A tiny 0.0250 microgram oil drop containing 15 excess electrons is suspended between to horizontally closely-spaced metal plates that carry same but opposite charges on their facing surfaces. The plates are both circular with a radius of 6.50 centimeter. (k = 1/4pi(epsilon)0 = 9.0 x 109 N·m2/C2, e = 1.6 x 10-19 C).
(a) Find how much excess charge must be on each plate to hold the oil drop steady?
(b) Determine which plate must be positive, the upper one or the lower one?