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Unformatted text preview: zapata (jz883) – HW #3 – Antoniewicz – (56495) 1 This printout should have 19 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The total charge that an automobile battery can supply without being recharged is given in terms of Amperehours. A typical 12V battery has a rating of 72 A · h . Suppose you forget to turn off the headlights in your parked automobile. If each of the two headlights draws 1 . 7 A , find out the time before your battery is “dead”. Correct answer: 21 . 1765 h. Explanation: Let : rating = 72 A · h and I = 1 . 7 A . If only one of the two headlights is left on, the time before the battery dies is given by T 1 = rating I = 72 A · h 1 . 7 A = 42 . 3529 h . Thus the required time is 21 . 1765 h . 002 10.0 points A commercial capacitor is constructed as in the figure. This particular capacitor is made from two stripes of aluminum separated by a strip of paraffincoated paper. Each strip of foil and paper is 5 . 88 cm wide. The foil is . 0049 mm thick, and the paper is 0 . 0258 mm thick and has a dielectric constant of 3 . 8. Metal Foil Paper What length should the strips be if a capac itance of 6 . 24 × 10 − 8 F is desired before the capacitor is rolled up? (Use the parallelplate formula. Adding a second strip of paper and rolling up the capacitor doubles its capaci tance by allowing both surfaces of each strip of foil to store charge.) Correct answer: 0 . 814142 m. Explanation: Let : k = 3 . 8 , w = 5 . 88 cm = 0 . 0588 m , ǫ = 8 . 85 × 10 − 12 C 2 / N · m 2 , d 1 = 0 . 0049 mm = 4 . 9 × 10 − 6 m , d 2 = 0 . 0258 mm = 2 . 58 × 10 − 5 m , and C = 6 . 24 × 10 − 8 F . Before the capacitor is rolled, the capacitor of this parallel plate capacitor is C = k ǫ A d = k ǫ w L d , where A is the surface area of one side of a foil strip. Thus the required length is L = C × d k ǫ w = (6 . 24 × 10 − 8 F) × (2 . 58 × 10 − 5 m) (3 . 8) (8 . 85 × 10 − 12 C 2 / N · m 2 ) (0 . 0588 m) = . 814142 m . 003 10.0 points An incandescent bulb has a resistance of 22 Ω when it is at room temperature (25 ◦ C) and 478 Ω when it is hot and delivering light to the room. The temperature coefficient of re sistivity of the filament is 0 . 005 ( ◦ C) − 1 , where the base resistance R is determined at 0 ◦ C....
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This note was uploaded on 10/01/2010 for the course PHYSICS PHY 303 L taught by Professor Antoniewicz during the Spring '10 term at Texas Pan American.
 Spring '10
 ANTONIEWICZ
 Physics, Charge

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