# 456 - Version 081/ABBAB – midterm 02 – turner –(56725...

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Unformatted text preview: Version 081/ABBAB – midterm 02 – turner – (56725) 1 This print-out should have 20 questions. Multiple-choice 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 kilowatt-hour, 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 parallel-plate 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. 1.12687e-11 2. 3.08666e-11 3. 8.86967e-11 4. 4.45357e-11 5. 1.43853e-11 6. 1.0662e-11 7. 2.77155e-11 8. 3.13416e-11 9. 1.09802e-11 10. 9.15348e-12 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 ǫ = 8 . 85419 × 10 − 12 C 2 / N · m 2 . The capacitance is C = ǫ A d , and the stored energy is W = 1 2 C V 2 = ǫ 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 μ F 3 μ F 3 μ F 3 μ F 3 μ F 3 μ F Find the equivalent capacitance of the cir- cuit. 1. C = 2 μ F correct 2. C = 4 μ F 3. C = 3 2 μ F 4. C = 4 3 μ F 5. C = 1 μ F 6. C = 2 3 μ F 7. C = 3 μ 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 μ F , C 2 = C = 3 μ F , C 3 = C = 3 μ F , C 4 = C = 3 μ F , C 5 = C = 3 μ F , C 6 = C = 3 μ F , and E B = 6 V ....
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456 - Version 081/ABBAB – midterm 02 – turner –(56725...

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