Expected Results
Following the previous simulated procedure using pennies, the slope of the line in Figure 122-2 is 2.7 grams, which is the mass of a single penny. This is a reasonable average for pennies minted before and after 1982. A more precise value can be established by sorting pennies into groups before and after 1982.
The charge of an electron determined by Millikan is –1.6 × 10–19 Coulombs.
Other Things to Try
Marbles can be used to simulate the logical process pursued by Millkan in a similar manner that was done with pennies. The marbles have a greater mass, which may make it easier to detect difference. However, finding a relationship graphically may be more difficult because of the variation in mass for a random set of marbles.
Why It Works
The size of an oil drop is found by observing its free-fall velocity in air. The oil drop is then given a charge by exposing it to ionizing radiation. The electric field that establishes equilibrium with gravity is related to the force. Although the exact number of electrons on any give oil drop cannot be determined directly, the common multiple leads us to identify the charge of a single electron.
The Point
The Millikan oil-drop experiment determines the charge of an electron by measuring the response of an oil drop charged by electrons in an electric field.
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