Download Milkan`s Oil Drop Experiment - International Leaders Academy

Chemistry Study sheet no. 2
2013/2014- First Semester
Chapter 4 : Atoms
Section 1 : The development of
atomic theory
Name : ______________________
Grade 9 / SAT
Day & Date : wednesday
06/11/1013
Milkan's Oil Drop Experiment
Background:
At the time of Millikan and Fletcher's oil drop experiments, the existence of subatomic particles was not
universally accepted. Experimenting with cathode rays in 1897, J. J. Thomson had discovered negatively
charged "corpuscles", as he called them, with a mass about 1840 times smaller than that of a hydrogen
atom.
An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron.
He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'
Millikan received the Nobel Prize for his work .
What is the Oil Drop Experiment:
The oil drop experiment was an experiment performed by Robert A. Millikan and Harvey Fletcher in
1909 to measure the elementary electric charge (the charge of the electron).
What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric
field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil
drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge
that the drop must have. By varying the charge on different drops, he noticed that the charge was always a
multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this
unit charge.
Experimental procedure:
1- Apparatus
Millikan’s and Fletcher's apparatus was designed as follows:
1- It incorporated a parallel pair of horizontal electrically charged brass plates (electrodes) with a
pinhole in the upper plate.
2- By applying a potential difference across the plates, a uniform electric field was created in the
space between them.
3- A ring of insulating material was used to hold the plates apart.
4- A hole was cut into the ring to allow viewing through a microscope.
5- A fine mist of oil droplets was sprayed into a chamber above the plates.
1
2- Method:
1- Initially the oil drops are allowed to fall between the plates with the electric field turned off. They
very quickly reach a terminal velocity because of friction with the air in the chamber.
2- Using the microscope, he measured their terminal velocity, and by use of a formula, calculated
the mass of each oil drop.
3- Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with xrays. This caused the air to become ionized, and electrons to attach themselves to the oil drops.
4- By attaching a battery to the plates above and below this bottom chamber, he was able to apply an
electric voltage.
5- The electric field produced in the bottom chamber by this voltage would act on the charged oil
drops; if the voltage was just right, the electromagnetic force would just balance the force of
gravity on a drop, and the drop would hang suspended in mid-air.
Conclusion
The experiment entailed balancing the downward gravitational force with the upward drag and electric
forces on tiny charged droplets of oil suspended between two metal electrodes. Since the density of the oil
was known, the droplets' masses, and therefore their gravitational and buoyant forces, could be
determined from their observed radii. Using a known electric field, Millikan and Fletcher could determine
the charge on oil droplets in mechanical equilibrium. By repeating the experiment for many droplets, they
confirmed that the charges were all multiples of some fundamental value, and calculated it to be
1.5924(17)×10−19 C, within 1% of the currently accepted value of 1.602176487(40)×10 −19 C. They
proposed that this was the charge of a single electron.
Study well.... 
Miss Maha Tomar
2
3