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28.4 RC Circuits So far we have analyzed direct current circuits in which the current is constant. In DC RC circuit. Charging a Capacitor Figure 28.19 shows a simple series RC circuit. Let us assume that the capacitor in this 4 Note that during charging, charges do 4 In previous discussions of capacitors, we assumed a steady-state situation, in which no current was present in any branch of the circuit containing a capacitor. Now we are considering the case before the SECTION 28.4 • RC Circuits 873 Example 28.10 A Multiloop Circuit (A) Under steady-state conditions, find the unknown currents I 1 , I 2 , and I 3 in the multiloop circuit shown in Figure 28.18. Solution First note that because the capacitor represents 1 reach point g, they all go toward point b through the 8.00-V battery; hence, I gb # I 1 . Labeling the currents as shown in Figure 28.18 and applying (1) I 1 % I 2 # I 3 Equation 28.10 applied to loops defcd and cfgbc, traversed (2) defcd 4.00 V $ (3.00 ')I 2 $ (5.00 ')I 3 # 0 (3) cfgbc (3.00 ')I 2 $ (5.00 ')I 1 % 8.00 V # 0 From Equation (1) we see that I 1 # I 3 $ I 2 , which, when substituted into Equation (3), gives (4) (8.00 ')I 2 $ (5.00 ')I 3 % 8.00 V # 0 Subtracting Equation (4) from Equation (2), we eliminate I 3 and find that Because our value for I 2 is negative, we conclude that the di- rection of I 2 is from c to f in the 3.00-' resistor. Despite this interpretation of the direction, however, we must continue 2 in subsequent calculations because our equations were established with our original Using I 2 # $ 0.364 A in Equations (3) and (1) gives (B) What is the charge on the capacitor? Solution We can apply Kirchhoff’s loop rule to loop bghab cap across the capacitor. We use this potential difference in the loop equation without reference $ 8.00 V % "V cap $ 3.00 V # 0 " V cap # 11.0 V Because Q # C "V cap (see Eq. 26.1), the charge on the capacitor is Q # (6.00 )F)(11.0 V) # Why is the left side of the capacitor positively charged? 66.0 )C 1.02 A I 3 # 1.38 A I 1 # $ 0.364 A I 2 # $ 4.00 V 11.0 ' # 4.00 V d c 5.00 Ω – + 8.00 V 3.00 Ω – + e I 3 f I 1 I 2 5.00 Ω h a g – + 3.00 V – + 6.00 F I = 0 b I 3 I 1 µ Figure 28.18 (Example 28.10) A multiloop circuit. Kirchhoff’s loop rule can be applied to any closed loop, including the one containing the capacitor. |