The parallel plates in a capacitor, with a plate area a $8\mathrm{.}50$ cm$^2$ an an air$-$filled seperation of $4\mathrm{.}90$ mm$,$ are charged by a $5\mathrm{.}90$ V battery. They are then disconnected from the battery and pulled apart $($without discharge$)$ to a speration of $8\mathrm{.}90$ mm$.$ Neglecting fringing, find $($a$)$ the potential difference between the plates, $($b$)$ the initial stored energy, $($c$)$ the final stored energy, and $($d$)$ the work required to separate the plates.