SM_chapter21 - Current and Direct Current Circuits CHAPTER...

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Current and Direct Current Circuits CHAPTER OUTLINE 21.1 Electric Current 21.2 Resistance and Ohm’s Law 21.3 Superconductors 21.4 A Structural Model for Electrical Conduction 21.5 Electric Energy and Power 21.6 Sources of emf 21.7 Resistors in Series and in Parallel 21.8 Kirchhoff’s Rules 21.9 RC Circuits 21.10 Context Connection The Atmosphere as a Conductor ANSWERS TO QUESTIONS SOLUTIONS TO PROBLEMS Section 21.1 Electric Current P21.1 I Q t Q I t 30.0 10 6 A  40.0 s 1.20 10 3 C N Q e 1.20 10 3 C 1.60 10 19 Ce lec t ron 7.50 10 15 electrons P21.4 q 4 t 3 5 t 6 A 2.00 cm 2 1.00 m 100 cm 2 2.00 10 4 m 2 (a) I 1.00 s dq dt t 1.00 s 12 t 2 5 t 1.00 s 17.0 A (b) J I A 17.0 A 2.00 10 4 m 2 85.0 kA m 2 P21.5 We use I nqAv d . Here n is the number of charge carriers per unit volume, and is identical to the number of atoms per unit volume. We assume a contribution of 1 free electron per atom in the relationship above. For aluminum, which has a molar mass of 27, we know that Avogadro’s number of atoms, N A , has a mass of 27.0 g. Thus, the mass per atom is
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27.0 g N A 27.0 g 6.02 10 23 4.49 10 23 ga tom . Thus, n density of aluminum mass per atom 2.70 g cm 3 4.49 10 23 n 6.02 10 22 atoms cm 3 6.02 10 28 atoms m 3 . Therefore, v d I nqA 5.00 A 6.02 10 28 m 3  1.60 10 19 C 4.00 10 6 m 2 1.30 10 4 ms or, v d 0.130 mm s . Section 21.2 Resistance and Ohm’s Law P21.6 I V R 120 V 240 0.500 A 500 mA P21.8 (a) Given M d V d A where d mass density, we obtain: A M d . Taking r resistivity, R r A r M d r d 2 M . Thus, MR r d 1.00 10 3 0.500 1.70 10 8 8.92 10 3 1.82 m . (b) V M d , or r 2 M d . Thus, r M  d 1.00 10 3 8.92 10 3 1.82 r 1.40 10 4 m . The diameter is twice this distance: diameter 280 m . P21.10 At the low temperature T C we write R C V I C R 0 1 T C T 0  where T 0 20.0 C . At the high temperature T h , R h V I h V 1 A R 0 1 T h T 0 .
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This note was uploaded on 01/30/2012 for the course PHYSICS 1B 1B taught by Professor Grosmain during the Winter '10 term at UCSD.

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SM_chapter21 - Current and Direct Current Circuits CHAPTER...

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