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PHY2054 Spring 2007 Prof. Eugene Dunnam
Prof. Paul Avery
Apr. 11, 2007 Exam 3 Solutions 1. An 8.0 mH inductor and a 2.50 resistor are wired in series to a 23.6 V ideal battery. A switch in the circuit is closed at time 0. at whighi'msjieennentisfirm a time 5.4 msec ger the
“FEELS mwithtlomjfldifierence in volts across the indqetiis: (94.4 M VFW“ * (2) 19.2
(3) 13.0
(4) 15.2
(5) 7.7 ThecurrentinanRL circuit, starting at0, is i=imx(l—e"/'), where r=L/R and im =V/R andVisthe voltage. Ihus VL =V—iR=V—V(l—e"/')==Ve"m We calculate r=
L / R = 3.2 msec. Plugging in yields VL = 4.4 V. 2. Refer to the previous problem. How much energy in joules is stored in the inductor after the
switch has been closed a long time? @315
(2) o
(3) 0.24
(4) 0.012
(5) 0.17 The energy stored in the inductor is U L =12-Li2 . T 0 find the current, we see that afier a long time
the current reaches itsfidl value eff = 23.6/2.5 = 9.44.4. Thus UL = 0.36 J. ...
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