A $5.80-\mu \mathrm{F}$, parallel-parallel-plate, air capacitor has a plate separation of 5.00 $\mathrm{mm}$ and is charged to a potential difference of 400 $\mathrm{V}$ . Calculate the energy density in the region between the plates, in units of $\mathrm{J} / \mathrm{m}^{3} .$