Robert Millikan was awarded the Nobel Prize in 1911 for determining the charge on the electron. He did that with his famous OIL DROP EXPERIMENT. The experiment was simple enough. Oil was sprayed out of an atomizer used for spraying perfume. This made a tiny mist of droplets. And much like you get a static charge when you walk across a carpet in socks, these droplets got a very tiny charge, of only a few electrons, when they slid across the spray nozzle of the perfume bottle. Then UV radiation was The small oil drops would pick up some unknown number of electrons due to friction between the oil and the nozzle. The drops of oil were allowed to fall onto a surface where there was a tiny hole. This would allow individual drops to fall below, where there were two parallel, oppositely-charged electric plates. One was positively-charged and the other was negatively-charged. Light shined upon the oil drops from the side caused them to show up like little stars when looking through a microscope used to view the region between the plates. The weight of an oil drop could be calculated by using the terminal velocity of the freely falling oil drop and the formula for terminal velocity of a sphere. Once the weight was determined, the voltage across the plates could be adjusted until the upward electric force exactly cancelled out the weight of the oil drop. When the exact balance was reached, the oil drop would remain suspended between the plates. Once the voltage is determined, the charge on the oil drop can be calculated. The problem is that it is not known how many electrons were on each drop. Here Millikan had to rely on hundreds of measurements of charges on different oil drops so he could look at all the charges and determine what the smallest multiple that each one differed by. That smallest amount turned out to be 1.6 x 10-19 Coulombs, the charge on one electron! The calculation: The force pulling down on the oil drop (ie, gravity) is proportional to the mass of the particle, while the force pulling up is proportional to the charge! That is critical. What Millikan did was he made tiny oil droplet with just a few more electrons on them than protons. The mass of the oil droplet can be easily determined by using a microscope and measuring the diameter of the droplet, and then using the density of oil. So he could find the force pulling down based on the size of the droplet. And then he could balance the droplet using the voltage on his metal plates. This would tell him that the force pulling up was equal to the force pulling down. And since he knows the force pulling down, he can infer the force pulling up. And from the voltage, and the force pulling up, and the spacing of his plates, he can infer the charge the particle has. https://youtube/watch?v=2HhaQtvICe8
Posted on: Mon, 08 Sep 2014 17:47:52 +0000
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