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Unformatted text preview: zhou (cz3574) – HW7 – mackie – (10621) 1 This printout should have 31 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A 19 V battery delivers 113 mA of current when connected to a 76 Ω resistor. Determine the internal resistance of the battery. Correct answer: 92 . 1416 Ω. Explanation: Let : V = 19 V , I = 113 mA = 0 . 113 A , and R = 76 Ω . The internal resistance is in series with the given resistor, so V = I ( R + r ) r = V I R = 19 V . 113 A 76 Ω = 92 . 1416 Ω . 002 (part 1 of 2) 10.0 points A string of 30 identical Christmas tree lights are connected in series to a 120 V source. The string dissipates 81 W. What is the equivalent resistance of the light string? Correct answer: 177 . 778 Ω. Explanation: Let : V = 120 V and P = 81 W . Resistance is R = V 2 P = (120 V) 2 81 W = 177 . 778 Ω . 003 (part 2 of 2) 10.0 points What is the resistance of a single light? Correct answer: 5 . 92593 Ω. Explanation: Let : n = 30 . R = n R 1 , so R 1 = R n = 177 . 778 Ω 30 = 5 . 92593 Ω . 004 10.0 points Consider the circuit shown in the figure. 25 V 27 Ω 2 Ω 39 Ω 12 Ω 18 Ω Find its equivalent resistance. Correct answer: 48 . 0621 Ω. Explanation: 25 V R a R b R c R d R e Let : R a = 27 Ω , R b = 2 Ω , R c = 39 Ω , R d = 12 Ω , and R e = 18 Ω . R a and R b are in parallel, so 1 R ab = 1 R a + 1 R b zhou (cz3574) – HW7 – mackie – (10621) 2 R ab = parenleftbigg 1 R a + 1 R b parenrightbigg − 1 R ab = parenleftbigg 1 27 Ω + 1 2 Ω parenrightbigg − 1 = 1 . 86207 Ω R d and R e are in parallel, so R de = parenleftbigg 1 R d + 1 R e parenrightbigg − 1 R de = parenleftbigg 1 12 Ω + 1 18 Ω parenrightbigg − 1 = 7 . 2 Ω These resistors are in series, so R eq = R ab + R c + R de = 1 . 86207 Ω + 39 Ω + 7 . 2 Ω = 48 . 0621 Ω . 005 10.0 points A resistor with an unknown resistance is con nected in parallel to a 7 Ω resistor. When both resistors are connected in parallel to an emf source of 13 V, the current across the un known resistor is measured with an ammeter to be 4 A. What is the resistance of the unknown re sistor? Correct answer: 3 . 25 Ω. Explanation: Let : Δ V = 13 V , and I 1 = 4 A . Δ V = IR R 1 = Δ V I 1 = 13 V 4 A = 3 . 25 Ω . 006 (part 1 of 2) 10.0 points The power supplied to the circuit shown in the figure is 10.0 W. E 2 . 0 Ω 19 . 0 Ω 7 . 0 Ω 4 . 4 Ω 2 . 0 Ω a) Find the equivalent resistance of the cir cuit. Correct answer: 3 . 65264 Ω. Explanation: E R 1 R 2 R 3 R 4 R 5 Let : R 1 = 2 . 0 Ω , R 2 = 19 . 0 Ω , R 3 = 7 . 0 Ω , R 4 = 4 . 4 Ω , and R 5 = 2 . 0 Ω . For resistors in parallel, 1 R eq,p = 1 R a + 1 R b ....
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 Spring '11
 Hiyishi
 Current, Correct Answer, Series and parallel circuits, Zhou

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