Chapter 33 Solutions 33.1 ∆v(t) = ∆Vmax sin(ω t) = 2 ∆Vrms sin(ω t) = 200 2 sin[2 π (100t)] = (283 V) sin (628 t) 33.2 ∆Vrms = (a) P= (120 V)2 (∆Vrms )2 →R= = 193 Ω R 75.0 W (b) R= (120 V)2 = 144 Ω 100 W Each meter reads the rms value. 33.3 ∆Vrms = I rms = 33.4 170 V = 120 V 2 (a) 100 V = 70.7 V 2 ∆Vrms 70.7 V = = 2.95 A 24.0 Ω R ∆vR = ∆Vmax sin ω t ∆vR = 0.250 ( ∆Vmax ) , so sin ω t = 0.250, or ω t = sin −1 (0.250) The smallest angle for which this is true is ω t = 0.253 rad. Thus, if t = 0.010 0 s , ω= (b) The second time when ∆vR = 0.250 ( ∆Vmax ) , ω t = sin −1 (0.250) again. For this occurrence, ω t = π − 0.253 rad = 2 .89 rad (to understand why this is true, recall the identity sin( π − θ ) = sin θ from trigonometry). Thus, t= 33.5 0.253 rad = 25.3 rad/s 0.010 0 s 2.89 rad = 0.114 s 25.3 rad s iR = I max sin ω t becomes (0.00700)ω = sin–1(0.600) = 0.644 Thus, and 0.600 = sin(ω 0.00700) ω = 91.9 rad/s = 2π f so f = 14.6 Hz © 2000 by Harcourt, Inc. All rights reserved. Chapter 33 Solutions 271 P = I rms ( ∆Vrms ) and ∆Vrms = 120 V for each bulb (parallel circuit), so: 33.6 I1 = I 2 = I3 = ∆Vrms 120 V P1 150 W = = 1.25 A , and R1 = = = 96.0 Ω = R2 1.25 A ∆Vrms 120 V I1 ∆Vrms P3 100 W 120 V = = 0.833 A , and R3 = = = 144 Ω 0.833 A ∆Vrms 120 V I3 ∆Vmax = 15.0 V 33.7 I max = and Rtotal = 8.20 Ω + 10.4 Ω = 18.6 Ω ∆Vmax 15.0 V = = 0.806 A = 2 I rms 18.6 Ω Rtotal 2 0.806 A 2 P speaker = I rms Rspeaker = (10.4 Ω) = 3.38 W 2 For Imax = 80.0 mA, 33.8 Irms = 80.0 mA 2 = 56.6 mA Vrms 50.0 V (XL)min = Irms = 0.0566 A = 884 Ω XL 884 Ω XL = 2π f L → L = 2π f ≥ 2π (20.0) ≥ 7.03 H 33.9 (a) XL = L= (b) 33.10 XL 13.3 = = 0.0424 H = 42.4 mH 2 π (50.0) ω XL = ω= ∆Vmax 100 = = 13.3 Ω 7.50 I max ∆Vmax 100 = = 40.0 Ω 2.50 I max XL 40.0 = = 942 rad/s L 42.4 × 10 − 3 At 50.0 Hz, I max = X L 60.0 Hz 50.0 = XL = 2 π ( 50.0 Hz)L = 2 π ( 50.0 Hz) ( 54.0 Ω) = 45.0 Ω 2 π (60.0 Hz) 60.0 ∆Vmax = XL 2 ( ∆Vrms ) = XL 2 (100 V ) = 3.14 A 45.0 Ω © 2000 by Harcourt, Inc. All rights reserved. 272 Chapter 33 Solutions iL (t ) = 33.11 [ (80.0 V ) sin (65.0 π )(0.0155) − π 2 ∆Vmax sin (ω t − π 2) = ωL (65.0 π rad s) 70.0 × 10−3 H ( ) ] iL (t ) = ( 5.60 A ) sin(1.59 rad) = 5.60 A ω = 2 π f = 2 π (60.0 / s) = 377 rad / s 33.12 XL = ω L = (377 / s)(0.0200 V ⋅ s / A) = 7.54 Ω I rms = ∆Vrms 120 V = = 15.9 A 7.54 Ω XL I max = 2 I rms = 2 (15.9 A) = 22.5 A 2 π (60.0) 1 s i(t) = I max sin ω t = (22.5 A)sin ⋅ = (22.5 A) sin 120° = 19.5 A s 180 V ⋅ s (19.5 A)2 = 3.80 J U = 21 Li 2 = 21 0.0200 A L= 33.13 N ΦB where Φ B is the flux through each turn. I N Φ B, max = 33.14 (a) XC = ( 2 ∆VL, rms 1 : 2π f C 2π f XC ∝ f > 41.3 Hz 1 , so X(44) = 21 X(22): XC < 87.5 Ω C I max = 2 I rms = 33.15 max = 120 V ⋅ s T ⋅ C ⋅ m N ⋅ m J = 0.450 T · m2 2 π (60.0) N ⋅ s J V ⋅ C 1 < 175 Ω 2 π f (22.0 × 10 −6 ) 1 <f 2 π (22.0 × 10 −6 )(175) (b) )= N Φ B, max = LI B, 2 ( ∆Vrms ) = 2 ( ∆Vrms ) 2 π f C XC (a) I max = 2 (120 V)2 π (60.0 / s)(2.20 × 10 − 6 C / V) = 141 mA (b) I max = 2 (240 V)2 π (50.0 / s)(2.20 × 10 − 6 F) = 235 mA ( XL ∆VL, max ω XL ) Chapter 33 Solutions 273 [ ] 33.16 Qmax = C ( ∆Vmax ) = C 2 ( ∆Vrms ) = 2 C ( ∆Vrms ) 33.17 I max = ( ∆Vmax )ω C = (48.0 V)(2 π )(90.0 s −1 )(3.70 × 10 − 6 F) = 100 mA 33.18 XC = 1 1 = = 2.65 Ω ω C 2 π (60.0 / s)(1.00 × 10 − 3 C / V) vC (t) = ∆Vmax sin ω t , to be zero at t = 0 iC = 33.19 (a) ∆Vmax sin(ω t + φ ) = XC 2 (120 V) 60 s -1 sin 2 π + 90.0° = (64.0 A)sin(120° + 90.0°) = – 32.0 A -1 2.65 Ω 180 s XL = ω L = 2π (50.0)(400 × 10- 3) = 126 Ω XC = 1 1 = = 719 Ω ω C 2 π (50.0)(4.43 × 10 −6 ) Z = R 2 + (XL − XC )2 = 500 2 + (126 − 719)2 = 776 Ω ∆Vmax = Imax Z = (250 × 10- 3)(776) = 194 V (b) φ = tan −1 ωL = 33.20 X L − XC 126 − 719 = tan −1 = – 49.9° 500 R 1 →ω = ωC Thus, the Current leads the voltage. 1 1 = = 1.75 × 10 4 rad / s −6 LC (57.0 × 10 )(57.0 × 10 −6 ) ω f = 2π = 2.79 kHz 33.21 (a) XL = ω L = 2π (50.0 s-1)(250 × 10-3 H) = 78.5 Ω (b) XC = (c) Z = R 2 + (XL − XC )2 = 1.52 kΩ (d) I max = (e) φ = tan −1 [ ] 1 = 2 π (50.0 s −1 )(2.00 × 10 −6 F) ωC −1 = 1.59 kΩ ∆Vmax 210 V = = 138 mA Z 1.52 × 10 3 Ω X L − XC −1 = tan (−10.1) = – 84.3° R © 2000 by Harcourt, Inc. All rights reserved. 274 Chapter 33 Solutions 33.22 (a) Z = R 2 + ( XL − XC ) = 68.0 2 + (16.0 − 101) = 109 Ω 2 XL = ω L = (100)(0.160) = 16.0 Ω XC = 1 1 = = 101 Ω ω C (100) 99.0 × 10 −6 ( ) (b) I max = ∆Vmax 40.0 V = = 0.367 A 109 Ω Z (c) tan φ = XL − XC 16.0 − 101 = = −1.25: 68.0 R φ = − 0.896 rad = − 51.3° Imax = 0.367 A ω = 100 rad/s φ = – 0.896 rad = – 51.3° XL = 2 π f L = 2 π (60.0)(0.460) = 173 Ω 33.23 XC = (a) 1 1 = = 126 Ω 2 π f C 2 π (60.0) 21.0 × 10 − 6 ( tan φ = ) XL − XC 173 Ω − 126 Ω = = 0.314 150 Ω R φ = 0.304 rad = 17.4° (b) 33.24 Since XL > XC , φ is positive; so voltage leads the current . XC = 1 1 = = 1.33 × 108 Ω 2 π f C 2 π (60.0 Hz)(20.0 × 10 −12 F) Z = (50.0 × 10 3 Ω)2 + (1.33 × 108 Ω)2 ≈ 1.33 × 108 Ω Irms = ∆Vrms 5000 V = = 3.77 × 10–5 A Z 1.33 × 108 Ω ( ∆Vrms )body = Irms Rbody = (3.77 × 10− 5 A)(50.0 × 10 3 Ω) = 1.88 V Chapter 33 Solutions 275 XC = 33.25 1 1 = = 49.0 Ω ω C 2 π (50.0)(65.0 × 10 −6 ) XL = ω L = 2 π (50.0)(185 × 10 −3 ) = 58.1 Ω Z = R 2 + (XL − XC )2 = (40.0)2 + (58.1 − 49.0)2 = 41.0 Ω I max = ∆Vmax 150 = = 3.66 A 41.0 Z (a) ∆VR = Imax R = (3.66)(40) = 146 V (b) ∆VL = Imax XL = (3.66)(58.1) = 212.5 = 212 V (c) ∆VC = Imax XC = (3.66)(49.0) = 179.1 V = 179 V (d) ∆VL – ∆VC = 212.5 – 179.1 = 33.4 V R = 300 Ω 33.26 XL = 200 Ω 500 −1 XL = ω L = 2 π s (0.200 H) = 200 Ω π XC = ( ) 1 500 −1 11.0 × 10 −6 F = 2 π s ωC π Z = R 2 + ( XL − XC ) = 319 Ω 2 33.27 (a) and −1 XL - XC = 109 Ω = 90.9 Ω φ = tan −1 XC = 90.9 Ω X L − XC = 20.0° R XL = 2 π (100 Hz)( 20.5 H) = 1.29 × 10 4 Ω Z= ∆Vrms 200 V = = 50.0 Ω 4.00 A I rms (XL − XC )2 = Z2 − R 2 = (50.0 Ω)2 − (35.0 Ω)2 XL − XC = 1.29 × 10 4 Ω − (b) 1 = ± 35.7 Ω 2 π (100 Hz)C ( { C = 123 nF or 124 nF ) ∆VL,rms = I rms XL = ( 4.00 A ) 1.29 × 10 4 Ω = 51.5 kV Notice that this is a very large voltage! © 2000 by Harcourt, Inc. All rights reserved. Z φ R = 300 Ω 276 Chapter 33 Solutions XL = ω L = [(1000 / s)(0.0500 H)] = 50.0 Ω 33.28 [ ] XC = 1/ ω C = (1000 / s)(50.0 × 10 − 6 F) −1 = 20.0 Ω Z = R 2 + (XL − XC )2 Z = (40.0)2 + (50.0 − 20.0)2 = 50.0 Ω (a) I rms = ( ∆Vrms ) / Z = 100 V / 50.0 Ω I rms = 2.00 A X − XC φ = Arctan L R φ = Arctan 33.29 30.0 Ω = 36.9° 40.0 Ω (b) P = ( ∆Vrms ) I rms cos φ = 100 V(2.00 A) cos 36.9° = 160 W (c) 2 P R = I rms R = (2.00 A)2 40.0 Ω = 160 W ω = 1000 rad/s, R = 400 Ω, ∆Vmax = 100 V, 1 ω L = 500 Ω , = 200 Ω ω C C = 5.00 × 10– 6 F, L = 0.500 H 2 1 Z = R2 + ω L − = 400 2 + 300 2 = 500 Ω ω C I max = ∆Vmax 100 = = 0.200 A 500 Z The average power dissipated in the circuit is P= (0.200 A)2 (400 Ω) = 8.00 W 2 I2 2 P = I rms R = max R 2 Chapter 33 Solutions 277 Goal Solution An ac voltage of the form ∆v = (100 V ) sin(1000 t ) is applied to a series RLC circuit. C = 5.00 µ F, and L = 0.500 H, what is the average power delivered to the circuit? G: Comparing ∆v = (100 V ) sin(1000 t ) with ∆Vmax = 100 V and If R = 400 Ω, ∆v = ∆Vmax sin ω t, we see that ω = 1000 s -1 Only the resistor takes electric energy out of the circuit, but the capacitor and inductor will impede the current flow and therefore reduce the voltage across the resistor. Because of this impedance, the average power dissipated by the resistor must be less than the maximum power from the source: P max = ( ∆Vmax )2 = (100 V)2 2( 400 Ω) 2R = 12.5 W 2 R, where I rms = ∆Vrms / Z. O: The actual power dissipated by the resistor can be found from P = I rms A : ∆Vrms = 100 = 70.7 V 2 In order to calculate the impedance, we first need the capacitive and inductive reactances: XC = 1 1 = = 200 Ω ω C (1000 s -1 )(5.00 × 10 −6 F) Then, I rms = and ( ) XL = ω L = 1000 s -1 (0.500 H) = 500 Ω Z = R 2 + (XL − XC )2 = (400 Ω)2 + (500 Ω − 200 Ω)2 = 500 Ω ∆Vrms 70.7 V = = 0.141 A 500 Ω Z and 2 P = I rms R = (0.141 A ) ( 400 Ω) = 8.00 W 2 L : The power dissipated by the resistor is less than 12.5 W, so our answer appears to be reasonable. As with other RLC circuits, the power will be maximized at the resonance frequency where XL = XC so that Z = R . Then the average power dissipated will simply be the 12.5 W we calculated first. 33.30 Z = R 2 + ( X L − XC ) 2 or ( X L − XC ) = (XL − XC ) = (75.0 Ω)2 − ( 45.0 Ω)2 φ = tan −1 I rms = Z2 − R2 = 60.0 Ω X L − XC 60.0 Ω = tan −1 = 53.1° 45.0 Ω R ∆Vrms 210 V = = 2.80 A 75.0 Ω Z P = ( ∆Vrms ) I rms cos φ = ( 210 V )( 2.80 A ) cos( 53.1˚ ) = 353 W © 2000 by Harcourt, Inc. All rights reserved. 278 Chapter 33 Solutions 33.31 (a) P = I rms (∆Vrms )cos φ = (9.00)(180) cos(– 37.0°) = 1.29 × 10 3 W 2 P = I rms R (b) tan φ = X L − XC R so 1.29 × 10 3 = (9.00)2 R becomes tan( − 37.0°) = R = 16.0 Ω so XL – XC = – 12.0 Ω XL = ω L = 2 π (60.0 / s)(0.0250 H) = 9.42 Ω *33.32 Z = R 2 + (XL − XC )2 = (20.0)2 + (9.42)2 Ω = 22.1 Ω ∆Vrms 120 V = = 5.43 A Z 22.1 Ω (a) I rms = (b) φ = tan −1 (9.42 / 20.0) = 25.2° (c) We require φ = 0. Thus, XL = XC: 9.42 Ω = and C = 281 µF (d) Pb = Pd or ( ∆Vrms )d = 33.33 X L − XC : 16 and power factor = cos φ = 0.905 so ( ∆Vrms )b ( Irms )b cos φb = R( ∆Vrms )b ( I rms )b cos φ b = 1 2 π (60.0 s −1 )C ( ∆Vrms )d 2 R (20.0 Ω)(120 V )( 5.43 A)(0.905) = 109 V Consider a two-wire transmission line: R1 I rms P 100 × 106 W = = = 2.00 × 10 3 A ∆Vrms 50.0 × 10 3 V ∆Vrms 2 2 loss = (0.010 0)P = I rms R line = I rms (2R1 ) Thus, R1 (0.010 0)P = (0.010 0)(100 × 10 = 2 2 I max But R1 = ρl or A Therefore d= 4ρ l = π R1 ( R1 6 2 2.00 × 10 A A= ( 3 RL ) W 2 ) = 0.125 Ω π d 2 ρl = R1 4 )( 4 1.70 × 10 − 8 Ω ⋅ m 100 × 10 3 m π (0.125 Ω) ) = 0.132 m = 132 mm Chapter 33 Solutions 279 33.34 Consider a two-wire transmission line: P = ∆Vrms I rms and power loss = R1 2 I rms R line 2 P P Thus, ( 2R1 ) = 100 ∆Vrms R1 = ρ d ( ∆Vrms ) = A 200 P R1 = or 2 or A= ∆Vrms ( ∆Vrms )2 R1 200 P 800ρ P d 2r = π ( ∆Vrms ) 2 One-half the time, the left side of the generator is positive, the top diode conducts, and the bottom diode switches off. The power supply sees resistance 1 1 2R + 2R −1 =R RL π ( 2r )2 200ρ P d = 4 ( ∆Vrms )2 and the diameter is 33.35 P = 100 ∆Vrms ( ∆V rms )2 and the power is R1 R RL R1 The other half of the time the right side of the generator is positive, the upper diode is an open circuit, and the lower diode has zero resistance. The equivalent resistance is then 1 1 Req = R + + 3R R −1 = 7R and 4 P= ( ∆V rms )2 The overall time average power is: 33.36 At resonance, 1 = 2π f L 2π f C Req [(∆V = rms 4( ∆V rms ) 7R )2 2 ][ R + 4( ∆V rms ) 7R 2 2 and 1 ( 2 π f )2 L =C The range of values for C is 46.5 pF to 419 pF 33.37 ω 0 = 2 π (99.7 × 106 ) = 6.26 × 108 rad / s = C= 1 LC 1 1 = = 1.82 pF ω 0 2 L (6.26 × 108 )2 (1.40 × 10 − 6 ) © 2000 by Harcourt, Inc. All rights reserved. ]= 11( ∆V rms ) 14 R 2 280 Chapter 33 Solutions L = 20.0 mH, C = 1.00 × 10–7, R = 20.0 Ω, ∆Vmax = 100 V 33.38 33.39 (a) The resonant frequency for a series –RLC circuit is (b) At resonance, I max = (c) From Equation 33.36, Q= (d) ∆VL, max = XL I max = ω 0 L I max = 2.24 kV f= 1 2π 1 = 3.56 kHz LC ∆Vmax = 5.00 A R ω 0L = 22.4 R The resonance frequency is ω 0 = 1 LC . Thus, if ω = 2ω 0 , L 2 XL = ω L = L = 2 LC C and XC = Z = R 2 + ( XL − XC ) = R 2 + 2.25( L C ) so I rms = 2 1 LC 1 L = = 2 C ωC 2C ∆Vrms = Z ∆Vrms R + 2.25( L C ) 2 and the energy dissipated in one period is Q = P ∆t: Q= ( ∆Vrms )2 R 2π = ( ∆Vrms )2 RC π ( R 2 + 2.25( L C ) ω R 2C + 2.25 L 4 π ( ∆Vrms ) RC LC 2 ) LC = 4R 2C + 9.00 L With the values specified for this circuit, this gives: 2 Q= 33.40 ( ) (10.0 × 10 H) F ) + 9.00(10.0 × 10 H) 4 π ( 50.0 V ) (10.0 Ω) 100 × 10 −6 F ( 4(10.0 Ω) 100 × 10 −6 2 The resonance frequency is ω 0 = 1 32 −3 12 −3 LC . L 2 XL = ω L = L = 2 LC C Then Z = R 2 + ( XL − XC ) = R 2 + 2.25( L C ) 2 = 242 mJ Thus, if ω = 2ω 0 , 1 LC 1 L = = 2 C ωC 2C and XC = so I rms = ∆Vrms = Z and the energy dissipated in one period is Q = P ∆t = ( ∆Vrms )2 R 2π = ( ∆Vrms )2 RC π ( R 2 + 2.25( L C ) ω R 2C + 2.25 L ) LC = 4 π ( ∆Vrms ) RC LC 2 4R 2C + 9.00 L ∆Vrms R + 2.25( L C ) 2 Chapter 33 Solutions 281 ω0 = For the circuit of problem 22, *33.41 For the circuit of problem 23, 1 = LC (160 × 10 1 −3 )( H 99.0 × 10 −6 F ( (a) 1 120 V = 9.23 V ∆V 2, rms = 13 ( ) (b) ∆V1, rms I1, rms = ∆V 2, rms I 2, rms ) −3 ω 0 L ( 251 rad s) 160 × 10 H = = 0.591 R 68.0 Ω Q= ω 0L L 1 L 1 460 × 10 −3 H = = = = 0.987 R R LC R C 150 Ω 21.0 × 10 −6 F (120 V)( 0.350 A) = (9.23 V)I 2, rms I 2, rms = (c) 42.0 W = 4.55 A for a transformer with no energy loss 9.23 V P = 42.0 W from (b) ( ∆Vout )max = N2 ( ∆Vin )max = N 33.43 1 ( ∆Vout )rms = 33.44 2000 (170 V) = 971 V 350 (971 V) = 687 V 2 (a) (∆V2, rms ) = NN21 (∆V1, rms ) (b) I1, rms ∆V1, rms = I 2, rms ∆V 2, rms (c) 0.950 I1, rms ∆V1, rms = I 2, rms ∆V 2, rms ( ) ( N2 = ( ) ( = 251 rad s Q= The circuit of problem 23 has a sharper resonance. 33.42 ) ) ) (2200)(80) = 1600 windings 110 I1, rms = (1.50)(2200) = 30.0 A 110 I1, rms = (1.20)(2200) = 25.3 A 110(0.950) © 2000 by Harcourt, Inc. All rights reserved. 282 Chapter 33 Solutions The rms voltage across the transformer primary is 33.45 ( N1 ∆V 2, rms N2 so the source voltage is ∆V s, rms = I1, rms Rs + The secondary current is (∆V2, rms ) , Then ∆V s, rms = and 33.46 Rs = ( ) N1RL N1RL ( N 2 ∆V 2, rms ( N2 ∆V1, rms N1 ) + ) ( I 2, rms ∆V 2, rms = 0.900 I1, rms ∆V1, rms ( ) ∆V 2, rms ) ) ) = 5(50.0 Ω) 80.0 V − 5(25.0 V) = 2(25.0 V) 2 87.5 Ω ) 120 V (120 V ) 24.0 Ω I 2, rms = 54.0 mA 10.0 × 10 3 V = 185 kΩ 0.054 A (c) Z2 = (a) R = (4.50 × 10 − 4 Ω / m)(6.44 × 10 5 m) = 290 Ω I 2, rms = ( N 2 ∆V 2, rms = I1, rms N1 RL N 2 ∆V 2, rms 10.0 × 10 3 V = = = 83.3 120 V N1 ∆V1, rms (b) ) ) N2 N1 ∆V 2, rms ∆V s, rms − N2 ∆V 2, rms = ( ( N1 ∆V 2, rms (a) ( ( N1 ∆V 2, rms N2 so the primary current is RL N 2 ∆V 2, rms Rs I 2, rms 10.0 × 10 3 V = 0.900 33.47 ) and I rms = 5.00 × 106 W P = = 10.0 A ∆Vrms 5.00 × 10 5 V 2 P loss = I rms R = (10.0 A)2 (290 Ω) = 29.0 kW (b) (c) P loss 2.90 × 10 4 = = 5.80 × 10 − 3 P 5.00 × 106 It is impossible to transmit so much power at such low voltage. Maximum power transfer occurs when load resistance equals the line resistance of 290 Ω, and is (4.50 × 10 3 V)2 = 17.5 kW, far below the required 5 000 kW 2 ⋅ 2(290 Ω) Chapter 33 Solutions 283 33.48 (a) (b) For the filter circuit, ∆Vout = ∆Vin At f = 600 Hz , XC = and ∆Vout = ∆Vin R + XC2 1 1 = = 3.32 × 10 4 Ω 2 π f C 2 π (600 Hz) 8.00 × 10 −9 F ( ) 3.32 × 10 4 Ω (90.0 Ω)2 + (3.32 × 10 4 Ω ) 2 ≈ 1.00 1 1 = = 33.2 Ω 3 2 π f C 2 π 600 × 10 Hz 8.00 × 10 −9 F At f = 600 kHz, XC = and ∆Vout = ∆Vin For this RC high-pass filter, 33.49 XC 2 ( )( 33.2 Ω (90.0 Ω)2 + (33.2 Ω)2 ∆Vout = ∆Vin ) = 0.346 R R + XC2 2 (a) (a) When then ∆Vout = 0.500, ∆Vin 0.500 Ω (0.500 Ω)2 + XC2 = 0.500 or XC = 0.866 Ω (b) If this occurs at f = 300 Hz, the capacitance is C= (b) 1 1 = = 6.13 × 10 − 4 F = 613 µ F 2 π f XC 2 π ( 300 Hz)(0.866 Ω) With this capacitance and a frequency of 600 Hz, XC = ( 1 2 π (600 Hz) 6.13 × 10 − 4 F ∆Vout = ∆Vin R R 2 + XC2 = ) (c) = 0.433 Ω 0.500 Ω (0.500 Ω)2 + (0.433 Ω)2 = 0.756 © 2000 by Harcourt, Inc. All rights reserved. Figures for Goal Solution 284 Chapter 33 Solutions Goal Solution The RC high-pass filter shown in Figure 33.22 has a resistance R = 0.500 Ω. (a) What capacitance gives an output signal that has one-half the amplitude of a 300-Hz input signal? (b) What is the gain ( ∆V out / ∆Vin ) for a 600-Hz signal? G: It is difficult to estimate the capacitance required without actually calculating it, but we might expect a typical value in the µ F to pF range. The nature of a high-pass filter is to yield a larger gain at higher frequencies, so if this circuit is designed to have a gain of 0.5 at 300 Hz, then it should have a higher gain at 600 Hz. We might guess it is near 1.0 based on Figure (b) above. O: The output voltage of this circuit is taken across the resistor, but the input sees the impedance of the resistor and the capacitor. Therefore, the gain will be the ratio of the resistance to the impedance. ∆Vout = ∆Vin A: (a) (b) R R + (1 ω C ) 2 2 When ∆V out / ∆Vin = 0.500 solving for C gives C= At 600 Hz, we have ω = ( 2 π rad) 600 s -1 so ∆Vout = ∆Vin 1 2 ∆Vin ωR −1 ∆Vout ( = 1 (2 π )(300 Hz)(0.500 Ω) (2.00)2 − 1 = 613 µ F ) 0.500 Ω (0.500 Ω)2 + (1200 π 1 rad / s) (613 µ F ) 2 = 0.756 L : The capacitance value seems reasonable, but the gain is considerably less than we expected. Based o n our calculation, we can modify the graph in Figure (b) to more transparently represent the characteristics of this high-pass filter, now shown in Figure (c). If this were an audio filter, it would reduce low frequency “humming” sounds while allowing high pitch sounds to pass through. A low pass filter would be needed to reduce high frequency “static” noise. 33.50 ∆V1 = I (r + R)2 + XL2 , and ∆V 2 = I R 2 + XL2 r = 20.0 Ω Thus, when ∆V1 = 2 ∆V 2 (r + R)2 + XL2 = 4( R 2 + XL2 ) or (25.0 Ω) which gives XL = 2 π f (0.250 H) = 2 + XL2 L = 250 mH ∆V2 ∆V1 = 4( 5.00 Ω) + 2 4XL2 R =5.00 Ω 625 − 100 Ω and f = 8.42 Hz 3 Chapter 33 Solutions 285 ∆Vout = ∆Vin *33.51 (a) R R + ( X L − XC ) 2 (8.00 Ω)2 1 = 4 At 200 Hz: 2 (8.00 Ω)2 + 400 π L − (8.00 Ω) At 4000 Hz: 1 400 π C 2 2 2 1 2 + 8000 π L − = 4(8.00 Ω) 8000 π C 400π L − For the high frequency half-voltage point, 8000π L − Solving Equations (1) and (2) simultaneously gives C = 54.6 µ F (b) When XL = XC , ∆Vout ∆Vout = = 1.00 ∆Vin ∆Vin max (c) XL = XC requires f0 = 1 2 π LC ∆Vout R 1 = = Z 2 ∆Vin (d) At 200 Hz, = 1 2π (5.80 × 10 and XC > X L , −4 )( H 5.46 × 10 − 5 F XL - XC so the phasor diagram is as shown: = 894 Hz R φ or Z φ At f 0 , XL = XC so ∆Vout and ∆Vin have a phase difference of 0° or and ∆Vout ∆Vin ∆Vout leads ∆Vin by 60.0° ∆Vout R 1 = = Z 2 ∆Vin [2] L = 580 µ H and so Thus, φ = cos −1 X L − XC > 0 1 = 60.0° 2 XL - XC ∆Vin Z or φ R φ ∆Vout ∆Vout lags ∆Vin by 60.0° (∆Vout, rms ) = ( 21 ∆Vin, rms ) P= 2 At 200 Hz and at 4 kHz, At f 0 , P = (f) ) [1] 1 = + 13.9 Ω 8000π C R 1 φ = − cos −1 = − cos −1 Z 2 At 4000 Hz, (e) 1 = − 13.9 Ω 400π C At the low frequency, XL − XC < 0 . This reduces to R 2 = R ( 1 1 ∆V in, max 2 2 (∆Vout,rms )2 = (∆Vin,rms )2 = 21 (∆Vin,max )2 = (10.0 V)2 = We take: Q = R ω 0L R R = R ( 2(8.00 Ω) ) −4 2 π f 0 L 2 π (894 Hz) 5.80 × 10 H = = 0.408 8.00 Ω R © 2000 by Harcourt, Inc. All rights reserved. R ) 2 = 6.25 W (10.0 V )2 = 8(8.00 Ω) 1.56 W 286 Chapter 33 Solutions 33.52 ( ∆Vout )1 = ( ∆Vin )1 Now 33.53 ∆Vout = ∆Vin For a high-pass filter, R 1 R2 + ωC ( ∆Vout )2 ( ∆Vin )2 and 2 ( ∆Vin )2 = ( ∆Vout )1 R 1 R2 + ωC R = ( ∆Vout )2 ( ∆Vin )1 so 2 1 R2 + ωC = 2 R2 1 R2 + ωC 2 = 1 1 1+ ω RC Rewrite the circuit in terms of impedance as shown in Fig. (b). Find: ∆Vout = From Figure (c), ∆V ab = ZR ∆V ab ZR + ZC ZR ∆Vin [1] [ ZC ] a ] (ZR + ZC ) ZR + ZC||(ZR + ZC ) ∆Vin ∆Vab ∆Vout ∆Vin Figure (b) 1 1 ZR + Z Z + Z R C C = −1 1 1 (ZR + ZC )ZR + Z + Z + Z C R C ZR ∆Vin ∆Vout = ∆Vin ∆Vout = ∆Vin 1 − R 2ω C j 3R − ωC − ĵ ωC R (3R) 2 1 + − R 2ω C ω C ∆Vab b R R 1 where we used = −j. j 1 2 3R − j+ R ω C j ω C = ZR Figure (c) −j where j= −1 ωC R a ZC ZC ∆Vout ZR ZC ZR = = ∆Vin ZC ( ZC + ZR ) + ZR ( ZR + 2ZC ) 3ZR + ZC + ( ZR )2 ZC Now, ZR = R and ZC = ∆Vout ZR b −1 or ∆Vout b Figure (a) ZR ZC || ( ZR + ZC ) [ ZC ZR ZC ZC || ( ZR + ZC ) ∆Vin ZR + ZC || ( ZR + ZC ) So Eq. [1] becomes ∆Vout = a 2 2 = ( Z 1.00 × 10 3 3.00 × 10 3 ) 2 + (1592 − 628) = 0.317 2 Chapter 33 Solutions 287 33.54 The equation for y = mx + b ) is: ∆v(t ) = [ ( ∆v)2 [ ( ∆v) 2 2( ∆Vmax ) t − ∆Vmax T ω0 = 2 ( ∆Vmax ) 1 T ∆v t dt = ( ) [ ] ∫ 0 T T ] = ] ( ∆Vmax )2 T [2t T − 1] 3 = ave ave ∆Vrms = 33.55 ∆v(t ) during the first period (using 2 T [ ( ∆v)2 ] ave = 3 ( ∆Vmax )2 3 2 ∫0 T t − 1 dt t=T = ( ∆Vmax )2 (+1)3 − (−1)3 = ( ∆Vmax )2 [ ] t=0 = 2 T2 6 3 ∆Vmax 3 1 1 = = 2000 s −1 −6 LC (0.0500 H)(5.00 × 10 F) so the operating frequency of the circuit is ω = Using Equation 33.35, P= P= ( ∆Vrms )2 Rω 2 ( R 2ω 2 + L2 ω 2 − ω 02 (400)2 (8.00)(1000)2 [ ω0 = 1000 s −1 2 (8.00)2 (1000)2 + (0.0500)2 (1.00 − 4.00) × 106 ] 2 ) (Q ≈ 12.5) 2 = 56.7 W Figure for Goal Solution Goal Solution A series RLC circuit consists of an 8.00-Ω resistor, a 5.00-µ F capacitor, and a 50.0-mH inductor. A variable frequency source applies an emf of 400 V (rms) across the combination. Determine the power delivered to the circuit when the frequency is equal to one half the resonance frequency. G: Maximum power is delivered at the resonance frequency, and the power delivered at other frequencies depends on the quality factor, Q. For the relatively small resistance in this circuit, we could expect a high Q = ω 0 L R . So at half the resonant frequency, the power should be a small 2 fraction of the maximum power, P av, max = ∆Vrms R = ( 400 V ) 8 Ω = 20 kW. 2 O: We must first calculate the resonance frequency in order to find half this frequency. Then the power delivered by the source must equal the power taken out by the resistor. This power can be found 2 R where I rms = ∆Vrms / Z. from P av = I rms © 2000 by Harcourt, Inc. All rights reserved. 288 Chapter 33 Solutions A : The resonance frequency is f0 = 1 = 2 π LC 2 π 1 (0.0500 H)( 5.00 × 10 −6 F ) = 318 Hz The operating frequency is f = f 0 / 2 = 159 Hz . We can calculate the impedance at this frequency: XL = 2 π f L = 2 π (159 Hz)(0.0500 H) = 50.0 Ω and XC = 1 1 = = 200 Ω 2 π f C 2 π (159 Hz) 5.00 × 10 -6 F ( ) Z = R 2 + (XL − XC )2 = 8.00 2 + (50.0 − 200)2 Ω = 150 Ω I rms = So, ∆Vrms 400 V = = 2.66 A 150 Ω Z The power delivered by the source is the power dissipated by the resistor: P av = I rms 2 R = (2.66 A)2 (8.00 Ω) = 56.7 W L : This power is only about 0.3% of the 20 kW peak power delivered at the resonance frequency. The significant reduction in power for frequencies away from resonance is a consequence of the relatively high Q -factor of about 12.5 for this circuit. A high Q is beneficial if, for example, you want to listen to your favorite radio station that broadcasts at 101.5 MHz, and you do not want to receive the signal from another local station that broadcasts at 101.9 MHz. R= ∆V 12.0 V I = 0.630 A = 19.0 Ω The impedance of the circuit is Z = ∆Vrms 24.0 V = = 42.1 Ω 0.570 A I rms The resistance of the circuit is 33.56 Z 2 = R 2 + ω 2 L2 L= 33.57 1 1 Z2 − R2 = (42.1)2 − (19.0)2 = 99.6 mH 377 ω (a) When ω L is very large, the bottom branch carries negligible current. Also, 1/ω C will be negligible compared to 200 Ω and 45.0 V/200 Ω = 225 mA flows in the power supply and the top branch. (b) Now 1/ω C → ∞ and ω L → 0 so the generator and bottom branch carry 450 mA Chapter 33 Solutions 289 33.58 (a) With both switches closed, the current goes only through generator and resistor. i(t) = ∆Vmax cos ω t R 1 ( ∆Vmax ) 2 R (b) P= (c) i(t) = 2 ∆Vmax cos [ω t + Arctan(ω L / R)] R 2 + ω 2 L2 1 ω L− 0 ω 0C 0 = φ = Arctan R (d) For We require ω 0 L = 1 , so ω0 C (e) At this resonance frequency, (f) U = 21 C ( ∆VC ) = 21 C I 2 XC 2 1 ω 02 L Z= R 2 2 U max = 21 CI max XC 2 = 21 C (g) C= U max = 1 LI 2 2 max = (h) Now ω = 2ω 0 = 1L 2 ( ∆Vmax )2 R2 1 = ω 0 2C 2 ( ∆Vmax )2 L 2R 2 ( ∆Vmax )2 R2 2 LC 1 L 1 L ωL − 2 C − 2 C 3 L ωC So φ = Arctan = Arctan = Arctan 2R C R R (i) Now ω L = 1 1 2 ωC ω= 1 2 LC = ω0 2 © 2000 by Harcourt, Inc. All rights reserved. 290 Chapter 33 Solutions 33.59 (a) As shown in part (b), circuit (a) is a high-pass filter and circuit (b) is a low-pass filter . ∆Vin (b) For circuit (a), For circuit (b), 33.60 (a) (b) I R, rms = ∆Vout = ∆Vin RL2 + ( XL − XC ) 2 = As ω → 0, ∆Vout ≈ ω RLC ≈ 0 ∆Vin As ω → ∞ , ∆Vout ≈1 ∆Vin ∆Vout = ∆Vin + ( X L − XC ) RL2 + (ω L) RL2 + (ω L − 1 ω C ) 2 Circuit (a) (high-pass filter) XC RL2 2 = As ω → 0, ∆Vout ≈1 ∆Vin As ω → ∞ , ∆Vout 1 ≈ 2 ≈0 ∆Vin ω LC ∆V out ∆Vin 1 ωC RL2 + (ω L − 1 ω C ) 2 Circuit (b) (low-pass filter) ∆Vrms 100 V = = 1.25 A 80.0 Ω R The total current will lag phasor diagram at the right. I L, rms = IR ∆V φ the applied voltage as seen in the IL I ∆Vrms 100 V = = 1.33 A XL 2 π 60.0 s −1 (0.200 H) ( Thus, the phase angle is: *33.61 RL2 + XL2 ∆Vout 2 ) I L, rms −1 1.33 A φ = tan −1 = tan 1.25 A = 46.7° I R, rms Suppose each of the 20 000 people uses an average power of 500 W. (This means 12 kWh per day, or $36 per 30 days at 10¢ per kWh). Suppose the transmission line is at 20 kV. Then I rms = (20 000)(500 W) P = ∆Vrms 20 000 V ~10 3 A If the transmission line had been at 200 kV, the current would be only ~10 2 A . Chapter 33 Solutions 291 L = 2.00 H, C = 10.0 × 10– 6 F, R = 10.0 Ω, ∆v(t) = (100 sin ω t) 33.62 (a) The resonant frequency ω0 produces the maximum current and thus the maximum power dissipation in the resistor. ω0 = 1 1 = = 224 rad/s LC (2.00)(10.0 × 10 − 6 ) ( ∆Vmax )2 (b) P= (c) I rms = 2R = ∆Vrms = Z 2 I rms R= ∆Vrms 1 R2 + ω L − ω C ( ) 1 2 I rms 2 (100)2 = 500 W 2(10.0) max 2 R ∆Vrms R and ( Irms )max = or ( ∆Vrms )2 R = 1 ( ∆Vrms )2 R Z2 R2 2 2 1 R2 + ω L − = 2R 2 ω C This occurs where Z 2 = 2R 2: ω 4 L2C 2 − 2Lω 2C − R 2ω 2C 2 + 1 = 0 L2C 2ω 4 − (2LC + R 2C 2 )ω 2 + 1 = 0 or [(2.00) (10.0 × 10 ) ]ω − [2(2.00)(10.0 × 10 −6 2 2 4 −6 ] ) + (10.0)2 (10.0 × 10 − 6 )2 ω 2 + 1 = 0 Solving this quadratic equation, we find that ω 2 = 51 130, ω 1 = 48 894 = 221 rad/s ω 2 = 51 130 = 226 rad/s and 48 894 R = 200 Ω, L = 663 mH, C = 26.5 µ F, ω = 377 s −1 , ∆Vmax = 50.0 V 33.63 ω L = 250 Ω, (a) I max = 1 ω C = 100 Ω, Z = R 2 + ( XL − XC ) = 250 Ω 2 ∆Vmax 50.0 V = = 0.200 A 250 Ω Z φ = tan −1 X L − XC = 36.8° R (∆V leads I) (b) ∆V R, max = I max R = 40.0 V at φ = 0° (c) ∆VC, max = (d) ∆VL, max = I maxω L = 50.0 V at φ = + 90.0° I max = 20.0 V at φ = – 90.0° ωC (I leads ∆V) (∆V leads I) © 2000 by Harcourt, Inc. All rights reserved. 292 Chapter 33 Solutions 2 P = I rms R= *33.64 2 ∆Vrms R, Z (120 V )2 Z 2 ( 40.0 Ω) : (120)2 ( 40.0) 250 = ( 40.0) 1= 250 W = so 2 1 + 2 π f (0.185) − 2 π f 65.0 × 10 − 6 ( ) 2 2304 f 2 1600 f 2 + 1.3511 f 4 − 5692.3 f 2 + 5 995 300 f2= 6396.3 ± (6396.3)2 − 4(1.3511)( 5 995 300) 2(1.3511) Z = R 2 + (ω L − 1 ω C ) 2 576 000 f 2 and 250 = so 1.3511 f 4 − 6396.3 f 2 + 5 995 300 = 0 ( 1600 f 2 + 1.1624 f 2 − 2448.5 ) 2 = 3446.5 or 1287.4 f = 58.7 Hz or 35.9 Hz 33.65 (a) From Equation 33.39, N1 ∆V1 = N 2 ∆V 2 Let output impedance Z1 = so that N1 Z1I1 = N 2 Z2 I 2 ∆V1 I1 and the input impedance Z2 = But from Eq. 33.40, N1 = N2 So, combining with the previous result we have (b) N1 = N2 IR = 33.66 Z1 = Z2 ∆Vrms ; R 8000 = 31.6 8.00 IL = ∆Vrms ; ωL IC = (a) I rms = I R2 + (IC − I L )2 = ∆Vrms (b) tan φ = ∆Vrms (ω C)−1 1 1 + ωC − R 2 ω L 1 IC − I L 1 1 = ∆Vrms − X X IR / R ∆V L rms C 1 1 tan φ = R − X X L C 2 Z1 Z2 ∆V 2 I2 I1 ∆V 2 N 2 = = I 2 ∆V1 N1 Chapter 33 Solutions 293 33.67 (a) I rms = ∆Vrms 1 1 + ωC − 2 ω L R 2 ∆Vrms → ( ∆Vrms )max when ω C = f= f= (b) 1 ωL 1 2 π LC 1 2 π 200 × 10 −3 H)(0.150 × 10 − 6 F) = 919 Hz IR = ∆Vrms 120 V = = 1.50 A 80.0 Ω R IL = ∆Vrms 120 V = = 1.60 A ωL (374 s −1 )(0.200 H) IC = ∆Vrms (ω C) = (120 V)(374 s −1 )(0.150 × 10 − 6 F) = 6.73 mA (c) I rms = I R2 + (IC − I L )2 = (1.50)2 + (0.00673 − 1.60)2 = 2.19 A (d) I − I 0.00673 − 1.60 φ = tan −1 C L = tan −1 = – 46.7° 1.50 IR The current is lagging the voltage . 33.68 (a) tan φ = ∆VL I (ω L) ω L = = ∆V R IR R Thus, R = (b) ( L ∆Vin ) 200 s (0.500 H) ωL = 173 Ω = tan( 30.0°) tan φ −1 ∆Vout ∆V R = = cos φ ∆Vin ∆Vin R ∆V = IZ ∆VL = IXL ∆Vout = ( ∆Vin ) cos φ = (10.0 V ) cos 30.0° = 8.66 V © 2000 by Harcourt, Inc. All rights reserved. φ ∆VR = IR ∆V out 294 Chapter 33 Solutions 33.69 (a) XL = XC = 1884 Ω L= f = 2000 Hz when XL 1884 Ω = = 0.150 H 2 π f 4000 π rad s XL = 2 π f (0.150 H) XC = C= and 1 (2π f )( 4.22 × 10 −8 F ) (b) 33.70 f (Hz) X L (Ω) XC (Ω) Z (Ω) 300 600 800 1000 1500 2000 3000 4000 6000 10000 283 565 754 942 1410 1880 2830 3770 5650 9420 12600 6280 4710 3770 2510 1880 1260 942 628 377 12300 5720 3960 2830 1100 40 1570 2830 5020 9040 ω0 = 1 = 1.00 × 106 rad s LC For each angular frequency, we find Z = R 2 + (ω L − 1/ ω C ) then I = (1.00 V ) / Z and P = I 2 (1.00 Ω) 2 The full width at half maximum is: ∆f = ∆ω (1.0005 − 0.9995)ω 0 = 2π 2π ∆f = 1.00 × 10 3 s −1 = 159 Hz 2π while 1.00 Ω R = = 159 Hz 2 π L 2 π 1.00 × 10 −3 H ( ) Z= 1 1 = (2π f )XC ( 4000π rad s)(1884 Ω) = 42.2 nF ( 40.0 Ω)2 + (XL − XC )2 Impedence, Ω ω ω0 ω L (Ω ) 0.9990 0.9991 0.9993 0.9995 0.9997 0.9999 1.0000 1.0001 1.0003 1.0005 1.0007 1.0009 1.0010 999.0 999.1 999.3 999.5 999.7 999.9 1000 1000.1 1000.3 1000.5 1000.7 1000.9 1001 1 ωC (Ω ) 1001.0 1000.9 1000.7 1000.5 1000.3 1000.1 1000.0 999.9 999.7 999.5 999.3 999.1 999.0 Z (Ω ) 2.24 2.06 1.72 1.41 1.17 1.02 1.00 1.02 1.17 1.41 1.72 2.06 2.24 P = I 2 R (W) 0.19984 0.23569 0.33768 0.49987 0.73524 0.96153 1.00000 0.96154 0.73535 0.50012 0.33799 0.23601 0.20016 Chapter 33 Solutions 295 ∆Vout = ∆Vin 33.71 (a) R R 2 + (1 ω C ) 2 = R C R 2 + (1 2 π f C ) 2 ∆Vin ∆Vout 1 1 = when =R 3 2 ∆Vin ωC Hence, f = ∆Vout R 1 ω = = 1.84 kHz 2 π 2 π RC 3 Log Gain versus Log Frequency (b) 0 -1 Log∆V out / ∆ V in -2 -3 -4 0 1 2 3 Log f © 2000 by Harcourt, Inc. All rights reserved. 4 5 6