A 0.80-μm-diameter oil droplet is observed between two parallel electrodes spaced 11 mm apart. The droplet hangs motionless if the upper electrode is 22.8 V more positive than the lower electrode. The density of the oil is 885 kg/m^3.
(a) What is the droplet's mass?
(b) What is the droplet's charge?
(c) Does the droplet have a surplus or a deficit of electrons? How many?

Explanation:

Given that,

Diameter of the oil droplet,

Radius,

Separation between the electrodes, d = 11 mm

The droplet hangs motionless if the upper electrode is 22.8 V more positive than the lower electrode.

The density of the oil is

(a) Density of oil is given by mas per unit volume.

(b) The electric force is balanced by the mass of the object. So,

qE = mg

E is electric field, E = V/d

(c) Charges come in integer multiples of the electronic charge units of e. So,

Hence, this is the required solution.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.