The Determination of the Charge on the Electron (Millikan) - Turning Points - A2 Physics - AQA

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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 


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