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meteorologist1

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A long thin coil of length l, cross-sectional area S, and n turns per unit length carries a current I. It is placed along the axis of a large circular ring of radius a, which is carrying a current I'. If d is the displacement of the center of the coil from the center of the ring along the coil axis, find the force on the coil as a function of d.

I'm not sure what formulas to use, and what I have to integrate to get the following result:

[tex] F = -\frac{\mu_0 II'nSa^2}{2} ((a^2 + (\frac{l}{2} - d)^2)^\frac{-3}{2} - (a^2 + (\frac{l}{2} + d)^2)^\frac{-3}{2})[/tex]

Thanks.