In 86, there is a description of the Millikan experiment

Chapter , Problem 87

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In 86, there is a description of the Millikan experiment used to determine the charge on the electron. During the experiment, a switch is used to reverse the direction of the electric field without changing its magnitude, so that one can measure the terminal speed of the microsphere both as it is moving upward and as it is moving downward. Let represent the terminal speed when the particle is moving up and the terminal speed when moving down. (a) If we let show that where is the microspheres net charge. For the purpose of determining what advantage does measuring both and have over measuring only one terminal speed? (b) Because charge is quantized, can only change by steps of magnitude , where is an integer. Using the data from 86, calculate . SSM