Question

Briefly describe Millikan’s oil drop experiment and what do the results tell us about electric charge.

Answer 1

Answer:

Apparatus:

Between 1909 and 1910 the American physicist Robert Millikan conducted a series of oil-drop experiments. Apparatus incorporated a parallel pair of horizontal metal plates. By applying a potential difference across the plates, a uniform electric field was created in the space between them. A ring of insulating material was used to hold the plates apart. Four holes were cut into the ring, three for illumination by a bright light, and another to allow viewing through a microscope.

A fine mist of oil droplets was sprayed into a chamber above the plates. Charging of oil-drops could be brought about by including an ionising radiation source (such as an X-ray tube). The droplets entered the space between the plates and, because they were charged, could be made to rise and fall by changing the voltage across the plates.

Result:

By comparing applied electric force with changes in the motion of the oil drops, American physicist Robert Millikan was able to determine the electric charge on each drop. He found that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron.

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