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Modern Physics

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PHY251 Homework Set 10 Reading: Chapter 11, 12 Homework: Chapter 11, Question 1, Problems 2,5,11,14 Chapter 12, Problems 7,23,31 Hints and Solutions Question XI.1 (10 points) One of the great puzzles in heredity before the quantum theory of molecules was invented was how the gene could retain its integrity generation after generation when it was kept at the human body temperature. How does the quantum theory of molecules resolve this problem? (Note that, according to very general ideas about statistical systems, all objects that are part of a thermal system move with energies on the order of kT , where k is Boltzmann's constant.) Hints: See Problem 4. Solution: The temperature of human body is T = 310 Kelvin which correspond to kT 2245 0.03 eV. The binding energy of molecules is much larger then this value, it is typically in order of tens of eV. In a group of DNA molecules only about one out of e 10/0.03 2245 6×10 144 molecules (i.e. none out of the typically 10 21 in the body) would have an energy sufficient to break up. The thermal energy which is accessible to molecules at the human body temperature is thus much smaller that the binding energy of molecules, therefore molecules are stable. Problem XI.2 (10 points) Consider the ground state of the silicon atom ( Z =14). What is the electronic configuration for this state. Solution: (1 s ) 2 (2 s ) 2 (2 p ) 6 (3 s ) 2 (3 p ) 2 Problem XI.5 (10 points) X-rays bombarding heavy atoms can be used to eject electrons from the 1 s shell in s file:///C|/DOCUME~1/PHY_CO~1/PHY251/PHY251_hw10_solutions.html (1 of 4) [5/14/2001 5:06:03 PM]
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atoms; indeed, this is the starting point for Moseley's experiments. Estimate the
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solutions 10 - s PHY251 Homework Set 10 Reading Homework...

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