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Unformatted text preview: hopkins (tlh982) – HW 05 – criss – (4908) 1 This printout should have 23 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A 3 . 1 cm length of tungsten filament in a small lightbulb has a resistance of 0 . 031 Ω. Find its diameter. (The resistivity is 5 . 6 × 10 − 8 Ω · m). Correct answer: 0 . 267023 mm. Explanation: Let : L = 3 . 1 cm = 0 . 031 m , R = 0 . 031 Ω , and ρ = 5 . 6 × 10 − 8 Ω · m . Resistance is R = ρL A = ρL π parenleftbigg d 2 parenrightbigg 2 = 4 ρL π d 2 , d = radicalbigg 4 ρL π R = radicalBigg 4 (5 . 6 × 10 − 8 Ω · m) (0 . 031 m) π (0 . 031 Ω) × parenleftbigg 1000 mm 1 m parenrightbigg = . 267023 mm . 002 10.0 points A length of copper wire has a resistance 32 Ω. The wire is cut into three pieces of equal length. The pieces are then connected as parallel lengths between points A and B. What resistance will this new “wire” of length L 3 have between points A and B? Correct answer: 3 . 55556 Ω. Explanation: Let : R = 32 Ω . The new wire has length L = L 3 and cross section A = 3 A , so its resistance is R = ρL A = ρ parenleftbigg L 3 parenrightbigg 3 A = 1 9 parenleftbigg ρL A parenrightbigg = R 9 = 32 Ω 9 = 3 . 55556 Ω . 003 10.0 points A 11 V battery delivers 116 mA of current when connected to a 74 Ω resistor. Determine the internal resistance of the battery. Correct answer: 20 . 8276 Ω. Explanation: Let : V = 11 V , I = 116 mA = 0 . 116 A , and R = 74 Ω . The internal resistance is in series with the given resistor, so V = I ( R + r ) r = V I R = 11 V . 116 A 74 Ω = 20 . 8276 Ω . 004 10.0 points A platinum resistance thermometer has a re sistance of 200 Ω when placed in a ice 0 ◦ C bath and 169 Ω when immersed in a crucible containing a melting substance. What is the melting point of the substance? The temperature coefficient of the platinum hopkins (tlh982) – HW 05 – criss – (4908) 2 is 0 . 00392( ◦ C) − 1 . Correct answer: 36 . 4408 ◦ C. Explanation: Let : T = 20 ◦ C , R 1 = 200 Ω , T 1 = 0 ◦ C , R = 169 Ω , and α = 0 . 00392( ◦ C) − 1 . The resistance at 20 ◦ C is R = R 1 1 + α ( T 1 T ) = 200 Ω 1 + [0 . 00392( ◦ C) − 1 ] (0 ◦ C 20 ◦ C) = 217 . 014 Ω , and R = R [1 + α ( T T )] = R + R α ( T T ) T T = R R αR T = T + R R αR T = 20 ◦ C + 169 Ω 217 . 014 Ω [0 . 00392( ◦ C) − 1 ](217 . 014 Ω) = 36 . 4408 ◦ C . 005 (part 1 of 4) 10.0 points An energy plant produces an output potential of 4300 kV and serves a city 176 km away. A highvoltage transmission line carries 690 A to the city. The effective resistance of a trans mission line [wire(s)] is 4 . 68 Ω / km times the distance from the plant to the city....
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This note was uploaded on 09/03/2009 for the course PHY PHY 2049 taught by Professor Robertr.criss during the Fall '08 term at University of South Florida.
 Fall '08
 RobertR.Criss
 Physics, Light

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