Teaching Physics with the Physics Suite

Edward F. Redish

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The size of an oil drop

In the Millikan oil drop experiment, an atomizer (a sprayer with a fine nozzle) is used to introduce many tiny droplets of oil between two oppositely charged parallel metal plates held in a vacuum chamber. Some of the droplets pick up one or more excess electrons. The charge on the plates is adjusted so that the electric force on the excess electrons exactly balances the weight of the droplet. The idea is to look for a droplet that has the smallest electric force and assume that it has only one excess electron. This lets the observer measure the charge on the electron.

Suppose the electric field of is set up by establishing a voltage difference of 10,000 Volts across the two plates which are separated by 35 cm. The charge on one electron is about 1.6x10-19 C. Estimate the radius of an oil drop for which its weight could be balanced by the electric force of this field on one electron.

The original apparatus is show in the figure at the right and a schematic is shown beneath it.

Page last modified April 15, 2010: E14