College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300



College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300
Textbook Problem

In the Millikan oil-drop experiment illustrated in Figure 15.21, an atomizer (a sprayer with a fine nozzle) is used to introduce many tiny droplets of oil between two oppositely charged parallel metal plates. 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 dial has the smallest electric force and assume it has only one excess electron. This strategy lets the observer measure the charge on the electron. Suppose we are using an electric field of 3 × 104 N/C. The charge on one electron is about 1.6 × 10−19 C. Estimate the radius of an oil drop of density 858 kg/m5 for which its weight could be balanced by the electric force of this field on one electron. (Problem 42 is courtesy of E.F. Redish. For more problems of this type, visit www.physics.umd.cdu/pcrg/.)

To determine
The radius of the oil drop.


Given info: The charge of the drop (q) is 1.6×1019C . The electric field (E) is 3×104N/C . The density of oil drop ( ρ ) is 858kgm3 .

The weight is balanced by the electric force.

Formula to calculate the weight is,


The electric force is,




On Re-arranging,

m=qEg       (I)

Formula to calculate the radius is,

r=[3m4πρ]13       (II)

Substitute Equation (I) in (II).


Substitute 1

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