hw11_solns - Answer Key – Homework 11 – David McIntyre...

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Unformatted text preview: Answer, Key – Homework 11 – David McIntyre 1 This print-out should have 36 questions, check that it is complete. Multiple-choice questions may continue on the next column or page: find all choices before making your selection. The due time is Central time. Chapters 27 and 28 problems. 001 (part 1 of 1) 0 points It is known that about one electron per atom of copper contributes to the current. The atomic mass of copper is 63 . 54 g and its den- sity is 8 . 92 g / cm 3 . Calculate the average drift speed of elec- trons traveling through a copper wire with a cross-sectional area of 1 mm 2 when carrying a current of 1 A (values similar to those for the electric wire to your study lamp). Correct answer: 7 . 38624 × 10- 5 m / s. Explanation: Given : N = 1 , M = 63 . 54 g , ρ = 8 . 92 g / cm 3 , A = 1 mm 2 = 1 × 10- 6 m 2 , I = 1 A , and q e = 1 . 602 × 10- 19 C / electron . We first calculate n , the number of current- carrying electrons per unit volume in copper. Assuming one free conduction electron per atom, n = N A ρ M , where N A is Avogadro’s number and ρ and M are the density and the atomic weight of copper, respectively n ≡ µ 1 electron atom ¶ N A ρ M . n = µ 1 electron atom ¶ 6 . 02 × 10 23 atoms 63 . 54 g · ( 8 . 92 g / cm 3 ) µ 10 6 cm 3 1 m 3 ¶ = 8 . 45112 × 10 28 electrons / m 3 . The drift speed is v d , v d = I n q e A = 1 A 8 . 45112 × 10 28 electrons / m 3 · 1 (1 . 602 × 10- 19 C / electron) · 1 (1 × 10- 6 m 2 ) = 7 . 38624 × 10- 5 m / s . 002 (part 1 of 3) 0 points The damage caused by electric shock depends on the current flowing through the body; 1 mA can be felt and 5 mA is painful. Above 15 mA, a person loses muscle control, and 70 mA can be fatal. A person with dry skin has a resistance from one arm to the other of about 100000 Ω. When skin is wet, the resistance drops to about 5300 Ω. What is the minimum voltage placed across the arms that would produce a current that could be felt by a person with dry skin? Correct answer: 100 V. Explanation: Given : I min = 1 mA , and R dry = 100000 Ω . The minimum voltage depends on the mini- mum current for a given resistance, so V min = I min R dry = (1 mA) µ 1 A 1000 mA ¶ (100000 Ω) = 100 V . 003 (part 2 of 3) 0 points Using the same electric potential as in Part 1, what would be the current if the person had wet skin? Correct answer: 18 . 8679 mA. Explanation: Given : V min = 100 V , and R wet = 5300 Ω . Answer, Key – Homework 11 – David McIntyre 2 For the voltage V from Part 1 and the resis- tance R wet , I = V min R wet = 100 V 5300 Ω µ 1000 mA 1 A ¶ = 18 . 8679 mA . 004 (part 3 of 3) 0 points What would be the minimum voltage that would produce a current that could be felt when the skin is wet?...
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hw11_solns - Answer Key – Homework 11 – David McIntyre...

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