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Unformatted text preview: Version 081/ABBAB midterm 02 turner (56725) 1 This printout should have 20 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points A certain coffeepot draws 4 A of current when it is operated on 120 V household lines. If electrical energy costs 10 cents per kilowatthour, how much does it cost to oper ate the coffeepot for 2 hours? 1. cost = 16 cents 2. cost = 4.8 cents 3. cost = 9.6 cents correct 4. cost = 8.0 cents 5. cost = 2.4 cents Explanation: Let : I = 4 A , V = 120 V , r = $0 . 1 / kWh , and t = 2 h . The power that the coffeepot consumes is P = I V , so the cost to operate the coffeepot for 2 hours is C = I V t r = (4 A) (120 V) (2 h) ($0 . 1 / kWh) parenleftbigg 1 kW 1000 W parenrightbigg = 9.6 cents . 002 10.0 points A parallelplate capacitor has 3 . 07 cm 2 plates that are separated by 9 . 34 mm with air be tween them. The permittivity of a vacuum is 8 . 85419 10 12 C 2 / N m 2 . If a 8 . 8 V battery is connected to this ca pacitor, how much energy does it store? 1. 1.12687e11 2. 3.08666e11 3. 8.86967e11 4. 4.45357e11 5. 1.43853e11 6. 1.0662e11 7. 2.77155e11 8. 3.13416e11 9. 1.09802e11 10. 9.15348e12 Correct answer: 1 . 12687 10 11 J. Explanation: Let : A = 3 . 07 cm 2 = 0 . 000307 m 2 , d = 9 . 34 mm = 0 . 00934 m , V = 8 . 8 V , and = 8 . 85419 10 12 C 2 / N m 2 . The capacitance is C = A d , and the stored energy is W = 1 2 C V 2 = A V 2 2 d = (8 . 85419 10 12 C 2 / N m 2 ) (0 . 000307 m 2 ) (8 . 8 V) 2 2 (0 . 00934 m) = 1 . 12687 10 11 J . 003 10.0 points Suppose that a voltage surge (increase) pro duces 127 V for a moment. By what percentage will the output of a 108 V, 124 W lightbulb increase, assuming its resistance does not change? 1. 150.133 2. 31.7278 3. 38.2802 Version 081/ABBAB midterm 02 turner (56725) 2 4. 27.4714 5. 104.356 6. 278.951 7. 92.6208 8. 73.934 9. 35.1406 10. 69.0 Correct answer: 38 . 2802%. Explanation: Let : V = 127 V , V = 108 V , and P = 124 W . Power is P = V 2 R % Change = parenleftbigg P P 1 parenrightbigg 100% = V 2 R V 2 R 1 100% = parenleftbigg V 2 V 2 1 parenrightbigg 100% = bracketleftbigg (127 V) 2 (108 V) 2 1 bracketrightbigg 100% = 38 . 2802% . 004 (part 1 of 2) 10.0 points Consider the system of capacitors connected to a battery as shown below. E B 3 F 3 F 3 F 3 F 3 F 3 F Find the equivalent capacitance of the cir cuit. 1. C = 2 F correct 2. C = 4 F 3. C = 3 2 F 4. C = 4 3 F 5. C = 1 F 6. C = 2 3 F 7. C = 3 F Explanation: E B C 1 C 2 C 3 C 4 C 5 C 6 c a b c d Let : C 1 = C = 3 F , C 2 = C = 3 F , C 3 = C = 3 F , C 4 = C = 3 F , C 5 = C = 3 F , C 6 = C = 3 F , and E B = 6 V ....
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 Spring '08
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