Chap 38 SM - Chapter 38 Solids Conceptual Problems 1 In the...

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Chapter 38 Solids Conceptual Problems 1 In the classical model of conduction, the electron loses energy on average during a collision because it loses the drift velocity it had acquired since the last collision. Where does this energy appear? Determine the Concept The energy lost by the electrons in collision with the ions of the crystal lattice appears as Joule heat ( I 2 R ). 2 A metal is a good conductor because the valence energy band for electrons is ( a ) empty, ( b ) partly filled, ( c ) filled, but there is only a small gap to a higher empty band, ( d ) completely filled, ( e ) None of the above. Determine the Concept If the valence band is only partially full, there are many available empty energy states in the band, and the electrons in the band can easily be raised to a higher energy state by an electric field. ( ) b is correct. 3 Thomas refuses to believe that a potential difference can be created simply by bringing two different metals into contact with each other. John talks him into making a small wager and is about to win the bet. ( a ) Which two metals from Table 38-2 would demonstrate his point most effectively? ( b ) What is the value of that contact potential? Picture the Problem The contact potential is given by e V 2 1 contact φ = , where 1 and 2 are the work functions of the two different metals in contact with each other. ( a ) Express the contact potential in terms of the work functions of the metals: e V 2 1 contact = Examining Table 38-2, we see that the greatest difference between the work functions will occur when potassium and nickel are joined. ( b ) Substitute numerical values and evaluate V contact : V 1 . 3 eV 1 . 2 eV 2 . 5 contact = = e V 4 ( a ) In Problem 3, which choices of different metals would make the least impressive demonstration? ( b ) What is the value of that contact potential? 193
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Chapter 38 194 Picture the Problem The contact potential is given by e V 2 1 contact φ = , where 1 and 2 are the work functions of the two different metals in contact with each other. ( a ) Express the contact potential in terms of the work functions of the metals: e V 2 1 contact = Examining Table 38-2, we see that the least difference between the work functions will occur when silver and gold are joined. ( b ) Substitute numerical values and evaluate : contact V V 1 . 0 eV 7 . 4 eV 8 . 4 contact = = e V 5 [SSM] When a sample of pure copper is cooled from 300 K to 4 K, its resistivity decreases more than the resistivity of a sample of brass when it is cooled through the same temperature difference. Why? Determine the Concept The resistivity of brass at 4 K is almost entirely due to the residual resistance, the resistance due to impurities and other imperfections of the crystal lattice. In brass, the zinc ions act as impurities in copper. In pure copper, the resistivity at 4 K is due to its residual resistance, which is very low if the copper is very pure.
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This note was uploaded on 02/28/2012 for the course PHYSICS 122 taught by Professor Goussiou during the Spring '12 term at University of Washington.

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Chap 38 SM - Chapter 38 Solids Conceptual Problems 1 In the...

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