Robert Millikan was awarded the Nobel Prize in 1911 for determining (through a now famous experiment) the charge on the electron. This is also known as the elementary charge or fundamental unit of charge. The experiment was simple enough. Oil was sprayed out of an atomizer used for spraying perfume. The small oil drops would pick up some unknown number of electrons due to friction between the oil and the nozzle. These oil drops were sprayed between two oppositely charged parallel plates. Light shined upon the oil drops from the side caused them to show up like little stars when looking through a microscope used to view the region between the plates. The weight of an oil drop could be calculated by using the terminal velocity of the freely falling oil drop and the formula for terminal velocity of a sphere. Once the weight was determined, the voltage across the plates could be adjusted until the upward electric force exactly cancelled out the weight of the oil drop. When the exact balance was reached, the oil drop would remain suspended between the plates.
Use the simulated Millikan Oil Drop experiment below to determine the potential needed to suspend the oil drop. The blue up and down arrows adjust the voltage. The green vector represents the weight of the oil drop and the red vector represents the electric force on the oil drop from the charged plates.
Once the voltage is determined, the charge on the oil drop can be calculated. The problem is that it is not known how many electrons were on each drop. Here Millikan had to rely on hundreds of measurements of charges on different oil drops so he could look at all the charges and determine what the smallest multiple that each one differed by. That smallest amount turned out to be 1.6 x 10-19 Coulombs, the charge on one electron!
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