Problem: .Millikan’s oil-drop experiment to measure the charge of the electron. Small drops of oil are allowed to fall between electrically charged plates. Millikan measured how varying the voltage between the plates affected the rate of fall. From these data he calculated the negative charge on the drops. Because the charge on any drop was always some integral multiple of 1.602 10-19C, Millikan deduced this value to be the charge of a single electron.Are the masses of the oil drops changed significantly when electrons accumulate on them?

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We have to determine whether the mass of an oil drop used in the Millikan oil drop experiment would be changed significantly by accumulation of electrons.


To solve this problem, we need to know the mass of an electron.

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Problem Details

An experimental apparatus has an upper chamber into which oil drops are sprayed over a plate with a positive charge. Drops can fall through a hole in the plate to a lower chamber, into which x-rays are beamed above a negatively charged plate. (1) X-ray irradiation causes drops to pick up electrons and become negatively charged. (2) The force of gravity pulls drops downward but is opposed by the electric field that pushes the negatively charged drops upward. A microscopic view shows drops of different sizes moving upward and downward.

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Millikan’s oil-drop experiment to measure the charge of the electron. Small drops of oil are allowed to fall between electrically charged plates. Millikan measured how varying the voltage between the plates affected the rate of fall. From these data he calculated the negative charge on the drops. Because the charge on any drop was always some integral multiple of 1.602 10-19C, Millikan deduced this value to be the charge of a single electron.

Are the masses of the oil drops changed significantly when electrons accumulate on them?