Lecture18 - Physics 344 Foundations of 21st Century...

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Physics 344 Foundations of 21 st Century Physics: Relativistic and Quantum Systems Instructor: Dr. Mark Haugan Office: PHYS 282 haugan@purdue.edu TA: Dan Hartzler Office: PHYS 7 dhartzle@purdue.edu Grader: Fan Chen Office: PHYS 222 chen926@purdue.edu Office Hours: If you have questions, just email us to make an appointment. We enjoy talking about physics! Help Session: Thursdays 2:00 – 4:00 in PHYS 154 Pick up your graded exam?
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Particle Physics and the Structure of Matter In previous courses we have used repeatedly the fact that ordinary matter consists of atoms to connect seemingly unrelated topics, for example, the springiness of a metal to the speed of sound in that metal and the fields of mechanics and thermal physics. We also used the fact that fact that an atom itself consists of a “cloud” of electrons bound to a tiny, massive nucleus consisting of much more tightly bound protons and neutrons to develop a microscopic understanding of the behavior of electric circuits. Although the idea that each sample of a chemical element was a collection of identical microscopic particles is much older, the hierarchical combination of the atomic model of matter and the nuclear model of atoms with atoms of different elements distinguished by number of electrons was still a work in progress less than 100 years ago. In this model, neutral atoms must have equal numbers of protons and electrons, but these do not account for the masses observed for atoms of different elements. The neutron, whose presence in nuclei explains the observed masses of atoms, was not discovered until 1932!
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A combination of chemical and physics research over the past 200 years led eventually to the acceptance of the reality of atoms. Einstein’s Nobel Prize was actually awarded for his 1905 statistical mechanics explanation of the observed properties of Brownian motion, work that finally convinced the last atomic skeptics. Last year you studied Rutherford scattering, so, you know that the nuclear model of atoms traces its origins to 1911. To explain the scattering of alpha particles by a gold foil that he observed, Rutherford needed each gold atoms’ positive charge to be concentrated in a tiny, massive nucleus. Today, the field of high-energy particle physics is a natural continuation of this and other research that established the nuclear model of the atom.
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This note was uploaded on 12/09/2011 for the course PHYS 344 taught by Professor Garfinkel during the Fall '08 term at Purdue University-West Lafayette.

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Lecture18 - Physics 344 Foundations of 21st Century...

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