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**1. Homework Statement**

Assume that the magnitude of the magnetic field outside a sphere of radius R is B = B0 (R/r)^2. Determine the total energy stored in the magnetic field outside the sphere.

**2. Homework Equations**

I think it's necessary to use the energy density equation.

u = B^2/(2*u0)

total energy = u * volume.

**3. The Attempt at a Solution**

By just plugging in the given data, I come up with (2*pi*B0^2*R^3)/(u0*3). Assuming that r = R. Or 2*pi*B0^2*R^4/(3r*u0) if I don't assume that. My book's answer is similar except that the 3 in the denom. is not there. So I think I need to use calculus but I don't quite see how to set it up...

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