Problem 31.

In a Millikan oil drop experiment a student sprayed oil droplets with a density of between two horizontal parallel plates that were 6.0 cm apart. The student adjusted the potential difference between the plates to 5000 V so that one of the drops became stationary. The diameter of this drop was measured to be . What was the magnitude of the charge on the drop.

Solution

In the stationary state the gravitational force should be equal to electric force. The gravitation force is

where d is the diameter of the droplet.

This force should be equal to electric force, which is

Then