W11Physics1CLec30Afkw

W11Physics1CLec30Afkw - Physics 1C Lecture 30A The Nucleus...

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Physics 1C Lecture 30A
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The Nucleus All nuclei are composed of protons and neutrons (they can also be called nucleons). The atomic number, Z , is the number of protons in the nucleus. The neutron number, N , is the number of neutrons in the nucleus. The mass number, A , is the number of nucleons in the nucleus ( A=N+Z ). In symbol form, it is: X A Z where X is the chemical symbol of the element.
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The Nucleus Isotopes of an element have the same Z but differing N (and thus A ) values. For example, both have 92 protons, but U-235 has 143 neutrons while U-238 has 146 neutrons. Nucleons and electrons are very, very light. For this reason, it is convenient to de±ne a new unit of mass known as the uni±ed mass unit, u. Where: 1 u = 1.660559x10 -27 kg (exactly 1/12 of the mass of one atom of the isotope C-12) 92 238 U 92 235 U
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The Nucleus Einstein’s mass-energy equivalence tells us that all mass has an energy equivalence. This is known as the rest energy. This means mass can be converted to some form(s) of useable energy. The equation is given by: From here we get that the energy equivalent of 1u of mass is: This gives us the relationship between mass and energy to also be: E R = mc 2
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The Nucleus This table gives the following masses for the selected particles: Note that the neutron is slightly heavier than the proton. Note: 2x938.28 + 2x939.57 = 3755.7 > 3728.40
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Size of the Nucleus Rutherford determined an approximate size of the nucleus with his thin-foil experiments. He assumed that the initial kinetic energy of the positively charged alpha particle (+2 e ) would be converted into Fnal electrical potential energy: ±rom here he could Fnd an upper limit for the size of the nucleus. KE i = PE f 1 2 m ! v 2 = k e q 1 q 2 r
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Size of the Nucleus He found that the radius of the nucleus could be no larger than about 3.2x10 -14 m (golden-foil experiments). Since Rutherford’s experiment, many experiments have been performed to measure the exact size and shape of the nucleus.
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This note was uploaded on 01/30/2012 for the course PHYS 1C 1C taught by Professor Wethien during the Spring '11 term at UCSD.

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W11Physics1CLec30Afkw - Physics 1C Lecture 30A The Nucleus...

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