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millikan

# millikan - THE MILLIKAN OIL-DROP EXPERIMENT REFERENCES...

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THE MILLIKAN OIL-DROP EXPERIMENT REFERENCES 1.R.A. Millikan, The Electron (photocopied excerpts available at the Resource Centre). 2.Instruction Manuals for Leybold-Heraeus apparatus (available at the Resource Centre). (2 weights) INTRODUCTION This experiment is one of the most fundamental of the experiments in the undergraduate laboratory. The experimental apparatus is patterned after the original apparatus, made and used by R.A. Millikan to show that electric charge exists as integral multiples of “e” the charge on a single electron. Historically, this experiment ranks as one of the greatest experiments of modern physics. GETTING STARTED This method first described in 1913, is based on the fact that different forces act on an electrically charged oil drop moving in the homogeneous electric field of a plate capacitor. When the plate capacitor’s electric field intensity is E , the following forces act on a droplet of charge Q : gravitational force m oil g , where m oil is the mass of the oil drop, buoyant force m air g, where m air is the mass of air displaced by the oil drop, electric force QE , and, only if the droplet, considered in this case as a sphere, moves against the ambient air: Stokes’ resistance force. Stokes’ Law states that for a spherical object of radius r moving through a fluid of viscosity η at a speed v under laminar flow conditions, the viscous force F on the object is given by F 6 π r η v . The viscous force always opposes the motion and is, of course, responsible for the steady terminal velocities observed when a drop falls in air.

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2 We suggest the following method to find the charge on an oil drop: (i) measure the fall velocity v 1 of a droplet in the free space (at zero voltage) and also (ii) the rise velocity v 2 of a droplet at a definite voltage. Write down the equation for a charged oil drop falling freely (in zero electric field) in air under gravity and the equation for the same drop moving in the opposite direction due to an applied electric field. Eliminate the radius of the drop from these two equations to obtain an expression for the charge on the drop in terms of measurable quantities and the constants given below.
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millikan - THE MILLIKAN OIL-DROP EXPERIMENT REFERENCES...

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