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SECTION 28.4 • RC Circuits 877 Quick Quiz 28.10 Consider the circuit in Figure 28.22 and assume that the battery has no internal resistance. Just after the switch is closed, the current in the bat- ! ! ! ! ! ! ε C R R Figure 28.22 (Quick Quiz 28.10) How does the current vary after the switch is closed? Conceptual Example 28.11 Intermittent Windshield Wipers Many automobiles are equipped with windshield wipers that Solution The wipers are part of an RC circuit whose time through a multiposition switch. As it increases with time, the Example 28.12 Charging a Capacitor in an RC Circuit An uncharged capacitor and a resistor are connected in 5 ' , find the time constant of the circuit, the maximum charge on the capacitor, the Solution The time constant of the circuit is - # RC # 5 ' )(5.00 . 10 $ 6 F) # 4.00 s. The maximum charge on the capacitor is Q # C # (5.00 )F)(12.0 V) # 0 # /R # (12.0 V)/(8.00 . 10 5 ' ) # 15.0 )A. Using these values and Equations 28.14 and 28.15, we find that Graphs of these functions are provided in Figure 28.24. (15.0 )A)e $ t/4.00 s I(t
) # (60.0 )C)(1 $ e $ t/4.00 s ) q(t
) # ! ! ! Interactive R ε C + – S 0 1 2 3 4 5 6 7 0 10 20 30 40 50 60 q( µ C) t (s) 0 1 2 3 4 5 6 7 0 5 10 15 I( µ A) t (s) (a) (b) µ µ Q = 60.0 µ C µ I 0 = 15.0 µ A µ t = τ At the Interactive Worked Example link at http://www.pse6.com, you can vary R, C, and ! and observe the charge and current as functions of time while charging or discharging the capacitor. Figure 28.23 (Example 28.12) The switch in this series RC circuit, open for times t * 0, is closed at t # 0. Figure 28.24 (Example 28.12) Plots of (a) charge versus time and (b) current versus time for the RC circuit shown in Figure 28.23, with # 12.0 V, R # 8.00 . 10 5 ' , and C # 5.00 ) F. ! |