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ch20-h3-solutions - liu(ql744 ch20-h3 chiu(56565 This...

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This print-out should have 7 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A 0 . 3 μ F capacitor is given a charge Q 0 . After 2 s, the capacitor’s charge is 0 . 5 Q 0 . What is the e ff ective resistance across this capacitor? Correct answer: 9 . 61797 M Ω . Explanation: Let : C = 0 . 3 μ F = 3 × 10 - 7 F and t = 2 s . The charge on the capacitor is Q ( t ) = Q 0 e - t/ τ . e t/ τ = Q 0 Q t τ = ln Q 0 Q τ = t ln Q 0 Q . The e ff ective resistance is R = τ C = t C ln Q 0 Q = 2 s (3 × 10 - 7 F) ln Q 0 0 . 5 Q 0 · 1 M Ω 10 6 Ω = 9 . 61797 M Ω . 002 (part 1 of 2) 10.0 points A 12 V automobile battery is connected to an electric starter motor. The current through the motor is 255 A. How much power does the motor dissipate? Correct answer: 3060 W. Explanation: Power is P = I V = (255 A) (12 V) = 3060 W . 003 (part 2 of 2) 10.0 points How much energy does the battery deliver to the motor during a 14 . 5 s period? Correct answer: 44370 J. Explanation: Let : V = 12 V , I = 255 A , and t = 14 . 5 s .
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