Answer
23 J
Work Step by Step
The magnetic energy density is given by equation 21–10.
$$u=\frac{Energy}{Volume}=\frac{1}{2}\frac{B^2}{\mu_o}$$
Solve for the stored energy.
$$Energy=(Volume) \frac{1}{2}\frac{B^2}{\mu_o}$$
$$=(\pi (0.010m)^2(0.36m)) \frac{1}{2}\frac{(0.72T)^2}{4 \pi \times10^{-7}(T \cdot m)/A)}$$
$$=23J$$