Millikan Oil Drop Experiment
From High School Online Collaborative Writing
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[edit] Millikan Oil Drop Experiment
-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'.
R.A. Millikan performed his famous experiment about 1910. It was fundamental in the process of describing the structure of the atom. J.J. Thomson had recently measured the ratio of charge to mass. This particular result allowed us then to know both the charge (from this experiment) and the mass by combining this result with Thomson's.
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.
The experiment is quite simple. The apparatus is located in a darkened room so as to avoid external light scattering. A container with a substance of known dimensions is atomized much like a spray of perfume. As the droplets of latex in this case are forced up a rubber tube they are ionized, either gaining electrons or losing some by friction with the rubber tube.
[edit] The Apparatus
[edit] The Procedure
1) A fine mist of oil was sprayed from an atomizer. Most of the droplets became negatively charged as they picked up some small number of electrons as they passed through its nozzle. Some of the drops then fell through a hole in the top plate and drifted into the region between the two parallel plates.
2) It was then lit from the side by an intense light, these drops glistened when the region was viewed through a telescope. Once a drop was located, the voltage was varied controlling the electric field, E, to slow down the drop's descent. When the drop reached terminal velocity (mg = qE) it was tracked through the remainder of its fall and a ratio of mass per unit charge was recorded.
3) Once the mass of the drop could be determined then the drop's electric charge could be calculated from the recorded electric field strengths, (q = mg / E). By timing the drop's motion, its terminal velocity was calculated (s = vt). Using equations from a detailed theory of air resistance, Millikan was able to determine each drop's radius. From the radius, he was able to calculate each drop's volume. Using the density of oil and the volume, he was able to determine the mass of each drop.
4) Once the mass of each drop was determined, Millikan, with the help of his graduate student H. Fletcher, showed that the charges of the droplets always carried a whole number multiple of a basic charge, qe = 1.6 x 10-19 C. Today, the accepted value for the fundamental unit of charge is e = 1.602 x 10-19 C.
[edit] The Result
Millikan determined the charge on a drop. Then he redid the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for q. The charge q on a drop was always a multiple of -1.6 x 10 -19 C, the charge on a single electron.
This number was the one Millikan was looking for, and it also showed that the value was quantized; the smallest unit of charge was this amount, and it was the charge on a single electron.
