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Simulation: Millikan’s Oil Drop Experiment
A simulation in physics is a representation of a real-world experiment (typically very difficult to
carry out) through the use of an analogous (but typically easier to carry out) experiment that
models the internal processes of the simulated one.
If we can’t do something for real, we do something “just like” it.
Background for Today’s Activity
In 1897, J. J. Thomson announced that cathode rays are negatively charged particles, which we
now call electrons (cathode rays—beams of electrons—are used to “draw” the pictures in tubetype televisions and computer monitors). Thomson realized the mass of an electron is about
1/1000th the mass of a hydrogen atom, but was not able to measure the charge of the electron.
How can you measure the properties of something with 1/1000th the mass of the lightest atom,
and is too small to be seen?
During a series of experiments between 1909 and 1913, Robert Millikan devised an experiment
to measure the charge of an electron. He sprayed very fine drops of oil into the space between
electrically-charged metal plates. Friction caused the drops to acquire a net electric charge.
A drop would fall under the influence of gravity, and its size could be determined by measuring
its speed of fall. Millikan could also apply a voltage to the metal plates, creating an electric force
in the opposite direction to the gravitational force. If the size of the drop was known, the charge
on a drop held stationary by the opposing electric and gravitational forces could be calculated.
After looking at many drops, Millikan came to a remarkable conclusion: the charge on every
drop was an integral multiple of a basic charge. This basic charge is the charge of a single
Millikan won a Nobel Prize for measuring the charge of something too small to be seen or
“counted” in any conventional way. Today it is commonplace for scientists and engineers to
manipulate objects too small to be seen. Today’s simulation will present such an experiment.
In our simulation, we will determine the mass of an object which we imagine is too small to be
seen or counted. You’ll have to use your imagination and pretend you can’t see or count.
We can, however, measure the mass of an unknown number of these objects, and use the result
to infer the mass of a single object.
By the way, Millikan’s original experiments on the charge of an electron generated a lot of
controversy. His second set of improved experiments produced a more accurate result, and won
him the Nobel prize. Interestingly, Millikan did “cosmetic surgery” on some of his data to
produce a “better” result. What do you think of that?
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You’ll be given simulated oil drops (envelopes) charged with electrons (filled with random
numbers of unknown objects). Your task is to determine the mass of the electron (the mass of the
unknown object).
I’ll see if you can develop, as a group, a procedure to determine the unknown mass. If necessary,
you can adapt the procedure on the Student Activity side of the “Good to the Last (Oil) Drop”
If necessary, use the detailed procedure below. No Peeking unless I give you permission!
Detailed Procedure
1. Create a data table with five columns: Mass with Envelope, Measured Mass, Sorted Mass,
Unique Masses, Difference in Mass.
2. Measure the mass of an envelope plus objects. Record the result the “Mass with Envelope”
3. Subtract the envelope mass (5.1 grams) from the “Mass with Envelope” mass. Record the
result in the “Measured Mass” column.
4. Repeat steps 2 and 3 for all your envelopes.
5. Fill in the “Sorted Mass” column of your data table by arranging all the masses in descending
6. Copy into the “Unique Mass” column only the unique masses from the “Sorted Mass” column.
Question: what should you do if two or more of the “Sorted Mass” masses are different but seem
to be almost the same? Answer: take their average and record that average in the “Unique Mass”
7. Make sure your “Unique Masses” are arranged in descending order. Subtract each mass from
the one above it and record the difference in the “Differences in Mass” column. Note that this
column will have one less entry than the “Unique Mass” column
8. Use your “Difference in Mass” data to determine the mass of a single “electron” (the unknown
object in the envelope).
“If the envelopes are identical, you don’t need to know their mass.” Explain why you agree or
disagree with that statement.
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Be aware that the measuring instrument may have errors, and that these errors may vary with
measured value.
Could you design a series of measurements that could help you understand the measuring
instrument errors.
Suggestions: measure the mass of one object multiple times. Measure the mass of a group of
several of the objects multiple times. Make multiple measurements of the masses of different
groups of several objects. Measure the mass of one object, add another object and measure the
combined mass, continue adding more of the object one at a time and measuring the combined
Something to Think About
“The most important thing we look for when we hire someone is the ability to take and analyze
data.” Dr. Ryan Feeler, Senior Engineer – Diode Lasers, Northrop Grumman / Cutting Edge
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