PX157 - D3c - magnetic field energy

P = V I = L \frac{d I}{d t} I = \frac{1}{2} L \frac{d I^{2}}{d t}

E = \int_{0}^{t_{f}} P d t = \frac{1}{2} L (I^{2} (t_{f}) - I^{2} (0))

assuming $I (0) = 0$ , ie: ramping up the current
in general, energy input/store:

\begin{aligned} U & = \frac{1}{2} L I^{2} \\ U & = \frac{1}{2 μ_{0}} B^{2} V o l . \end{aligned}