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Unformatted text preview: segovia (cs39966) Homework 6 Spurlock (14441) 1 This printout should have 21 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points How much energy is stored in a 73 mH induc tor at the instant when the current is 3 . 2 A? Correct answer: 0 . 37376 J. Explanation: Let : L = 73 mH = 0 . 073 H and I = 3 . 2 A . The energy is U = 1 2 L I 2 = 1 2 (0 . 073 H)(3 . 2 A) 2 = . 37376 J . 002 10.0 points An inductor of 240 turns has a radius of 7 cm and a length of 29 cm. The permeability of free space is 1 . 25664 10 6 N / A 2 . Find the energy stored in it when the cur rent is 0 . 6 A. Correct answer: 0 . 000691596 J. Explanation: Let : r = 7 cm = 0 . 07 m , = 29 cm = 0 . 29 m , N = 240 turns , I = 0 . 6 A , and = 1 . 25664 10 6 N / A 2 . The inductance is L = N 2 A = N 2 r 2 = (1 . 25664 10 6 N / A 2 ) (240 turns) 2 (0 . 07 m) 2 . 29 m = 0 . 0038422 H . The energy stored in an inductor is U = 1 2 L I 2 = 1 2 (0 . 0038422 H)(0 . 6 A) 2 = . 000691596 J . 003 10.0 points A slinky toy spring has a radius of 3 . 4 cm and an inductance of 330 H when extended to a length of 0 . 6 m. How many turns are in the spring? Correct answer: 208 . 293 turns. Explanation: Let : r = 3 . 4 cm = 0 . 034 m , L = 330 H = 0 . 00033 H , and = 0 . 6 m . From L = N 2 A , we find N = radicalBigg L A = radicalBigg (0 . 00033 H)(0 . 6 m) (1 . 25664 10 6 n / A 2 ) (0 . 034 m) 2 = 208 . 293 turns . 004 (part 1 of 3) 10.0 points An inductor and a resistor are connected with a double pole switch to a battery as shown in the figure. The switch has been in position b for a long period of time. 200 mH 6 . 13 7 . 6 V S b a segovia (cs39966) Homework 6 Spurlock (14441) 2 If the switch is thrown from position b to position a (connecting the battery), how much time elapses before the current reaches 155 mA? Correct answer: 4 . 35739 ms. Explanation: Let : R = 6 . 13 , L = 200 mH , and E = 7 . 6 V . L R E S b a The time constant of an RL circuit is = L R = . 2 H 6 . 13 = 0 . 0326264 s . The final current reached in the circuit is I = E R = 7 . 6 V 6 . 13 = 1 . 2398 A . The switch is in position a in an RL circuit connected to a battery at t = 0 when I = 0. Then the current vs. time is I = I parenleftBig 1 e t / parenrightBig . Solving the above expression for t , when I = I 1 gives t 1 = ln parenleftbigg 1 I 1 I parenrightbigg = (0 . 0326264 s) ln parenleftbigg 1 . 155 A 1 . 2398 A parenrightbigg = 4 . 35739 ms . 005 (part 2 of 3) 10.0 points What is the maximum current in the inductor a long time after the switch is in position a ?...
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This note was uploaded on 11/10/2011 for the course PHYSICS 303L taught by Professor Hoffman during the Fall '11 term at University of Texas at Austin.
 Fall '11
 HOFFMAN
 Energy, Work

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