spring_08_exam3_sol_phy2054

# spring_08_exam3_sol_phy2054 - PHY2054 Spring 2008 Prof P...

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PHY2054 Spring 2008 1 Prof. P. Kumar Prof. P. Avery April 3, 2008 Exam 3 Solutions 1. ( Ch. 20 ) In the circuit shown, L = 56 mH, R = 4.6 Ω and V = 12.0 V. The switch S has been open for a long time then is suddenly closed at t = 0. At what value of t (in msec) will the current in the inductor reach 1.1 A? (1) 6.67 (2) 10.5 (3) 2.88 (4) 19.0 (5) None of these The current in an RL circuit, starting at 0, is ( ) / max 1 t i i e τ = , where / L R τ = and max / i V R = and V is the voltage. Here i max = 12.0 / 4.6 = 2.609 and τ = 0.056 / 4.6 = 12.17 msec. The solution is ( ) max ln 1 / 6.67 t i i τ = − = msec. 2. Refer to the previous problem. What is the total energy stored in the inductor a long time after the switch is closed? (1) 0.19 J (2) 0.048 J (3) 0.76 J (4) 0.034 J (5) None of these After a long time, the inductor can be ignored and the current in the inductor is i = 12.0/4.6 = 2.61 A. The energy in the inductor is 2 1 2 0.19 L U Li = = J.

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