Module 7
1. Module 7
1.11. Page 4
Module 7—Investigating the Nature of the Atom
Lesson Summary
Millikan’s experimental design was based on measuring the charge of many oil droplets and comparing them, reasoning that the smallest difference in charge among all the droplets would be due to the presence of one extra electron. That small difference in charge would then be equal to the charge of a single electron—the elementary unit of charge. Using the charge on numerous oil droplets and through careful analysis, Millikan discovered that the charges of the oil droplets were always integer multiples of 1.602 × 10–19 C. He reasoned this must be the charge of a single electron, a value that is referred to as the elementary charge.
The charge on each oil drop was determined by measuring, based on the motion of the droplet, the amount of electrical force that acts on it in a uniform electrical field. Oil droplets charged by friction were injected into a sealed vessel with electrodes on either end. Applying a voltage across the electrodes produced an electrical force that acted on the droplet, causing it to move upwards at a constant speed. Measuring the fall and rise speeds, Millikan determined the radius, volume, mass, and, ultimately, the charge of the droplet. By comparing many hundreds of droplets, the smallest integer difference in charge revealed the charge on one electron.
Although time-consuming, his experiment was instrumental in establishing not only the value of the elementary charge, but also the quantized nature of electric charge. It also confirmed the idea that the atom was not the smallest form of matter and that it had divisible parts.
Lesson Glossary
elementary unit of charge: the charge of an electron or a proton, 1.60 × 10–19 C