814_Physics ProblemsTechnical Physics

814_Physics ProblemsTechnical Physics - 154 P28.66 Direct...

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154 Direct Current Circuits P28.66 Start at the point when the voltage has just reached 2 3 V and the switch has just closed. The voltage is 2 3 V and is decaying towards 0 V with a time constant RC 2 ∆∆ Vt Ve C tRC af = L N M O Q P 2 3 2 . We want to know when C will reach 1 3 V . Therefore, 1 3 2 3 2 VV e = L N M O Q P or e = 2 1 2 or tR C 12 2 = ln . V R 1 R 2 V + C V c Voltage controlled switch FIG. P28.66 After the switch opens, the voltage is 1 3 V , increasing toward V with time constant RRC + bg : V C tR RC =− L N M O Q P −+ 2 3 . When V C = 2 3 2 3 2 3 V e or e = 1 2 . So R C 21 2 2 =+ ln and Tt t R RC =+= + 1 2 22 ln . P28.67 (a) First determine the resistance of each light bulb: P = V R 2 R V == = af a f 120 60 0 240 V W . . We obtain the equivalent resistance R eq of the network of light bulbs by identifying series and parallel equivalent resistances: FIG. P28.67 RR eq + 1 23 1 11 240 120 360
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This note was uploaded on 12/14/2011 for the course PHY 203 taught by Professor Staff during the Fall '11 term at Indiana State University .

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