hw 12-solutions

# hw 12-solutions - howard (cah3459) hw 12 turner (56705)...

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1 This print-out should have 16 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 (part 1 of 2) 10.0 points The current in a conductor varies over time as shown in the fgure below. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Current(A) Time(s) a) How much charge passes through a cross section oF the conductor in the time interval t = 0 s to t = 3 s? Correct answer: 11 C. Explanation: Let : Δ t =3 . 0s . I = Δ Q Δ t Δ Q = I Δ t = (2 A)(1 s) + (4 A)(1 s) + (6 A)(0 . 5 s) + (4 A)(0 . 5 s) = 11 C . 002 (part 2 of 2) 10.0 points b) What constant current would transport the same total charge during the 3 s interval as does the actual current? Correct answer: 3 . 66667 A. Explanation: I = Δ Q Δ t = 11 C 3s = 3 . 66667 A . 003 10.0 points An aluminium wire with a cross-sectional area oF 5 . 7 × 10 - 6 m 2 carries a current oF 5 . 4 A. ±ind the driFt speed oF the electrons in the wire. The density oF aluminium is 2 . 7g / cm 3 . (Assume that two electrons is supplied by each atom). Correct answer: 4 . 90804 × 10 - 5 m / s. Explanation: Let: A =5 . 7 × 10 - 6 m 2 , I =5 . 4 A = 5 . 4C / s , ρ =2 . 7g / cm 3 =2 . 7 × 10 6 g / m 3 , and n e = electrons / atom = 2 . Assuming n e Free conduction electrons per atom, the density oF charge carriers is n e times the density oF aluminium atoms: n = n e density mass / atom = n e ρ M/N A = n e N A ρ M . Thus the driFt speed is v d = I nq e A = IM n e N A ρ q e A = (5 . 4C / s) (26 . 98 g) (2) (6 . 02 × 10 23 ) (2 . 7 × 10 6 g / m 3 ) × 1 (1 . 602 × 10 - 19 C) (5 . 7 × 10 - 6 m 2 ) = 4 . 90804 × 10 - 5 m / s . 004 10.0 points A current oF 1 A ²ows in a copper wire 8 mm in diameter. The density oF valence electrons in copper is roughly 9 × 10 28 m - 3 . ±ind the driFt speed oF these electrons. Correct answer: 1

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## This note was uploaded on 10/11/2010 for the course PHY 56715 taught by Professor Turner during the Spring '10 term at University of Texas at Austin.

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hw 12-solutions - howard (cah3459) hw 12 turner (56705)...

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