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Millikan Oil Drop Experiment
Ck12 Science
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Printed: January 5, 2016
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Millikan Oil Drop Experiment
1
Millikan Oil Drop
Experiment
• Explain how Millikan used electric fields to find the charge of the electron.
This is the original equipment used by Robert Millikan and Harvey Fletcher in 1909 at the Univeristy of Chicago
to measure the electric charge on a single electron. With incredible perserverence, they were able to determine the
charge to within 1% of the currently accepted value.
Millikan Oil Drop Experiment
The sketch below shows a diagram of the Millikan oil drop experiment. Two parallel charged plates are placed
with the positively charged plate above and the negatively charged plate below. The positively charged plate has a
pinhole opening in its center. An atomizer is used to spray drops of oil into the apparatus. The friction of the oil
passing through the narrow opening puts a charge on many of the oil drops. As the oil drops fall, one or more pass
through the pinhole opening and enter the area between the plates.
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The charge on the plates is adjustable. By measuring the terminal velocity of the oil drops with the electric field
off, Millikan could determine the mass of the drops. Millikan and his graduate assistant were able to determine the
force of the field on the drops when it was turned on by comparing the velocity of the drops with the field on to their
velocity with the field off. This is easily determined when the oil drop is stationary; namely, when the downward
gravitational force exactly equals the upward electrical force on the drop.
Example Problem: An oil drop weighs 1.9 × 10−14 N. It is suspended in an electric field whose intensity is
4.0 × 104 N/C. Since the oil drop is suspended, the gravitational force, 1.9 × 10−4 N, is equal to the electrical force,
FE = Eq. What is the charge on the oil drop?
Solution: q =
weight
E
=
1.9×10−14 N
4.0×104 N/C
= 4.8 × 10−19 C
The charge on this particular oil drop was 4.8 × 10−19 C. In doing his experiment, however, Millikan faced a
problem. When the oil is sprayed through the atomizer, some oil drops are negatively charged, but we don’t know
how many extra electrons the drops acquire. The charge on this oil drop could be the result of having one extra
electron, or having five extra electrons.
In order to determine the charge on one electron, the oil drop experiment was carried out many times, and the charges
on many oil drops was determined. The smallest charge was found to be 1.6 × 10−19 C, and all the other charges
on oil drops were found to be whole number multiples of 1.6 × 10−19 C. In the example problem above, we would
conclude that the oil drop held three extra electrons.
Summary
• Using a uniform electric field between two charged parallel plates and oil drops, Millikan determined the
charge on a single electron.
• The friction of the oil passing through the narrow opening puts charges on many of the oil drops.
• When an oil drop is stopped by adjusting the charge on the plates, its weight, mg, downward equals the
electrical force, Eq , upward.
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Chapter 1. Millikan Oil Drop Experiment
Practice
Questions
The video below covers the Millikan oil drop experiment in more detail. Use this resource to answer the following
questions.
http://www.youtube.com/watch?v=XMfYHag7Liw
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/64694
1. What was the purpose of the X-rays passing through the instrument?
2. At which university was the oil drop experiment conducted?
Review
Questions
1.
2.
3.
4.
Give reasons for each drop requiring a different voltage to balance.
Why do some oil drops not react to adjustments in voltage?
Why do some drops fall faster instead of slower when you increase the voltage?
In another universe, where the unit for charge is the “mork,” a physicist performed the Millikan oil drop
experiment and measured the following charges on a series of oil drops.
TABLE 1.1:
Trial
1
2
3
Charge
2.62 × 10−13 mork
3.93 × 10−13 mork
1.31 × 10−13 mork
Trial
4
5
Charge
5.14 × 10−13 mork
6.55 × 10−13 mork
5. Which of the following is the best choice for the charge on an electron in morks?
1.
2.
3.
4.
5.
2.62 × 10−13 mork
6.55 × 10−13 mork
1.31 × 10−13 mork
1.6 × 10−13 mork
None of these.
6. An oil drop weighs 1.9 × 10−15 N and is suspended in an electric field of 6.0 × 103 N/C.
1. What is the charge on the drop?
2. How many excess electrons does it carry?
• Millikan oil drop experiment: The purpose of Robert Millikan’s oil drop experiment (1909) was to measure
the electric charge of the electron. He did this by carefully balancing the gravitational and electric forces on
tiny charged droplets of oil suspended between parallel charged metal plates.
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References
1. . http://commons.wikimedia.org/wiki/File:Millikan%E2%80%99s_oil-drop_apparatus_1.jpg .
2. Zachary Wilson. CK-12 Foundation .
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