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Planck's Constant
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Although many people believe that Planck's constant, h, was the first-discovered universal constant, in reality, it follows four others, as explained in Chapter 2. Also, it was invented, not discovered. If you are already acquainted with Planck's constant, no doubt you learned that it is a fundamental, universal physical constant of nature (FUPCON). This description seems to mean that it has existed in nature from time immemorial, and Planck just happened to discover it at the opportune time--during the birth of quantum theory. In reality, he invented it so that he could convert a particular proportion into an equation. Therefore, the term, fundamental, universal physical constant of nature, or FUPCON, is a misnomer for constant of proportionality, which, itself, is a misnomer for collection of quantum-attribute factors.
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During the past 15 years, undergraduate physics students have attempted to find Planck's constant by measuring the "turn-on voltage" of light emitting diodes (LED). It has been assumed that this voltage is approximately equal to the potential difference of the LED's band gap. This potential, when plotted as a function of frequency of the light emitted, has a slope interpreted as h/e. The theoretical basis of this experiment is flawed. Curiously enough, the erroneous analysis turns out to produce exactly the correct results for values of h/e. Major changes to the theoretical interpretation and minor changes to the experimental procedure result in a more complete undergraduate experiment producing significant and accurate data.
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The key to achieving a highly accurate value for Planck’s Constant is to include a wide range of frequencies in the data. The ultraviolet LED should be viewable with the naked eye... the infrared LED is not. One way to get the data for this point is to use a digital camera that will “convert” infrared light to visible light. The camera on most cell phones will work well. Hold the camera over the LED and slowly increase the potential on the power supply. When the LED begins to emit light you will be able to view the light through the camera’s display.
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Perhaps the foremost scientists of the 20th century was Niels Bohr, the first to apply Planck's quantum idea to problems in atomic physics. In the early 1900's, Bohr proposed a quantum mechanical description of the atom to replace the early model of Rutherford.
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During the First World War Planck's oldest son, Karl, was killed in action at Verdun, and Erwin was taken prisoner by the French in 1914. Grete died in 1917 while giving birth to her first child. Her sister died two years later the same way, after marrying Grete's widower. Both granddaughters survived and were named after their mothers. Planck endured these losses stoicly.
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With these measurements, the researchers calculated a value for Planck's constant of 6.62606891 x 10-34 Joule-seconds, with an uncertainty of 89 parts per billion. They plan to immerse their watt balance in a vacuum enclosure (shown in yellow) which will further increase the precision of their experiment. In addition, this setup ultimately promises to lead to a definition of the kilogram based on quantum units, rather than one based on the stalwart physical artifact currently stored in France.
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Planck's Constant