A long air-filled solenoid has $1000$ turns, a length of $50\ \text{cm}$ and a cross-sectional area of $10\ \text{cm}^2$. Take $\mu_0 = 4\pi \times 10^{-7}\ \text{T m/A}$. When a steady current of $2\ \text{A}$ flows in it, the energy stored in its magnetic field is closest to