Millikan's Oil Drop Experiment

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Millikan's Oil Droplet Experiment
Robert Millikan set out to discover the charge on an electron in 1909 as the smallest unit charge a
particle could have.
Apparatus :
Oil droplets
Oil Charged plate
Charged Plate
Method :
The atomiser was used to spray small droplets of oil into the chamber where a few would then fall
through into the lower chamber. Millikan would measure the terminal velocity of an individual droplet in
order to work out its volume and the force of gravity acting upon it. Millikan used low volatility oil so that
no oil evaporated while he was taking measurements
Next he would apply a charge to the oil droplets but ionising the air around them using x-rays, or the
droplets were charged due to the friction as the rubbed the barrel of the atomiser as they left it.
The battery was then connected to the plates above and below the chamber and the voltage supplied
changed until the oil droplet was hovering, the force from the electric field equal and opposite to the
force of gravity.
Millikan could then calculate the charge on the oil droplet
Calculations :
When calculating the force of gravity upon each oil droplet and therefore the volume, Millikan also had to
take into account the viscous force from the air - or air resistance - on the droplet as well, which could be
calculated using Stoke's law :
Where = the viscosity of the fluid, units
r = the radius of the object
v = the velocity of the object
When the droplet reaches terminal velocity, calculated by
air resistance is equal to the downwards force of gravity, so
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Since the mas of the drop is the volume multiplied by the density of the oil this equation equals
Millikan had already measured and in previous experiments and could calculate r for each oil droplet.
The upward force from the electric field was given by
Where Q = charge on the oil drop
V = potential difference between the plates
d = distance between plates
When the particle is stationary
And so Millikan could work out the charge on each oil droplet.…read more


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