An air$-$filled spherical capacitor is constructed with an inner$-$shell radius of $6\mathrm{.}80$ cm and an outer$-$shell radius of $14\mathrm{.}4$ cm$.$

$($a$)$ Calculate the capacitance of the device.

pF

$($b$)$ What potential difference between the spheres results in a $4\mathrm{.}00-$C charge on the capacitor?

kV