Evans SPH 4U1

Unit 6: Electricity

Note 6: Millikan Experiment

Reference: Chapter 7.5

http://www.bun.falkenberg.se/gymnasium/amnen/fysik/millikaneng.html

Recall Terminal Velocity:

Terminal velocity is the speed where a falling body stops accelerating due to air resistance (i.e. magnitude of air resistance force = magnitude of force of gravity)

Millikan's Experiment:

He discovered that there was an elementary charge of e = 1.602 x 10-19C.

He oppositely charged two horizontal and parallel plates.  In the upper plate was a series of very small holes. He sprayed a mist of tiny oil droplets into the upper plate from above.  Initially, there was no charge on the plates. Most of the oil droplets would fall onto the top plate but some would fall through the holes into the space between the plates. The tiny oil droplets quickly reach their terminal velocity (note that mist particles fall very slowly)..  Through the use of magnifying lenses he was able to measure the terminal velocity of the oil droplets.  By knowing the terminal velocity, he was able to determine the oil droplet's mass.

After measuring the velocity of the oil drop, and while it was still slowly falling in the space between the plates, Millikan then applied a potential difference between the plates.  The electric force acting on the charged oil drop affected the terminal speed at which it fell. Using the difference in the terminal speed and the amount of electric field, he was able to calculate the amount of charge on the falling oil drop.

He measured the charge on thousands of oil drops.  The charge on each drop was found to be a multiple of a basic quantity of charge (a quantized amount) This basic quantity of charge is called the elementary charge, e, and is equal to the magnitude of the charge on the electron,

1.602x10-19C..

Simplified:

For a positive charged drop, the electric force is

When the droplet no longer falls from one plate to the other (i.e. it is in balance),

Recall from 7.4

is the electric potential difference between the plates and r is the separation between the plates.

Therefore,

Example:

In a Millikan experiment, two horizontal plates are 3.0 cm apart. A latex sphere, of mass 1.9 x 10-15kg, remains stationary when the potential difference between the plates is 500V with the upper plate positive.

a) Is the sphere positively or negatively charged? ( negative)

b) What is the magnitude of the charge on the sphere? (1.1x10-18C)

c) How many excess or deficit electrons does the sphere have? (7 excess)

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