Millikan’s Oil Drop Experiment

What is the charge on a single electron? The charge on an electron is an important number called the fundamental charge. The charge on any object consists of an integer multiple of the fundamental charge.

Apparatus Millikan’s apparatus consists two charged metal plates that have a variable voltage applied to them. The negative plate is on the bottom and the positive plate is on the top. A very precise pump is placed so that it will create a small drop of oil that will fall into the space between the plates. This drop becomes negatively charged as it falls from the pump. The drop will be repelled by the negative plate and attracted to the positive plate. Millikan varied the voltage between the plates until the drop was floating in a stationary position between the plates.

Floating: Eq = mg q = mg/E Forces Eq mg When the drop was stationary the force of gravity will be equal and opposite to the force of the electric field. Simple algebra can be used to calculate the charge on the drop. The question then becomes – how many excess electrons are on the drop? mg

Trial 1 q = 1.2 C + + + + + + + + + + - - - - - - - - - - - - - + + + + + + + + + + Millikan answered that question by performing numerous trials and recording his results. The numbers here are much larger than the numbers encountered in the experiment. - - - - - - - - - - - - -

Trial 2 q = 2.4 C + + + + + + + + + + - - - - - - - - - - - - -

Trial 3 q = 4.8 C + + + + + + + + + + - - - - - - - - - - - - -

Trial 4 q = 7.2 C + + + + + + + + + + - - - - - - - - - - - - -

Results Trial 1 1.2 C Trial 2 2.4 C Trial 3 4.8 C Trial 4 7.2 C Common 1.2 C Once the results of all the trials are known, it is possible to calculate the charge on a single electron. Since there is always an integer number of electrons, all the results must be integer multiples of the fundamental charge. By finding the least common factor of all the results one finds the fundamental charge.

Charge on an Electron 1.6 x 10-19 C The actual charge on an electron is incredibly small. Notice that if you have a trillion electrons the total charge is still less than a microcoulomb.