A tiny 0.0250 -microgram oil drop containing 15 excess electrons is suspended between to horizontally closely-spaced metal plates that carry equal but opposite charges on their facing surfaces. The plates are both circular with a radius of 6.50cm. (k = 1/4pi(epsilon)0 = 9.0 x 10^9 N x m^2/C^2, e = 1.6 x 10^-19 C)
a) How much excess charge must be on each plate to hold the oil drop steady?
b) Which plate must be positive, the upper one or the lower one?