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Practice HW 7 solution

# Practice HW 7 solution - song(shs546 Practice HW 7...

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song (shs546) – Practice HW 7 Solutions – weathers – (22202) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A 16 pF capacitor is connected across a 77 V source. What charge is stored on it? Correct answer: 1 . 232 × 10 - 9 C. Explanation: Let : C = 16 pF = 1 . 6 × 10 - 11 F and V = 77 V . The capacitance is C = q V q = C V = (1 . 6 × 10 - 11 F) (77 V) = 1 . 232 × 10 - 9 C 002(part1of3)10.0points The Coulomb constant is 8 . 98755 × 10 9 N · m 2 / C 2 . If a drop of a conducting liquid had a ca- pacitance of 0 . 7 pF in air, what is its radius? Correct answer: 0 . 00629129 m. Explanation: Let : k e = 8 . 98755 × 10 9 N · m 2 / C 2 , C = 0 . 7 pF = 7 × 10 - 13 F and V = 115 V . If the charge on the drop is Q and its radius is R , then the potential difference between the surface and infinity is V = k e Q R . Thus the capacitance of the drop is C = Q V = R k e R = k e C = ( 8 . 98755 × 10 9 N · m 2 / C 2 ) × (7 × 10 - 13 F) = 0 . 00629129 m . 003(part2of3)10.0points If its radius is 3 . 2 mm, what is its capacitance? Correct answer: 3 . 56048 × 10 - 13 F. Explanation: Let : r = 3 . 2 mm = 0 . 0032 m . C = r k e = 0 . 0032 m 8 . 98755 × 10 9 N · m 2 / C 2 = 3 . 56048 × 10 - 13 F . 004(part3of3)10.0points What is the charge on the smaller drop if its potential is 115 V? Correct answer: 4 . 09455 × 10 - 11 C. Explanation: Q = C V = ( 3 . 56048 × 10 - 13 F ) (115 V) = 4 . 09455 × 10 - 11 C .

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