HW03-solutions

# HW03-solutions - kazi(mik88 HW03 tsoi(57210 This print-out...

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kazi (mik88) – HW03 – tsoi – (57210) 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(part1of2)10.0points The potential difference in a simple circuit is 6 V and the resistance is 47 Ω. What current I flows in the circuit? Correct answer: 0 . 12766 A. Explanation: Let : V = 6 V and R = 47 Ω . The current is I = V R = 6 V 47 Ω = 0 . 12766 A . 002(part2of2)10.0points How many electrons pass a given point in the circuit in 4 min? Correct answer: 1 . 91489 × 10 20 . Explanation: Let : I = 0 . 12766 A , t = 4 min = 240 s , and q e = 1 . 6 × 10 - 19 C . I = q t , where q is in coulombs and t in seconds; that is, q = I t, and the total charge for n electrons is q = n q e , so n = q q e = I t q e = (0 . 12766 A) (240 s) 1 . 6 × 10 - 19 C = 1 . 91489 × 10 20 . 003(part1of2)10.0points Consider two conductors 1 and 2 made of the same ohmic material; i.e. , ρ 1 = ρ 2 . Denote the length by , the cross sectional area by A . The same voltage V is applied across the ends of both conductors. The field E is inside of the conductor. V 1 vector E 1 I 1 1 r 1 V 2 vector E 2 I 2 2 r 2 If A 2 = 2 A 1 , ℓ 2 = 2 1 and V 2 = V 1 , find the ratio E 2 E 1 of the electric fields. 1. E 2 E 1 = 2 2. E 2 E 1 = 1 16 3. E 2 E 1 = 8 4. E 2 E 1 = 1 3 5. E 2 E 1 = 1 4 6. E 2 E 1 = 1 12 7. E 2 E 1 = 1 8. E 2 E 1 = 1 2 correct 9. E 2 E 1 = 1 8 10. E 2 E 1 = 4 Explanation: E 2 E 1 = V 2 2 V 1 1 = 1 2 = 1 2 1 = 1 2 . 004(part2of2)10.0points Determine the ratio I 2 I 1 of the currents .

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kazi (mik88) – HW03 – tsoi – (57210) 2 1. I 2 I 1 = 2 2. I 2 I 1 = 1 3 3. I 2 I 1 = 1 8 4. I 2 I 1 = 1 4 5. I 2 I 1 = 1 16 6. I 2 I 1 = 4 7. I 2 I 1 = 8 8. I 2 I 1 = 1 correct 9. I 2 I 1 = 1 2 10. I 2 I 1 = 1 12 Explanation: I 2 I 1 = V 2 R 2 V 1 R 1 = R 1 R 2 = ρ 1 parenleftbigg 1 A 1 parenrightbigg ρ 2 parenleftbigg 2 A 2 parenrightbigg = 1 A 1 2 1 2 A 1 = 1 . 005 10.0points Which of the following copper conductor con- ditions has the least resistance? 1. thick, short, and cool correct 2. thin, long, and hot 3. thick, long, and hot 4. thin, long, and cool 5. thick, long, and cool 6. thin, short, and cool 7. thick, short, and hot 8. thin, short, and hot Explanation: We know that the resistance R of a conduc- tor is determined by the resistivity ρ , cross- sectional area A and length : R = ρ A . Thus we need smaller ρ and and larger A . A cool copper conductor has a lower resistivity than a hot one does, namely a smaller ρ . A thicker conductor means a larger A . A short conductor gives a smaller .
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