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# The Millikan oil drop experiment

last edited by 5 years, 2 months ago

4.2.4.P15 and 8.2.P6

 In 1909, Millikan and Fletcher ionized tiny oil droplets in air and watched them fall and rise in an electric field (see simplified model at right). They were able to roughly determine the magnitude of the charge of an electron which we know now is e = 1.6 x 10–19 Coulombs. Note that these droplets were small enough to reach terminal velocity quickly without an electric field present, but also small enough that the force of buoyancy was not an issue. Also, they are usually too far apart to affect each other much.

a. Using arrows (→, ←, ↑, ↓, N for none, or K for insufficient information) identify which direction the electric field must be pointing for the droplets to rise if they are negatively charged. (You may simply model the two charged plates as one positive charge and one negative charge respectively, but it is up to you to decide which one is above the oil drop and which one is below.)

b. If, by adjusting the electric field, the experimenter could get a droplet to hover in one spot in an electric field of 100 N/C, calculate what the magnitude of the charge/mass ratio |q/m| is for this droplet. Show all work.

Alan Peel 11/13/14