The diagram shows an oil droplet that has become charged by gaining five electrons.

Question 1

The diagram shows an oil droplet that has become charged by gaining five electrons. The droplet remains stationary between charged plates.

What is the magnitude and direction of the electrostatic force on the oil droplet?

A 5.0 × 10^–15 N upwards

B 5.0 × 10^–15 N downwards

C 5.0 × 10^–13 N upwards

D 5.0 × 10^–13 N downwards

Reference: Past Exam Paper – June 2008 Paper 1 Q31

Solution 1:

Answer: C.

Consider the forces acting on the charged oil droplet.

The weight of the oil droplets causes a downward force.

Being negatively charged (due to the electrons), the oil droplet is under an upward electrostatic force attracting it to the positive plate.

Electrostatic force = EQ        

where E is the electric field strength and Q is the total charge of the droplet

Electric field strength = V / d = 5000 / (0.8×10^-2)

Total charge = 5 × charge of an electric = 5 × (1.6×10^-19)

Electrostatic force = EQ = (V/d) × Q = 5.0 × 10^-13 N