$23\mathrm{.}70($a$)$ If a spherical raindrop of radius $0\mathrm{.}650$ mm carries a charge of

$$

$3\mathrm{.}60$

$$

pC

uniformly distributed over its volume, what is the potential at its surface? $($Take the potential to be zero at an infinite distance from the raindrop.$)($b$)$ Two identical raindrops, each with radius and charge specified in part $($a$),$ collide and merge into one larger raindrop. What is the radius of this larger drop, and what is the potential at its surface, if its charge is uniformly distributed over its volume?