and outside of this wire.
I know I need to use biot-savart law, but im not sure how to do this with a non uniform density

  • physics - ,

    Integrate the current from r = 0 to r = r, using the current density.

    I(r) = Integral (r=0 to r) of J*2*pi*r dr

    = I r/R where R is the radius of the wire


    Then use Ampere's law that says the integral of H (=B/mu) around the loop equals the current flowing through the loop. The value of B will be uniform around that circular loop because of symmetry.

    B(r) = mu*2*pi*(r/R)*(I/r)
    = mu*2*pi*(I/R)
    which is independent of r.

    Outside the wire, use Ampere's law again:

    B(r) = mu*2*pi(*I/r)

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