A Tiny 00250-µG Oil Drop Containing 15 Excess Electrons Is Suspended Between

Short Answer

A tiny 0.0250-µg oil drop containing 15 excess electrons is suspended between two horizontally closely-spaced metal plates that carry equal but opposite charges on their facing surfaces. The plates are both circular with a radius of 6.50 cm. (k = 1/4πε₀ = 9.0 × 10⁹ N ∙ m²/C², e = 1.6 × 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?