$($a$)$ State briefly why capacitors are used in electric systems. If a $68F$ parallel air capacitor separated by $2$ mm between the plates and $48$ V is applied across the plates, $(1)$ Find the charge deposited on the plates, the electric field between them and the energy stored in the capacitor, $($ii$)$ If the distance between the plates is reduced to $1$ mm and a dielectric material of $lo{n}_r=5\mathrm{.}0$ is inserted between the plates, Find the new capacitance $C^{\text{'}},$ Charge $Q^{\text{'}},$ elecric field and the energy stored. $[$Hints: $[$:$Q=CV$;$E=\frac{V}{d}$;$W=\frac{1}{2}C{V}^2$