In the exam you are expected to know about:
determination of Q
Condition for holding a charged oil droplet, of charge Q, stationary between oppositely charged parallel plates;
(QV)/d = mg
Motion of a falling oil droplet with and without an electric field;
terminal speed, Stokes’ Law for the viscous force on an oil droplet used to calculate the droplet radius
F = 6phrv
Quantisation of electric charge.
Robert Millikan used a simple experiment that served to confirm the unit electronic charge as 1.6 × 10-19 C. He sprayed oil drops into a space between two charged plates. Each tiny oil droplet was charged up by friction as it left the sprayer. The theory was simple; the attractive electrostatic force between the droplet and the positively charged plate would balance out the weight of the droplet. His apparatus was like this:
He would select a particular oil drop and hold it stationary by altering the voltage between the two plates.
The forces on the stationary drop are like this:
We know that:
the electric force = electric field × charge (F = Eq)
the electric field strength in a uniform field, E = V/d
We can see that if the forces are balanced;
This method leaves us with a problem, calculating the mass of a single, tiny oil drop is too difficult to directly do accurately. Millikan used the following method to calculate the mass; he turned off the plates and watched the oil drop. Very quickly the oil drop reached terminal speed.
So we can write:
mg = drag force
The drag force can be worked out indirectly using Stoke's Law, which describes the force acting on a sphere falling at terminal speed through a …