Download The Millikan Oil Drop Experiment

The Millikan Oil Drop Experiment
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In 1910, Robert Millikan of the University of Chicago published the details of an experiment that proved beyond doubt that charge was carried by discrete positive and negative entities each of which had an equal magnitude.
He was also able to measure the unitary charge (which we now recognise to be the electron) accurately. This experiment is considered to be one of the most significant ever carried out, and Millikan received the Nobel Prize in 1923.
The oil drop experiment:
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The atomizer produced a fine spray of oil droplets with a radius of about 1 µm. Many of the droplets were charged. Occasionally a droplet fell through the pinhole. If the droplet was charged, it could be brought to a halt and held stationary by applying a voltage across the metal plates.
When the droplet was stationary, the force exerted by the electrostatic field, EQ (where E is the field and Q is the charge on the droplet), was equal to the weight of the droplet, mg, where m is its mass and g the acceleration due to gravity.
E can be calculated by measuring the voltage across the plates.
q
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Here was the data, Millikan noticed that the change in charge was always a mutliple of 1.6 X 10
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C
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Example #1
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In an oil drop experiment, a droplet contained a charge of 3.6 X 10 C.
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If the electric field between the plates is 6.2 X 10 N/C, what is the mass of the oil drop?
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Ex #2
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An oil drop of mass 9.8 X 10 kg is suspended between two horizontal plates. If the electric field strength is
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2.0 X 10 V/m, what is the magnitude of the charge on the oil drop?
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Force #2
Force #1
Example #3
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Example #3
In a Millikan oil drop experment a student sprayed oil droplets
with a density of 7.8 x 102 kg/m3 between two horizontal plates
that were seperated by 4.0 cm. The student adjusted the potential difference on the plates to read 4.6 x 103 V so that the
drops became stationary. The diameter of the drop was measured to be 2.4 x 10­6 m. What is the charge on the drop?
(Hint: you need to rememeber two formulas from grade 9 science)
(volume of a sphere, and the density formula
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Example PDE question
A Millikan Experiment
+ + + + + + + + + + + + + + + + + + + + + + + + Fe
mass
d = 0.10 m
Fg
­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ A charged particle that has a mass of 2.8 X 10­16 kg accelerates upward
at 3.6 m/s2 in the electric field between two horizontal plates that have a seperation of 0.10 m. The potential difference across the plates is 400 V.
The experiment is performed on Earth's surface at sea level
a) Draw the electric field lines showing the electric field between the plates
b) Calculate the magnitude and direction of the force on the particle
c) Determine the charge on the particle
d) Determine the time required for particle to move from the lower
plate to the upper plate if the particle begins from rest
d) Calculate the minimum voltage required to start the particle moving
from the lower plate to the upper plate
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