There is a spherical capacitor. Assume that the two spherical shells of the capacitor have radius

aa$=90$nm and bb$=100$nm$,$ the interior is dielectric for which rr$=3\mathrm{.}0.$

$($a$)$ Calculate the capacitance C$.$

$($b$)$ If the membrane potential is $70$mV$,$ calculate the energy stored in the cell membrane.

$($The membrane potential is $70$mV$$ here means that the potential of the outer shell is

$70$mV while the inner shell is grounded.$)$