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Millikan’s Experiment

Background: Early 1800’s-Dalton- Theory of the Atom. 1896- J.J. Thomson- determined that cathode rays were made up of negatively charged particles -measured the charge-to-mass ratio of an electron. Millikan’s Experiment.

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Millikan’s Experiment

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  1. Background: Early 1800’s-Dalton- Theory of the Atom 1896- J.J. Thomson- determined that cathode rays were made up of negatively charged particles -measured the charge-to-mass ratio of an electron Millikan’s Experiment 1907- Rutherford’s Gold Foil experiment-discovery of nucleus 1909- Robert Millikan-Oil Drop Experiment: measured the charge on an electron (the fundamental unit charge, e) Millikan’s Experiment: -produced a small oil droplet with an atomizer, allowed it to fall freely and then under the influence of a constant electric field 1) Mass Determination allowed the drop to fall freely until it reached terminal velocity – this allowed him to determine the mass of the oil drop Fv (viscous drag) Constant velocity Millikan’s Experiment... Fg

  2. 2) Charge Determination -Turned on voltage across charged parallel plates as the drop fell The electric force used to balance the gravitational force! + Recall the electric field between the plates is given by:  =  V d  V d F  Fg - Millikan determined the charges on the drops varied in integral multiples of a fundamental value, e: q = ne where n = 1, 2, 3, 4, etc. FE= Fg q  = mg q V = mg d Through analysis of many experiments he determined the fundamental charge, e = 1.60 x 10 -19 C q = mgd  V

  3. Example: Two horizontal plates are 3.0 cm apart. A latex sphere (m=1.5 x 10-15 kg) remains stationary when a potential difference of 345 V is placed across the plates. The upper plate is positive: a) Sketch the system. Is the sphere positive or negative? b) Find the charge magnitude. c) What is the number of excess or deficit electrons on the sphere? Answer: a) negative, b) q = 1.28 x 10-18 C c) n =7.98 ~ 8

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