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9D Chapter 4-7 Notes

9D Chapter 4-7 Notes - Class Notes for Modern Physics Part...

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Class Notes for Modern Physics, Part 2 J. Gunion U.C. Davis 9D, Spring Quarter J. Gunion
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The particle nature of matter Most of you are already convinced that matter is composed of particles, but it is useful to at least briefly recall how our current understanding arose historically. There were four major items in making this case: 1. Around 1833, Faraday performed a series of electrolysis experiments. these established three basic things: (a) that matter consists of molecules and that molecules consist of atoms; (b) that charge is quantized, because only integral numbers of charges are transferred between the electrolysis electrodes; (c) and that the subatomic parts of atoms carry positive and negative charges. However, he was unable to directly determine the masses of these subatomic particles, but it seemed clear that they were related to the atomic weights that were known from chemistry. Also, the absolute size of the charge of these subatomic particles could not be determined from electrolysis — only that charge was quantized. J. Gunion 9D, Spring Quarter 1
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Considerable time would pass before the next major input. 2. Around 1897, Thomson identified cathode rays as something with the same sign as the negative charges seen by Faraday. And, he found that all negative particles emitted from a cathode had identical e/m e values, where e was the charge. He postulated that whatever this object was, it was probably a fundamental constituent of matter. We know it as the electron. A few years later, he was able to use measurements in a viscous cloud chamber to roughly determine the magnitude of the charge separately. He found that “ e is the same in magnitude as the charge carried by the hydrogen atom in the electrolysis of solutions.” 3. In 1909, Millikan was able to obtain a much more precise measurement of the electronic charge. This could be combined with the e/m e value obtained by Thomson to obtain a value for m e that was about 1000 times smaller than the mass of the Hydrogen atom (the latter being close to the proton mass, m p ), which had been known from atomic weights and chemistry. J. Gunion 9D, Spring Quarter 2
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4. Finally, in 1913, Rutherford and co-workers established the nuclear model of the atom by scattering fast-moving α particles (charged Helium nuclei) from metal foil targets. He showed that atoms consist of a compact positively charged nucleus (with diameter about 10 - 14 m ) surrounded by a swarm of orbiting electrons (with the electron cloud diameter being of order 10 - 10 m .) Here, I will try to say a few additional words about the Thomson and Rutherford experiments. Please read the material in the book on the Millikan experiment. Thomson The apparatus and schematic are shown below. We consider a B field pointing into the page and a E field in the plane of the page. When present, these produce forces F E = - eE (upwards) and F B = - e ( v × B ) (downwards) on the e - .
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