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3 Measurement of the Electron Charge

Modern Physics

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PHY251 - Measurement of Electron Charge PHY 251 Measurement of the Electron Charge Introduction The purpose of this experiment is to measure the smallest unit into which electric charge can be divided, that is, the charge of an electron e . The method is the one proposed by R.A. Millikan in 1910. A small sphere of mass m having a charge q can be suspended in air by applying an electric field of field strength E to balance the gravitational force on it. We then have m g = q E . We neglect here the (very) small buoyant force. The charge q will in general not be the electron charge but rather an integral multiple of it: q = n e , with n = 1, 2, 3, ... When the measurement is repeated several times, e can be found as the largest common denominator of the measured charges q . In the absence of an electric field, the electrons will reach a constant terminal velocity v T after a short time. The viscous force balances the gravitational force, so that the net force acting on the droplet is zero and we have: m g = K v T ,
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