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Unit: Atoms, Molecules, and Ions Module: Early Atomic Theory [page 1 of 1]
· Robert Millikan determined the charge of an electron by observing oil droplets in
an electric field.
· Today, the charge of an electron is accepted as 1.60 x 10
an electron is accepted as 9.11 x 10 kg.
C, and the mass of
Robert Millikan determined the charge of an
electron by observing oil droplets in an electric field.
In the Millikan oil drop experiment, oil is atomized
into small droplets, and then allowed to fall through
a hole into the chamber. Droplets fall because of
gravity. An x-ray beam knocks electrons off of air
molecules, and some of those electrons become
attached to the oil droplets. The negatively charged
oil droplets feel an upward force from the electrical
With the proper choice of an electric field, FC (the
Coulomb force) will exactly balance FG (the
gravitational force), and the droplets will become
motionless. This means that q = mgd/V, where q is
the charge of a particular droplet, m is the mass of
that droplet, g is the acceleration due to gravity, d
is the distance between the plate and the droplet,
and V is the electrical potential. Since the mass of
the droplet can be determined by measuring the
terminal velocity when the electric field is turned
off, the only unknown is q.
Through repeated measurements, Millikan found
that the charge of each droplet was an integer
multiple of 1.59 x 10 C. This was the charge of
an individual electron.
J. J. Thomson had previously shown that the
charge-to-mass ratio of an electron was
1.76 x 10 C/kg. Using this ratio and the
experimentally determined charge, the mass could
also be calculated.
Today, the charge of an electron is accepted as
1.60 x 10 C, and the mass of an electron is
accepted as 9.11 x 10 kg.
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Copyright 2001, Thinkwell Corp. All Rights Reserved. 6905.doc rev 05/31/2001