A Tiny 00250- $\mu \mathrm { g }$ Oil Drop Containing 15 Excess Electrons Is Suspended Between Two

question 27

Short Answer

A tiny 0.0250- $\mu \mathrm { g }$ 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 $6.50 \mathrm {~cm} . \left( k = 1 / 4 \pi \varepsilon _ { 0 } = 9.0 \times \right.$ $\left. 10 ^ { 9 } \mathrm {~N} \cdot \mathrm { m } ^ { 2 } / \mathrm { C } ^ { 2 } , e = 1.6 \times 10 ^ { - 19 } \mathrm { C } \right)$
(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?

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