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Test: ACT Science
The Millikin oil drop experiment is among the most important experiments in the history of science. It was used to determine one of the fundamental constants of the universe, the charge on the electron. For his work, Robert Millikin won the Nobel Prize in Physics in 1923.
Millikin used an experimental setup as follows in Figure 1. He opened a chamber of oil into an adjacent uniform electric field. The oil droplets sank into the electric field once the trap door opened, but were then immediately suspended by the forces of electricity present in the field.
Figure 1:
By determining how much force was needed to exactly counteract the gravity pulling the oil droplet down, Millikin was able to determine the force of electricity. This is depicted in Figure 2.
Using this information, he was able to calculate the exact charge on an electron. By changing some conditions, such as creating a vacuum in the apparatus, the experiment can be modified.
Figure 2:
When the drop is suspended perfectly, the total forces up equal the total forces down. Because Millikin knew the electric field in the apparatus, the force of air resistance, the mass of the drop, and the acceleration due to gravity, he was able to solve the following equation:
Table 1 summarizes the electric charge found on oil drops in suspension. Millikin correctly concluded that the calculated charges must all be multiples of the fundamental charge of the electron. A hypothetical oil drop contains some net charge due to lost electrons, and this net charge cannot be smaller than the charge on a single electron.
Table 1:
|
Trial # |
Electric Charge Calculated in Coulombs (C) |
Vacuum Used? |
|
1 |
1.602176487 x 10-8 |
No |
|
2 |
1.602176487 x 10-2 |
Yes |
|
3 |
1.602176487 x 10-6 |
No |
|
4 |
1.602176487 x 10-4 |
Yes |
| 1. | Based only on the information in the passage, which of the following could be the charge of one electron? I. 1.602176487 x 10-6 C II. 1.602176487 x 10-2 C III. 1.602176487 × 10-19 C IV. 1.602176487 × 10-17 C |
I and III
I and II
I, II, III, and IV
III and IV
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