Physics parallel plates question ?

Question

Kind of confused about what to do here:

A pair of parallel plates are placed 0.0200 m apart and the top plate is positive. When the potential difference across the plates is 240.0 V, an oil drop of mass 2.0 × 10^-11 kg gets suspended between the plates. (e = 1.6 × 10-19 C)

(a) Draw a free-body diagram for the charge.

(b) What is the charge on the oil drop?

(c) Is there an excess or deficit of electrons on the oil drop? How many electrons are in excess or deficit?

Answer 1

(a) "suspended" means that the weight (m*g, down) is equal to the electrostatic force (q*E, up). Simple enough.

(b) field E = V/d = 240.0V / 0.0200m = 12000 N/C

q = mg/E = 2.0e-11kg * 9.8m/s² / 1200N/C = 1.6e-14 C

(c) "top plate is positive" means that the field points down; in order to get an upward force on the droplet (required to offset the downward weight force), the droplet must be negatively charged. Therefore we have an excess of electrons.

n = 1.6e-14C / 1.6e-19C/e ≈ 1e5 electrons

(100,000 electrons)

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