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solutions 8

# solutions 8 - Patel Kinal Homework 8 Due 7:00 pm Inst...

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Patel, Kinal – Homework 8 – Due: Feb 19 2008, 7:00 pm – Inst: Weathers 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. The due time is Central time. 001 (part 1 of 2) 10 points The quantity of charge passing through a sur- face of area 1 . 96 cm 2 varies with time as q = q 1 t 3 + q 2 t + q 3 , where q 1 = 4 . 7 C / s 3 , q 2 = 3 . 7 C / s, q 3 = 2 . 4 C, and t is in seconds. What is the instantaneous current through the surface at t = 0 . 8 s? Correct answer: 12 . 724 A. Explanation: Let : q 1 = 4 . 7 C / s 3 , q 2 = 3 . 7 C / s , q 3 = 2 . 4 C , and t = 0 . 8 s . I d q dt = 3 q 1 t 2 + q 2 = 3 ( 4 . 7 C / s 3 ) (0 . 8 s) 2 + 3 . 7 C / s = 12 . 724 A . 002 (part 2 of 2) 10 points What is the value of the current density at t = 0 . 8 s? Correct answer: 64918 . 4 A / m 2 . Explanation: Let : a = 1 . 96 cm 2 = 0 . 000196 m 2 and t = 0 . 8 s . J I A = 12 . 724 A 0 . 000196 m 2 = 64918 . 4 A / m 2 . keywords: 003 (part 1 of 1) 10 points A conductor with cross-sectional area 7 cm 2 carries a current of 12 A. If the concentration of free electrons in the conductor is 9 × 10 28 electrons / m 3 , what is the drift velocity of the electrons? Correct answer: 0 . 00118886 mm / s. Explanation: Let : I = 12 A , n = 9 × 10 28 electrons / m 3 , q e = 1 . 60218 × 10 - 19 C , and A = 7 cm 2 = 0 . 0007 m 2 .

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