Download Exploration of the Millikan Oil-Drop Experiment

Exploration of the Millikan Oil-Drop Simulation
The purpose of this exploration is to become familiar with the simulation of a simplified version
of the Millikan oil-drop experiment and to review the underlining physics principles necessary to
understand his experiment. As you work through this exercise, have the applet open in a separate
window (if necessary, click on the link MillikanLike.swf). If you need help understanding the
apparatus and variables, see the Help menu for assistance before starting this exploration.
Once you have finished this exploration, you should be able to conduct a simulation of
Millikan’s experiment and complete a formal lab report (see Simulate Experiment under the
Resources menu of the applet).
Note: The simulation is a model and, like all models, has some limitations. Millikan’s original
experiment is very tedious and time consuming. This applet includes a number of simplifications:
 only one drop is observed rather than many drops;
 each drop has a random charge assuming some charging method; Millikan used friction
of drops from an atomizer spray and ionizing X-rays to charge the drops;
 drops fall in a vacuum rather than in air, but the principle of a zero net force still applies
as in the original experiment;
 “real-time” graphing of velocity is available versus Millikan’s timing of a drop moving,
up or down, through a marked distance;
 mass of a drop is given versus Millikan’s method of measuring the terminal velocity and
using the density of the oil and Stoke’s law to calculate the mass;
Velocity of an Oil-Drop
Millikan would have sprayed numerous oil drops into the upper chamber and observed many oil
drops falling through the opening. Part of Millikan’s experiment involved observing an oil drop
falling with no electric field present. In the Millikan applet, select “Force Directions” under the
Options menu. With the voltage set at zero, click “Start”.
1. Name the force(s) acting on the oil drop. What equation would be used to calculate the
value of this force?
Click “Rewind” and select “v-t Graph” under the Options menu. Click “Start” to release the
drop.
2. Describe and explain the shape of the graph.
3. Click the slope tool icon on the graph and click on several points along the line. What
does this value represent?
Effect of an Electric Field
Click “Reset” and have both “Force Directions” and “v-t Graph” selected under the Options
menu. Move the Voltage slider to about one-half of the maximum value. Click “Start” and
observe the oil drop.
4. Describe the shape of the graph. Use the slope tool to provide some quantitative
observations as part of your description.
5. What is the sign of the charge on the oil drop? How do you know?
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6. Why is there no electric force on the drop in the upper chamber? Verify your answer by
selecting “Electric Field Direction” under the Options menu.
7. What equation would be used to calculate the magnitude of the electric force acting on
the drop in the lower chamber?
Balanced Forces
In Millikan’s original experiment and in this simulation, the voltage between the plates is
adjusted to produce balanced forces on a charged oil drop; that is, Fnet  0 .
8. If the electric and gravitational forces are balanced, predict the shape of the v-t graph.
What would be the slope of this line?
Click “Reset” and make sure that both “Force Directions” and “v-t Graph” are selected under the
Options menu. Move the Voltage slide to about one-half of the maximum value. Click “Start”
and then click “Pause” when the oil drop has fallen part way between the plates.
9. Note the force arrows. Is the magnitude of the electric force larger or smaller than the
gravitational force?
10. Does the voltage have to be increased or decreased to make the electric force equal in
magnitude to the gravitational force? Justify your answer with appropriate equations.
Move the voltage slider so that the force arrows appear to be the same length. Click “Start” to let
the drop continue falling and then immediately click “Pause”. Check the slope of the graph to see
if the forces are balanced.
Continue moving the voltage slider and making finer adjustments by entering voltage values
using the box (#) until you obtain a slope equal to zero. When you have balanced the forces,
click “Record Data” to record your results in the Evidence table.
Note: If at any time the drop reaches the bottom before you have balanced the forces, click
“Rewind” to keep the same charged drop and voltage setting.
11. When the electric and gravitational forces are balanced, show how the charge on the oil
drop is calculated.
Determining the Smallest Unit of Charge
Millikan’s experiment took many years of work. He measured thousands of oil drops of various
sizes and charges and used different oils. The evidence of the movement of the oil drops in an
electric field clearly shows that they are charged, but there is no way to know how many units of
charge are present on a particular oil drop. Millikan knew this and purposely changed the charge
on particular oil drops by irradiating them with X-rays. Ultimately his aim was to obtain a set of
results from which a pattern could be deduced.
Millikan suspected that the charges he obtained were fairly large multiples of the smallest unit
charge. This means that it is not so easy to see whether they are integer multiples of the same
smallest unit. To understand how Millikan did his analysis, consider the following analogy:
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A group of sealed boxes each contain an unknown number of identical metal balls. The mass
of each box is measured. What is the mass of one metal ball?
Box
1
Mass (g) 32.2
2
16.1
3
25.3
4
29.9
5
9.2
6
41.4
7
13.8
8
23.0
9
6.9
10
34.5
The solution is to realize that the difference in the masses of two boxes would likely
represent a smaller number of metal balls than any individual box contains.
12. To make this analysis even more obvious, rearrange the masses in order from highest
to lowest.
13. Subtract the adjacent masses (e.g., 41.4 g – 34.5 g) to obtain nine mass differences.
14. What is the likely mass of one metal ball?
15. Test this conclusion by determining the number of balls present in each box. Are the
results consistent with the smallest mass found in the previous question?
16. What assumption is being made in this analogy?
Significance of Millikan’s Oil-Drop Experiment
At the time of Millikan’s experiments, there was still widespread disagreement about the
existence of subatomic particles and many scientists also thought that electric charge was a
continuous variable. Millikan’s oil-drop experiment, together with Thomson’s e/m results,
helped considerably to establish the electron as a subatomic particle with a definite electrical
charge. Millikan’s experiment was the first reliable and unambiguous determination of the
charge on an electron, now known as the elementary charge. The elementary charge is now
recognized as one of the fundamental physical constants.
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