Practice Homework 17 2010

# Practice Homework 17 2010 - choe (dec865) Homework 17...

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Unformatted text preview: choe (dec865) Homework 17 Weathers (22202) 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points An automobile starter motor draws a current of 2 . 3 A from a 18 . 1 V battery when operating at normal speed. A broken pulley locks the ar- mature in position, and the current increases to 14 . 1 A. What was the back emf of the motor when operating normally? Correct answer: 15 . 1475 V. Explanation: Let : I = 14 . 1 A , I = 2 . 3 A , and E = 18 . 1 V . When not rotating, E = I R , and from this, R = E I = 18 . 1 V 14 . 1 A = 1 . 28369 , When rotating, E E back = I R , or E back = E I R = 18 . 1 V (2 . 3 A) (1 . 28369 ) = 15 . 1475 V . 002 10.0 points In an RL series circuit, an inductor of 4 . 43 H and a resistor of 6 . 49 are connected to a 21 . 4 V battery. The switch of the circuit is initially open. Next close the switch and wait for a long time. Eventually the current reaches its equilibrium value. At this time, what is the corresponding energy stored in the inductor? Correct answer: 24 . 0831 J. Explanation: Let : L = 4 . 43 H , R = 6 . 49 , and E = 21 . 4 V . The current in an RL circuit is I = E R parenleftBig 1 e Rt/L parenrightBig . The final equilibrium value of the current, which occurs as t , is I = E R = 21 . 4 V 6 . 49 = 3 . 29738 A . The energy stored in the inductor carrying a current 3 . 29738 A is U = 1 2 L I 2 = 1 2 (4 . 43 H) (3 . 29738 A) 2 = 24 . 0831 J . 003 10.0 points A uniform electric field of magnitude 1 . 02 10 6 V / m throughout a cylindrical volume re- sults in a total energy of 1 . 53 J. What magnetic field over this same region stores the same total energy? Correct answer: 0 . 00340235 T. Explanation: Let : E = 1 . 02 10 6 V / m . The electric field density is u E = E 2 2 . The magnetic field density is u B = B 2 2 . We are looking for B such that U B = U M u B V = u E V . choe (dec865) Homework 17 Weathers (22202) 2 In the above equation, V is the volume of the region under consideration. The speed of light c = 1 , so B 2 = E 2 B = E = E c = 1 . 02 10 6 V / m 2 . 99792 10 8 m / s = . 00340235 T . 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. 214 mH 7 . 44 7 . 9 V S b a If the switch is thrown from position b to position a (connecting the battery), how much time elapses before the current reaches 148 mA? Correct answer: 4 . 31753 ms. Explanation: Let : R = 7 . 44 , L = 214 mH , and E = 7 . 9 V ....
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Practice Homework 17 2010 - choe (dec865) Homework 17...

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