Download 7.6 - Millikan Oil Drop Experiment

7.6 The Milikan Experiment: The Elementary Charge
Elementary Values
 What is the smallest denomination of money that you can possess?
 What is the smallest quantity of rice that you can eat?
 What is the smallest amount of charge an object can possess?
1 penny
1 grain
1 electron
Milikan’s Experiment
 Milikan’s experiment answered two questions about charge:
o Does there exist a smallest unit of electric charge? (i.e. a fundamental charge)
o What is this elementary charge and what is its magnitude (in coulombs)?
 We know now that the electron is the smallest unit of charge.
 Millikan reasoned that the elementary charge would be the charge on an electron.
Milikan’s Oil Drop Experiment
 When tiny oil drops are sprayed from an atomizer, they get charged by friction.
 Millikan created a uniform electric field using two charged plates.
 By varying the voltage, he could suspend the charged oil drops in mid-air.
 For a positively charged drop of mass m and charge q, the electric force
acts upward if the lower plate is positively charged.

FE
q

 If the drop is balanced:

FE

Fg
q
mg
or
q
mg
V
d
 However, remember that the electric field in the region between two plates is equal to:
Therefore
q
mg d
The charge on a balanced drop can be determined if mass is known.
V
 How did Millikan determine the mass of an oil drop?
o He measured its terminal speed (free fall) as the plates are disconnected from the battery.
 Millikan repeated the experiment several times until he had a long list of values for charge.
 These values were all multiples of the elementary charge.
 The currently accepted value for the elementary charge is: e = 1.602 x 10-19 C
 An object with an excess (or deficit) of electrons has a charge q that is given by:
Where N is the # of excess or deficit electrons and e is 1.602 x 10-19 C
q
Ne
Example: In a Millikan-type experiment, two horizontal plates are 2.5 cm apart. A latex sphere of mass 1.5 x
10-15 kg remains stationary when the potential difference between the plates is 460 V with the upper plate
positive.
a) Is the sphere charged positively or negatively?
b) Calculate the magnitude of the charge on the sphere.
c) How many excess or deficit electrons did the latex sphere in example 1 have?
Your Work: Page 365 #1,3,5