Download Millikan`s Oil Drop Experiment

Find the charge of an electron - Millikan’s Oil Drop Experiment
Millikan (1910) reasoned that he could find the charge on an electron by finding
the smallest number of which the charges on several oil drops are a multiple.
q = ne, where q is the charge on the droplet, n is the number
of electrons in excess or deficit, and e is the charge
of the electron (presumable a constant)
…but how to find the charge on the
drop?
…and how to find the mass of the
drop?
Millikan suspended charged oil droplets
(charged by friction as they left the
nozzle of an atomizer) in an electric
field between two plates. The voltage
that provided sufficient force to balance
the oil drop against gravity would be
proportional to the charge on the oil
drop, which would be a multiple of the
number of electrons providing that
charge.
Millikan measured the terminal velocity
and diameter of individual droplets
before suspending them in the electric
field.
The mass m is related to terminal
v ACd
m t
2g
2
velocity vt as
, where ρ
is the density of the air, A is the crosssectional area of the droplet, and Cd is
the drag coefficient.
Millikan used a low power microscope to watch the charged oil droplets rise and
fall and finally become suspended in the electric field as he varied the potential
difference between the two plates.
We will simulate Millikan’s experiment using an applet in which we will find the
velocity of the rising and falling droplets. The mass is inferred from the speed of
the droplet rising and falling in the field. The charge is inferred from the
acceleration of the droplet in the field. We will compare the inferred charges on
the droplets and attempt to identify the lowest common factor, or the elementary
charge, the charge of the electron.
Find, label, and define or relate in the sketch of Millikan’s electrical microbalance
below the quantities r, ΔVb, m, FE, FG, q, and e.
When the oil droplet is
suspended by the
electrical force of the field
between the plates
balancing the weight of
the charged droplet, then
FE=Fg
so, qε = mg
but since  
V
,
r
the charge on an oil droplet is q 
mgr
V
where q is the charge on the droplet, m is the mass of the
oil drop, r is the separation of the plates, and ΔV is the
voltage that balances the droplet against gravity.
Simulate Millikan’s experiment.
1. Load up the Java applet which simulates Millikan’s experiment at
http://projects.cbe.ab.ca/sss/science/physics/map_north/applets/millikan/millik
an.html
2. Practice with the applet before trying to collect data. Once you’re good and
ready, begin collecting.
3. Use division and subtraction to determine the lowest common factor of the
charges on the droplets, or the elementary charge.