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Unformatted text preview: 126 CHAPTER 18 18.44 From Q = Qmax ( 1 − e −t τ ) , the ratio Q Qmax at t = 2τ is found to be
Q
1
= 1 − e −2τ τ = 1 − 2 = 0.865 , or Q is 86.5% of Qmax at t = 2τ
Qmax
e 18.45 The resistive network between a an b reduces, in the stages shown below, to an
equivalent resistance of Req = 7.5 Ω .
2.4 W 5.1 W a 18.46 a a 1.8 W
b 2.4 W 2.4 W 3.6 W 8.6 W 1.8 W 3.5 W 1.5 W 3.6 W b a 3.6 W b b (a) The circuit reduces as shown below to an equivalent resistance of Req = 14 Ω . 5.0 W
28 V 3.0 W 10 W 3.0 W 10 W 4.0 W 5.0 W 4.0 W 2.0 W 28 V 3.0 W 10 W 3.0 W 10 W 4.0 W 5.0 W 3.0 W 10 W 5.0 W 4.0 W 2.0 W 28
V 10 W 2.0 W Figure 1 28
V 7.5 W Figure 2 5.0 W
10 W 28
V 10 W Figure 3 Figure 5 Figure 4 (b) The power dissipated in the circuit is P = 5.0 W
4.0 W 4.0 W ( ∆V )
Req 2 = I 5.0 W ( 28 V )
14 Ω 28
V 14
W Figure 6 2 = 56 W (c) The current in the original 5.0 Ω resistor (in Figure 1) is the total current supplied
by the battery. From Figure 6, this is
I= ∆V 28 V
=
= 2.0 A
Req 14 Ω ...
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 Spring '09
 Physics, Current, Work

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