Solution Manual for Electric Circuits 10th Edition by James Nilsson, Susan Reidel by welldoneassistant

INSTRUCTOR SOLUTIONS MANUAL for

Circuit Variables

Assessment Problems AP 1.1 Use a product of ratios to convert two-thirds the speed of light from meters per second to miles per second: 1 in 1 ft 1 mile 124,274.24 miles 2 3 × 108 m 100 cm · · · · = 3 1s 1m 2.54 cm 12 in 5280 feet 1s

Now set up a proportion to determine how long it takes this signal to travel 1100 miles: 1100 miles 124,274.24 miles = 1s xs Therefore, x=

1100 = 0.00885 = 8.85 × 10−3 s = 8.85 ms 124,274.24

AP 1.2 To solve this problem we use a product of ratios to change units from dollars/year to dollars/millisecond. We begin by expressing $10 billion in scientific notation: $100 billion = $100 × 109 Now we determine the number of milliseconds in one year, again using a product of ratios: 1 year 1 day 1 hour 1 min 1 sec 1 year · · · · = 365.25 days 24 hours 60 mins 60 secs 1000 ms 31.5576 × 109 ms Now we can convert from dollars/year to dollars/millisecond, again with a product of ratios: $100 × 109 1 year 100 · = = $3.17/ms 9 1 year 31.5576 × 10 ms 31.5576 1–1 © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

1–2

CHAPTER 1. Circuit Variables

AP 1.3 Remember from Eq. (1.2), current is the time rate of change of charge, or i = dq In this problem, we are given the current and asked to find the total dt charge. To do this, we must integrate Eq. (1.2) to find an expression for charge in terms of current: q(t) =

Z t

i(x) dx

We are given the expression for current, i, which can be substituted into the above expression. To find the total charge, we let t → ∞ in the integral. Thus we have qtotal = =

Z ∞ 0

20e−5000x dx =

20 −5000x ∞ 20 e = (e−∞ − e0) −5000 −5000 0

20 20 (0 − 1) = = 0.004 C = 4000 µC −5000 5000

AP 1.4 Recall from Eq. (1.2) that current is the time rate of change of charge, or i = dq . In this problem we are given an expression for the charge, and asked to dt find the maximum current. First we will find an expression for the current using Eq. (1.2): i=

dq d 1 1 t = − + 2 e−αt 2 dt dt α α α

d 1 d t −αt d 1 −αt = − e − e 2 dt α dt α dt α2

t 1 1 −αt e − α e−αt − −α 2 e−αt = 0− α α α

= −

1 1 −αt e +t+ α α

= te−αt Now that we have an expression for the current, we can find the maximum value of the current by setting the first derivative of the current to zero and solving for t: di d = (te−αt) = e−αt + t(−α)eαt = (1 − αt)e−αt = 0 dt dt Since e−αt never equals 0 for a finite value of t, the expression equals 0 only when (1 − αt) = 0. Thus, t = 1/α will cause the current to be maximum. For this value of t, the current is i=

1 −α/α 1 e = e−1 α α

© 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Problems

1–3

Remember in the problem statement, α = 0.03679. Using this value for α, i=

1 e−1 ∼ = 10 A 0.03679

AP 1.5 Start by drawing a picture of the circuit described in the problem statement:

Also sketch the four figures from Fig. 1.6:

[a] Now we have to match the voltage and current shown in the first figure with the polarities shown in Fig. 1.6. Remember that 4A of current entering Terminal 2 is the same as 4A of current leaving Terminal 1. We get (a) v = −20 V, (c) v = 20 V,

i = −4 A; (b) v = −20 V, i = −4 A;

(d) v = 20 V,

i = 4A i = 4A

[b] Using the reference system in Fig. 1.6(a) and the passive sign convention, p = vi = (−20)(−4) = 80 W. Since the power is greater than 0, the box is absorbing power. [c] From the calculation in part (b), the box is absorbing 80 W. AP 1.6 [a] Applying the passive sign convention to the power equation using the voltage and current polarities shown in Fig. 1.5, p = vi. To find the time at which the power is maximum, find the first derivative of the power with respect to time, set the resulting expression equal to zero, and solve for time: p = (80,000te−500t)(15te−500t) = 120 × 104 t2 e−1000t dp = 240 × 104 te−1000t − 120 × 107 t2e−1000t = 0 dt

1–4

CHAPTER 1. Circuit Variables Therefore, 240 × 104 − 120 × 107 t = 0 Solving, t=

240 × 104 = 2 × 10−3 = 2 ms 120 × 107

[b] The maximum power occurs at 2 ms, so find the value of the power at 2 ms: p(0.002) = 120 × 104 (0.002)2 e−2 = 649.6 mW [c] From Eq. (1.3), we know that power is the time rate of change of energy, or p = dw/dt. If we know the power, we can find the energy by integrating Eq. (1.3). To find the total energy, the upper limit of the integral is infinity: wtotal =

Z ∞ 0

120 × 104 x2e−1000x dx ∞

120 × 104 −1000x e [(−1000)2 x2 − 2(−1000)x + 2) = (−1000)3 0 =0−

120 × 104 0 e (0 − 0 + 2) = 2.4 mJ (−1000)3

AP 1.7 At the Oregon end of the line the current is leaving the upper terminal, and thus entering the lower terminal where the polarity marking of the voltage is negative. Thus, using the passive sign convention, p = −vi. Substituting the values of voltage and current given in the figure, p = −(800 × 103 )(1.8 × 103 ) = −1440 × 106 = −1440 MW Thus, because the power associated with the Oregon end of the line is negative, power is being generated at the Oregon end of the line and transmitted by the line to be delivered to the California end of the line.

Problems

1–5

Chapter Problems

P 1.1

(260 × 106 )(540) = 104.4 gigawatt-hours 109

P 1.2

(480)(320) pixels 2 bytes 30 frames · · = 9.216 × 106 bytes/sec 1 frame 1 pixel 1 sec (9.216 × 106 bytes/sec)(x secs) = 32 × 230 bytes

P 1.3

32 × 230 = 3728 sec = 62 min ≈ 1 hour of video 9.216 × 106

[a]

20,000 photos x photos = 3 (11)(15)(1) mm 1 mm3 x=

[b]

(20,000)(1) = 121 photos (11)(15)(1)

16 × 230 bytes x bytes = 3 (11)(15)(1) mm (0.2)3 mm3 x=

(16 × 230 )(0.008) = 832,963 bytes (11)(15)(1) 5280 ft 2526 lb 1 kg · · = 20.5 × 106 kg 1 mi 1000 ft 2.2 lb

P 1.4

(4 cond.) · (845 mi) ·

P 1.5

Volume = area × thickness Convert values to millimeters, noting that 10 m2 = 106 mm2 106 = (10 × 106 )(thickness) 106 ⇒ thickness = = 0.10 mm 10 × 106

P 1.6

[a] We can set up a ratio to determine how long it takes the bamboo to grow 10 µm First, recall that 1 mm = 103 µm. Let’s also express the rate of growth of bamboo using the units mm/s instead of mm/day. Use a product of ratios to perform this conversion: 250 mm 1 day 1 hour 1 min 250 10 · · · = = mm/s 1 day 24 hours 60 min 60 sec (24)(60)(60) 3456 Use a ratio to determine the time it takes for the bamboo to grow 10 µm: 10/3456 × 10−3 m 10 × 10−6 m = 1s xs

10 × 10−6 = 3.456 s 10/3456 × 10−3

1–6

CHAPTER 1. Circuit Variables

[b] P 1.7

1 cell length 3600 s (24)(7) hr · · = 175,000 cell lengths/week 3.456 s 1 hr 1 week

[a] First we use Eq. (1.2) to relate current and charge: i=

dq = 0.125e−2500t dt

Therefore, dq = 0.125e−2500t dt To find the charge, we can integrate both sides of the last equation. Note that we substitute x for q on the left side of the integral, and y for t on the right side of the integral: Z q(t) q(0)

dx = 0.125

Z t 0

e−2500y dy

We solve the integral and make the substitutions for the limits of the integral: t

q(t) − q(0) = 0.125

e−2500y = 50 × 10−6 (1 − e−2500t) −2500 0

But q(0) = 0 by hypothesis, so q(t) = 50(1 − e−2500t) µC [b] As t → ∞, qT = 50 µC. [c] q(0.5 × 10−3 ) = (50 × 10−6 )(1 − e(−2500)(0.0005)) = 35.675 µC. P 1.8

First we use Eq. (1.2) to relate current and charge: i=

dq = 20 cos 5000t dt

Therefore, dq = 20 cos 5000t dt To find the charge, we can integrate both sides of the last equation. Note that we substitute x for q on the left side of the integral, and y for t on the right side of the integral: Z q(t) q(0)

dx = 20

Z t 0

cos 5000y dy

We solve the integral and make the substitutions for the limits of the integral, remembering that sin 0 = 0: q(t) − q(0) = 20

sin 5000y 5000

= 0

20 20 20 sin 5000t − sin 5000(0) = sin 5000t 5000 5000 5000

But q(0) = 0 by hypothesis, i.e., the current passes through its maximum value at t = 0, so q(t) = 4 × 10−3 sin 5000t C = 4 sin 5000t mC

Problems P 1.9

1–7

[a] First we use Eq. (1.2) to relate current and charge: i=

dq = 40te−500t dt

Therefore, dq = 40te−500t dt To find the charge, we can integrate both sides of the last equation. Note that we substitute x for q on the left side of the integral, and y for t on the right side of the integral: Z q(t) q(0)

dx = 40

Z t 0

ye−500y dy

We solve the integral and make the substitutions for the limits of the integral: t

e−500y (−500y − 1) = 160 × 10−6 e−500t(−500t − 1) + 160 × 10−6 q(t) − q(0) = 40 2 (−500) 0 = 160 × 10−6 (1 − 500te−500t − e−500t) But q(0) = 0 by hypothesis, so q(t) = 160(1 − 500te−500t − e−500t) µC [b] q(0.001) = (160)[1 − 500(0.001)e−500(0.001) − e−500(0.001) = 14.4 µC. 35 × 10−6 C/s = 2.18 × 1014 elec/s 1.6022 × 10−19 C/elec

P 1.10

P 1.11

w = qV = (1.6022 × 10−19 )(6) = 9.61 × 10−19 = 0.961 aJ

P 1.12

[a]

p = vi = (40)(−10) = −400 W Power is being delivered by the box. [b] Entering [c] Gaining P 1.13

[a] p = vi = (−60)(−10) = 600 W, so power is being absorbed by the box. [b] Entering

1–8

CHAPTER 1. Circuit Variables [c] Losing

P 1.14

Assume we are standing at box A looking toward box B. Use the passive sign convention to get p = vi, since the current i is flowing into the + terminal of the voltage v. Now we just substitute the values for v and i into the equation for power. Remember that if the power is positive, B is absorbing power, so the power must be flowing from A to B. If the power is negative, B is generating power so the power must be flowing from B to A. [a] p = (30)(6) = 180 W

P 1.15

180 W from A to B

[b] p = (−20)(−8) = 160 W

160 W from A to B

[c] p = (−60)(4) = −240 W

240 W from B to A

[d] p = (40)(−9) = −360 W

360 W from B to A

[a] In Car A, the current i is in the direction of the voltage drop across the 12 V battery(the current i flows into the + terminal of the battery of Car A). Therefore using the passive sign convention, p = vi = (30)(12) = 360 W. Since the power is positive, the battery in Car A is absorbing power, so Car A must have the ”dead” battery. [b] w(t) =

Z t 0

w(60) =

p dx;

Z 60 0

1 min = 60 s

360 dx

w = 360(60 − 0) = 360(60) = 21,600 J = 21.6 kJ P 1.16

p = vi;

Z t 0

p dx

Since the energy is the area under the power vs. time plot, let us plot p vs. t.

Problems

1–9

Note that in constructing the plot above, we used the fact that 40 hr = 144,000 s = 144 ks p(0) = (1.5)(9 × 10−3 ) = 13.5 × 10−3 W p(144 ks) = (1)(9 × 10−3 ) = 9 × 10−3 W 1 w = (9 × 10−3 )(144 × 103 ) + (13.5 × 10−3 − 9 × 10−3 )(144 × 103 ) = 1620 J 2 P 1.17

p = (12)(100 × 10−3 ) = 1.2 W;

w(t) = P 1.18

Z t 0

p dt

4 hr ·

w(14,400) =

Z 14,400 0

3600 s = 14,400 s 1 hr

1.2 dt = 1.2(14,400) = 17.28 kJ

[a] p = vi = (15e−250t )(0.04e−250t) = 0.6e−500t W p(0.01) = 0.6e−500(0.01) = 0.6e−5 = 0.00404 = 4.04 mW [b] wtotal =

Z ∞ 0

p(x) dx =

Z ∞ 0

0.6e−500x dx =

0.6 −500x ∞ e −500 0

= −0.0012(e−∞ − e0) = 0.0012 = 1.2 mJ P 1.19

[a] p = vi = (0.05e−1000t )(75 − 75e−1000t) = (3.75e−1000t − 3.75e−2000t) W dp = −3750e−1000t + 7500e−2000t = 0 dt 2 = e1000t

ln 2 = 1000t

so thus

2e−2000t = e−1000t p is maximum at t = 693.15 µs

pmax = p(693.15 µs) = 937.5 mW [b] w =

Z ∞

= P 1.20

[3.75e−1000t − 3.75e−2000t] dt =

3.75 −1000t 3.75 −2000t ∞ e − e −1000 −2000 0

3.75 3.75 − = 1.875 mJ 1000 2000

[a] p = vi = 0.25e−3200t − 0.5e−2000t + 0.25e−800t p(625 µs) = 42.2 mW [b]

w(t)

Z t 0

(0.25e−3200t − 0.5e−2000t + 0.25e−800t )

= 140.625 − 78.125e−3200t + 250e−2000t − 312.5e−800t µJ w(625 µs)

= 12.14 µJ

[c] wtotal = 140.625 µJ

1–10 P 1.21

CHAPTER 1. Circuit Variables

[a]

p =

dp dt

[(1500t + 1)e−750t](0.04e−750t)

(60t + 0.04)e−1500t

60e−1500t − 1500e−1500t (60t + 0.04)

−90,000te−1500t

dp = 0 when t = 0 Therefore, dt so pmax occurs at t = 0. [b] pmax

= [(60)(0) + 0.04]e0 = 0.04 = 40 mW

[c] w

Z t Z0 t 0

pdx 60xe−1500x dx +

Z t 0

0.04e−1500x dx t

60e−1500x e−1500x (−1500x − 1) + 0.04 (−1500)2 −1500 0 0 When t = ∞ all the upper limits evaluate to zero, hence 60 0.04 w= + = 53.33 µJ. 4 225 × 10 1500 =

P 1.22

[a]

p = vi = [(3200t + 3.2)e−1000t][(160t + 0.16)e−1000t] = e−2000t[512,000t2 + 1024t + 0.512] dp dt

= e−2000t[1,024,000t + 1024] − 2000e−2000t [512,000t2 + 1024t + 0.512] = −e−2000t[1024 × 106 t2 + 1,024,000t]

dp Therefore, = 0 when t = 0 dt so pmax occurs at t = 0. [b] pmax

= e−0 [0 + 0 + 0.512] = 512 mW

[c] w

= =

Z t Z0 t 0

pdx 512,000x2 e−2000x dx +

Z t 0

1024xe−2000x dx + t

512,000e−2000x [4 × 106 x2 + 4000x + 2] + −8 × 109 0 t t 1024e−2000x 0.512e−2000x (−2000x − 1) + 4 × 106 −2000 0 0

Z t 0

0.512e−2000x dx

Problems

1–11

When t → ∞ all the upper limits evaluate to zero, hence (512,000)(2) 1024 0.512 w= + + 9 6 8 × 10 4 × 10 2000 w = 128 × 10−6 + 256 × 10−6 + 256 × 10−6 = 640 µJ. P 1.23

[a] We can find the time at which the power is a maximum by writing an expression for p(t) = v(t)i(t), taking the first derivative of p(t) and setting it to zero, then solving for t. The calculations are shown below: p

= 0 t < 0,

p = 0 t > 40 s

p dp dt dp dt t1

= vi = t(1 − 0.025t)(4 − 0.2t) = 4t − 0.3t2 + 0.005t3 W

0 ≤ t ≤ 40 s

= 4 − 0.6t + 0.015t2 = 0.015(t2 − 40t + 266.67) when t2 − 40t + 266.67 = 0

= 0

= 8.453 s;

t2 = 31.547 s

(using the polynomial solver on your calculator) p(t1 )

= 4(8.453) − 0.3(8.453)2 + 0.005(8.453)3 = 15.396 W

p(t2 )

= 4(31.547) − 0.3(31.547)2 + 0.005(31.547)3 = −15.396 W

Therefore, maximum power is being delivered at t = 8.453 s. [b] The maximum power was calculated in part (a) to determine the time at which the power is maximum: pmax = 15.396 W (delivered) [c] As we saw in part (a), the other “maximum” power is actually a minimum, or the maximum negative power. As we calculated in part (a), maximum power is being extracted at t = 31.547 s. [d] This maximum extracted power was calculated in part (a) to determine the time at which power is maximum: pmax = 15.396 W (extracted) [e] w =

Z t 0

pdx =

Z t 0

(4x − 0.3x2 + 0.005x3 )dx = 2t2 − 0.1t3 + 0.00125t4

w(0)

w(30)

= 112.5 J

w(10)

112.5 J

w(40)

= 0J

w(20) = 200 J To give you a feel for the quantities of voltage, current, power, and energy and their relationships among one another, they are plotted below:

1–12

P 1.24

CHAPTER 1. Circuit Variables

[a] v(10 ms) = 400e−1 sin 2 = 133.8 V i(10 ms) = 5e−1 sin 2 = 1.67 A p(10 ms) = vi = 223.80 W

Problems

[b]

= vi = 2000e−200t sin2 200t 1 −200t 1 = 2000e − cos 400t 2 2 = 1000e−200t − 1000e−200t cos 400t

Z ∞

1000e

−200t

dt −

∞

e−200t = 1000 −200 0 (

Z ∞ 0

1000e−200t cos 400t dt

e−200t [−200 cos 400t + 400 sin 400t] −1000 2 + (400)2 (200) 200 = 5−1 = 5 − 1000 4 × 104 + 16 × 104 = 4 J

w P 1.25

1–13

)∞ 0

[a] p = vi = 2000 cos(800πt) sin(800πt) = 1000 sin(1600πt) W Therefore, pmax = 1000 W [b] pmax (extracting) = 1000 W [c] pavg

2.5×10−3 1 1000 sin(1600πt) dt 2.5 × 10 −3 0 2.5×10−3 250 5 − cos 1600πt [1 − cos 4π] = 0 4 × 10 = 1600π π 0

= =

[d] pavg

= =

P 1.26

[a] q

15.625×10−3 1 1000 sin(1600πt) dt 15.625 × 10−3 0 15.625×10−3 40 3 − cos 1600πt 64 × 10 = [1 − cos 25π] = 25.46 W 1600π π 0

area under i vs. t plot

1 (8)(12,000) + (16)(12,000) + 12 (16)(4000) 2

48,000 + 192,000 + 32,000 = 272,000 C

[b] w

p dt =

vi dt

v = 250 × 10−6 t + 8 0 ≤ t ≤ 12,000s:

0 ≤ t ≤ 16 ks

= 24 − 666.67 × 10−6 t

= 192 + 666.67 × 10−6 t − 166.67 × 10−9 t2

Z 12,000 0

(192 + 666.67 × 10−6 t − 166.67 × 10−9 t2 ) dt

= (2304 + 48 − 96)103 = 2256 kJ

1–14

CHAPTER 1. Circuit Variables 12,000 s ≤ t ≤ 16,000 s: i = p = w2

wT P 1.27

64 − 4 × 10−3 t 512 − 16 × 10−3 t − 10−6 t2 Z 16,000 12,000

(512 − 16 × 10−3 t − 10−6 t2) dt

(2048 − 896 − 789.33)103 = 362.667 kJ

w1 + w2 = 2256 + 362.667 = 2618.667 kJ

[a] 0 s ≤ t < 10 ms: v = 8 V;

i = 25t A;

p = 200t W

i = 0.5 − 25t A;

p = 200t − 4 W

i = −250 mA;

p=0W

10 ms < t ≤ 30 ms: v = −8 V; 30 ms ≤ t < 40 ms: v = 0 V; 40 ms < t ≤ 60 ms: v = 8 V;

i = 25t − 1.25 A; p = 200t − 10 W

t > 60 ms: v = 0 V;

i = 250 mA;

p=0W

[b] Calculate the area under the curve from zero up to the desired time: w(0.01)

1 (2)(0.01) = 10 mJ 2

w(0.03)

w(0.01) − 12 (2)(0.01) + 12 (2)(0.01) = 10 mJ

w(0.08)

w(0.03) − 12 (2)(0.01) + 12 (2)(0.01) = 10 mJ

Problems P 1.28

1–15

[a]

[b] i(t) =

10 + 0.5 × 10−3 t mA, 0 ≤ t ≤ 10 ks

i(t) =

15 mA,

10 ks ≤ t ≤ 20 ks

i(t) =

25 − 0.5 × 10−3 t mA, 20 ks ≤ t ≤ 30 ks

i(t) =

t > 30 ks

p = vi = 120i so p(t)

1200 + 0.06t mW,

0 ≤ t ≤ 10 ks

p(t)

1800 mW,

10 ks ≤ t ≤ 20 ks

p(t)

3000 − 0.06t mW, 20 ks ≤ t ≤ 30 ks

p(t)

t > 30 ks

1–16

CHAPTER 1. Circuit Variables [c] To find the energy, calculate the area under the plot of the power: 1 w(10 ks) = (0.6)(10,000) + (1.2)(10,000) = 15 kJ 2 w(20 ks) = w(10 ks) + (1.8)(10,000) = 33 kJ 1 w(10 ks) = w(20 ks) + (0.6)(10,000) + (1.2)(10,000) = 48 kJ 2

P 1.29

We use the passive sign convention to determine whether the power equation is p = vi or p = −vi and substitute into the power equation the values for v and i, as shown below: pa

−vaia = −(40)(−4 × 10−3 ) = 160 mW

vbib = (−24)(−4 × 10−3 ) = 96 mW

−vcic = −(−16)(4 × 10−3 ) = 64 mW

−vdid = −(−80)(−1.5 × 10−3 ) = −120 mW

veie = (40)(2.5 × 10−3 ) = 100 mW

pf = vf if = (120)(−2.5 × 10−3 ) = −300 mW Remember that if the power is positive, the circuit element is absorbing power, whereas is the power is negative, the circuit element is developing power. We can add the positive powers together and the negative powers together — if the power balances, these power sums should be equal: X Pdev = 120 + 300 = 420 mW; X Pabs = 160 + 96 + 64 + 100 = 420 mW Thus, the power balances and the total power absorbed in the circuit is 420 mW. P 1.30

−vaia = −(−3000)(−250 × 10−6 ) = −0.75 W

−vbib = −(4000)(−400 × 10−6 ) = 1.6 W

−vcic = −(1000)(400 × 10−6 ) = −0.4 W

vdid = (1000)(150 × 10−6 ) = 0.15 W

veie = (−4000)(200 × 10−6 ) = −0.8 W

pf = vf if = (4000)(50 × 10−6 ) = 0.2 W Therefore, X

Pabs = 1.6 + 0.15 + 0.2 = 1.95 W

Pdel = 0.75 + 0.4 + 0.8 = 1.95 W =

Pabs

Thus, the interconnection does satisfy the power check.

Problems P 1.31

1–17

[a] From the diagram and the table we have pa

−vaia = −(46.16)(6) = −276.96 W

vbib = (14.16)(4.72) = 66.8352 W

vcic = (−32)(−6.4) = 204.8 W

−vdid = −(22)(1.28) = −28.16 W

veie = (−33.6)(−1.68) = 56.448 W

vf if = (66)(0.4) = 26.4 W

vg ig = (2.56)(1.28) = 3.2768 W

−vhih = −(−0.4)(0.4) = 0.16 W

Pdel

= 276.96 + 28.16 = 305.12 W

Pabs

= 66.8352 + 204.8 + 56.448 + 26.4 + 3.2768 + 0.16 = 357.92 W

Therefore,

Pdel 6=

Pabs and the subordinate engineer is correct.

[b] The difference between the power delivered to the circuit and the power absorbed by the circuit is −305.12 + 357.92 = 52.8 W One-half of this difference is 26.4 W, so it is likely that pf is in error. Either the voltage or the current probably has the wrong sign. (In Chapter 2, we will discover that using KCL at the node connecting components f and h, the current if should be −0.4 A, not 0.4 A!) If the sign of pf is changed from negative to positive, we can recalculate the power delivered and the power absorbed as follows: X

Pdel

= 276.96 + 28.16 + 26.4 = 331.52 W

Pabs = 66.8352 + 204.8 + 56.448 + 3.2768 + 0.16 = 331.52 W Now the power delivered equals the power absorbed and the power balances for the circuit. P 1.32

[a] Remember that if the circuit element is absorbing power, the power is positive, whereas if the circuit element is supplying power, the power is negative. We can add the positive powers together and the negative powers X together — if the power balances, these power sums should be equal: Psup = 600 + 50 + 600 + 1250 = 2500 W; X Pabs = 400 + 100 + 2000 = 2500 W Thus, the power balances. [b] The current can be calculated using i = p/v or i = −p/v, with proper application of the passive sign convention:

1–18

P 1.33

CHAPTER 1. Circuit Variables

−pa /va = −(−600)/(400) = 1.5 A

pb /vb = (−50)/(−100) = 0.5 A

pc /vc = (400)/(200) = 2.0 A

pd /vd = (−600)/(300) = −2.0 A

pe /ve = (100)/(−200) = −0.5 A

−pf /vf = −(2000)/(500) = −4.0 A

pg /vg = (−1250)/(−500) = 2.5 A

[a] If the power balances, the sum of the power values should be zero: ptotal = 0.175 + 0.375 + 0.150 − 0.320 + 0.160 + 0.120 − 0.660 = 0 Thus, the power balances. [b] When the power is positive, the element is absorbing power. Since elements a, b, c, e, and f have positive power, these elements are absorbing power. [c] The voltage can be calculated using v = p/i or v = −p/i, with proper application of the passive sign convention:

P 1.34

pa /ia = (0.175)/(0.025) = 7 V

pb /ib = (0.375)/(0.075) = 5 V

−pc /ic = −(0.150)/(−0.05) = 3 V

pd /id = (−0.320)/(0.04) = −8 V

−pe /ie = −(0.160)/(0.02) = −8 V

pf /if = (0.120)/(−0.03) = −4 V

pg /ig = (−0.66)/(0.055) = −12 V

vaia = (120)(−10) = −1200 W

−vbib = −(120)(9) = −1080 W

vcic = (10)(10) = 100 W

−vdid = −(10)(−1) = 10 W

veie = (−10)(−9) = 90 W

−vf if = −(−100)(5) = 500 W

vgig = (120)(4) = 480 W

vhih = (−220)(−5) = 1100 W

Pdel = 1200 + 1080 = 2280 W

Problems

1–19

Pabs = 100 + 10 + 90 + 500 + 480 + 1100 = 2280 W X X Therefore, Pdel = Pabs = 2280 W

Thus, the interconnection now satisfies the power check. P 1.35

[a] The revised circuit model is shown below:

[b] The expression for the total power in this circuit is va ia − vb ib − vf if + vg ig + vh ih = (120)(−10) − (120)(10) − (−120)(3) + 120ig + (−240)(−7) = 0 Therefore, 120ig = 1200 + 1200 − 360 − 1680 = 360 so 360 =3A 120 Thus, if the power in the modified circuit is balanced the current in component g is 3 A.

ig =

Circuit Elements

Assessment a Problems AP 2.1

[a] Note that the current ib is in the same circuit branch as the 8 A current source; however, ib is defined in the opposite direction of the current source. Therefore, ib = −8 A Next, note that the dependent voltage source and the independent voltage source are in parallel with the same polarity. Therefore, their voltages are equal, and ib −8 vg = = = −2 V 4 4 [b] To find the power associated with the 8 A source, we need to find the voltage drop across the source, vi . Note that the two independent sources are in parallel, and that the voltages vg and v1 have the same polarities, so these voltages are equal: vi = vg = −2 V Using the passive sign convention, ps = (8 A)(vi ) = (8 A)(−2 V) = −16 W Thus the current source generated 16 W of power. 2–1 © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

2–2

CHAPTER 2. Circuit Elements

AP 2.2

[a] Note from the circuit that vx = −25 V. To find α note that the two current sources are in the same branch of the circuit but their currents flow in opposite directions. Therefore αvx = −15 A Solve the above equation for α and substitute for vx, α=

−15 A −15 A = 0.6 A/V = vx −25 V

[b] To find the power associated with the voltage source we need to know the current, iv . Note that this current is in the same branch of the circuit as the dependent current source and these two currents flow in the same direction. Therefore, the current iv is the same as the current of the dependent source: iv = αvx = (0.6)(−25) = −15 A Using the passive sign convention, ps = −(iv )(25 V) = −(−15 A)(25 V) = 375 W. Thus the voltage source dissipates 375 W. AP 2.3

[a] The resistor and the voltage source are in parallel and the resistor voltage and the voltage source have the same polarities. Therefore these two voltages are the same: vR = vg = 1 kV

Problems

2–3

Note from the circuit that the current through the resistor is ig = 5 mA. Use Ohm’s law to calculate the value of the resistor: vR 1 kV R= = = 200 kΩ ig 5 mA Using the passive sign convention to calculate the power in the resistor, pR = (vR )(ig ) = (1 kV)(5 mA) = 5 W The resistor is dissipating 5 W of power. [b] Note from part (a) the vR = vg and iR = ig . The power delivered by the source is thus psource −3 W psource = −vg ig so vg = − =− = 40 V ig 75 mA Since we now have the value of both the voltage and the current for the resistor, we can use Ohm’s law to calculate the resistor value: R=

40 V vg = 533.33 Ω = ig 75 mA

The power absorbed by the resistor must equal the power generated by the source. Thus, pR = −psource = −(−3 W) = 3 W [c] Again, note the iR = ig . The power dissipated by the resistor can be determined from the resistor’s current: pR = R(iR )2 = R(ig )2 Solving for ig , pr 480 mW = = 0.0016 R 300 Ω Then, since vR = vg i2g =

ig =

vR = RiR = Rig = (300 Ω)(40 mA) = 12 V

√ 0.0016 = 0.04 A = 40 mA

vg = 12 V

AP 2.4

[a] Note from the circuit that the current through the conductance G is ig , flowing from top to bottom, because the current source and the

2–4

CHAPTER 2. Circuit Elements conductance are in the same branch of the circuit so must have the same current. The voltage drop across the current source is vg , positive at the top, because the current source and the conductance are also in parallel so must have the same voltage. From a version of Ohm’s law, ig 0.5 A = = 10 V G 50 mS Now that we know the voltage drop across the current source, we can find the power delivered by this source: vg =

psource = −vg ig = −(10)(0.5) = −5 W Thus the current source delivers 5 W to the circuit. [b] We can find the value of the conductance using the power, and the value of the current using Ohm’s law and the conductance value: pg = Gvg2

9 pg = 2 = 0.04 S = 40 mS 2 vg 15

ig = Gvg = (40 mS)(15 V) = 0.6 A [c] We can find the voltage from the power and the conductance, and then use the voltage value in Ohm’s law to find the current: pg = Gvg2 Thus

so vg =

vg2 =

pg 8W = = 40,000 G 200 µS

40,000 = 200 V

ig = Gvg = (200 µS)(200 V) = 0.04 A = 40 mA AP 2.5 [a] Redraw the circuit with all of the voltages and currents labeled for every circuit element.

Write a KVL equation clockwise around the circuit, starting below the voltage source: −24 V + v2 + v5 − v1 = 0 Next, use Ohm’s law to calculate the three unknown voltages from the three currents: v2 = 3i2 ;

v5 = 7i5;

v1 = 2i1

Problems

2–5

A KCL equation at the upper right node gives i2 = i5 ; a KCL equation at the bottom right node gives i5 = −i1; a KCL equation at the upper left node gives is = −i2. Now replace the currents i1 and i2 in the Ohm’s law equations with i5: v2 = 3i2 = 3i5;

v5 = 7i5 ;

v1 = 2i1 = −2i5

Now substitute these expressions for the three voltages into the first equation: 24 = v2 + v5 − v1 = 3i5 + 7i5 − (−2i5) = 12i5 Therefore i5 = 24/12 = 2 A [b] v1 = −2i5 = −2(2) = −4 V [c] v2 = 3i5 = 3(2) = 6 V

[d] v5 = 7i5 = 7(2) = 14 V [e] A KCL equation at the lower left node gives is = i1. Since i1 = −i5, is = −2 A. We can now compute the power associated with the voltage source: p24 = (24)is = (24)(−2) = −48 W Therefore 24 V source is delivering 48 W. AP 2.6 Redraw the circuit labeling all voltages and currents:

We can find the value of the unknown resistor if we can find the value of its voltage and its current. To start, write a KVL equation clockwise around the right loop, starting below the 24 Ω resistor: −120 V + v3 = 0 Use Ohm’s law to calculate the voltage across the 8 Ω resistor in terms of its current: v3 = 8i3 Substitute the expression for v3 into the first equation: −120 V + 8i3 = 0

i3 =

120 = 15 A 8

2–6

CHAPTER 2. Circuit Elements Also use Ohm’s law to calculate the value of the current through the 24 Ω resistor: i2 =

120 V = 5A 24 Ω

Now write a KCL equation at the top middle node, summing the currents leaving: −i1 + i2 + i3 = 0

i1 = i2 + i3 = 5 + 15 = 20 A

Write a KVL equation clockwise around the left loop, starting below the voltage source: −200 V + v1 + 120 V = 0

v1 = 200 − 120 = 80 V

Now that we know the values of both the voltage and the current for the unknown resistor, we can use Ohm’s law to calculate the resistance: R =

v1 80 = 4Ω = i1 20

AP 2.7 [a] Plotting a graph of vt versus it gives

Note that when it = 0, vt = 25 V; therefore the voltage source must be 25 V. Since the plot is a straight line, its slope can be used to calculate the value of resistance: ∆v 25 − 0 25 R= = = = 100 Ω ∆i 0.25 − 0 0.25 A circuit model having the same v − i characteristic is a 25 V source in series with a 100Ω resistor, as shown below:

Problems

2–7

[b] Draw the circuit model from part (a) and attach a 25 Ω resistor:

To find the power delivered to the 25 Ω resistor we must calculate the current through the 25 Ω resistor. Do this by first using KCL to recognize that the current in each of the components is it , flowing in a clockwise direction. Write a KVL equation in the clockwise direction, starting below the voltage source, and using Ohm’s law to express the voltage drop across the resistors in the direction of the current it flowing through the resistors: 25 −25 V + 100it + 25it = 0 so 125it = 25 so it = = 0.2 A 125 Thus, the power delivered to the 25 Ω resistor is p25 = (25)i2t = (25)(0.2)2 = 1 W. AP 2.8 [a] From the graph in Assessment Problem 2.7(a), we see that when vt = 0, it = 0.25 A. Therefore the current source must be 0.25 A. Since the plot is a straight line, its slope can be used to calculate the value of resistance: ∆v 25 − 0 25 = = = 100 Ω ∆i 0.25 − 0 0.25 A circuit model having the same v − i characteristic is a 0.25 A current source in parallel with a 100Ω resistor, as shown below: R=

[b] Draw the circuit model from part (a) and attach a 25 Ω resistor:

Note that by writing a KVL equation around the right loop we see that the voltage drop across both resistors is vt. Write a KCL equation at the top center node, summing the currents leaving the node. Use Ohm’s law

2–8

CHAPTER 2. Circuit Elements to specify the currents through the resistors in terms of the voltage drop across the resistors and the value of the resistors. vt vt −0.25 + so 5vt = 25, thus vt = 5 V + = 0, 100 25 p25 =

vt2 = 1 W. 25

AP 2.9 First note that we know the current through all elements in the circuit except the 6 kΩ resistor (the current in the three elements to the left of the 6 kΩ resistor is i1 ; the current in the three elements to the right of the 6 kΩ resistor is 30i1 ). To find the current in the 6 kΩ resistor, write a KCL equation at the top node: i1 + 30i1 = i6k = 31i1 We can then use Ohm’s law to find the voltages across each resistor in terms of i1 . The results are shown in the figure below:

[a] To find i1, write a KVL equation around the left-hand loop, summing voltages in a clockwise direction starting below the 5V source: −5 V + 54,000i1 − 1 V + 186,000i1 = 0 Solving for i1 54,000i1 + 186,000i1 = 6 V

240,000i1 = 6 V

Thus, i1 =

6 = 25 µA 240,000

[b] Now that we have the value of i1 , we can calculate the voltage for each component except the dependent source. Then we can write a KVL equation for the right-hand loop to find the voltage v of the dependent source. Sum the voltages in the clockwise direction, starting to the left of the dependent source: +v − 54,000i1 + 8 V − 186,000i1 = 0

Problems

2–9

Thus, v = 240,000i1 − 8 V = 240,000(25 × 10−6 ) − 8 V = 6 V − 8 V = −2 V We now know the values of voltage and current for every circuit element. Let’s construct a power table: Element

Current

Voltage

Power

(µA)

(V)

Equation

(µW)

p = −vi

−125

54 kΩ

1.35

p = Ri2

33.75

p = −vi

−25

6 kΩ

775

4.65

p = Ri2

3603.75

Dep. source

750

−2

p = −vi

1500

1.8 kΩ

750

1.35

p = Ri2

1012.5

750

p = −vi

−6000

[c] The total power generated in the circuit is the sum of the negative power values in the power table: −125 µW + −25 µW + −6000 µW = −6150 µW Thus, the total power generated in the circuit is 6150 µW. [d] The total power absorbed in the circuit is the sum of the positive power values in the power table: 33.75 µW + 3603.75 µW + 1500 µW + 1012.5 µW = 6150 µW Thus, the total power absorbed in the circuit is 6150 µW. AP 2.10 Given that iφ = 2 A, we know the current in the dependent source is 2iφ = 4 A. We can write a KCL equation at the left node to find the current in the 10 Ω resistor. Summing the currents leaving the node, −5 A + 2 A + 4 A + i10Ω = 0

i10Ω = 5 A − 2 A − 4 A = −1 A

Thus, the current in the 10 Ω resistor is 1 A, flowing right to left, as seen in the circuit below.

2–10

CHAPTER 2. Circuit Elements

[a] To find vs , write a KVL equation, summing the voltages counter-clockwise around the lower right loop. Start below the voltage source. −vs + (1 A)(10 Ω) + (2 A)(30 Ω) = 0

vs = 10 V + 60 V = 70 V

[b] The current in the voltage source can be found by writing a KCL equation at the right-hand node. Sum the currents leaving the node −4 A + 1 A + iv = 0

iv = 4 A − 1 A = 3 A

The current in the voltage source is 3 A, flowing top to bottom. The power associated with this source is p = vi = (70 V)(3 A) = 210 W Thus, 210 W are absorbed by the voltage source. [c] The voltage drop across the independent current source can be found by writing a KVL equation around the left loop in a clockwise direction: −v5A + (2 A)(30 Ω) = 0

v5A = 60 V

The power associated with this source is p = −v5A i = −(60 V)(5 A) = −300 W This source thus delivers 300 W of power to the circuit. [d] The voltage across the controlled current source can be found by writing a KVL equation around the upper right loop in a clockwise direction: +v4A + (10 Ω)(1 A) = 0

v4A = −10 V

The power associated with this source is p = v4A i = (−10 V)(4 A) = −40 W This source thus delivers 40 W of power to the circuit. [e] The total power dissipated by the resistors is given by (i30Ω )2(30 Ω) + (i10Ω)2 (10 Ω) = (2)2 (30 Ω) + (1)2 (10 Ω) = 120 + 10 = 130 W

Problems

2–11

Problems P 2.1

[a] Yes, independent voltage sources can carry the 5 A current required by the connection; independent current source can support any voltage required by the connection, in this case 5 V, positive at the bottom. [b] 20 V source:

absorbing

15 V source:

developing (delivering)

5 A source:

developing (delivering)

[c] P20V

(20)(5) = 100 W (abs)

P15V

−(15)(5) = −75 W (dev/del)

P5A

Pabs =

−(5)(5) = −25 W

(dev/del)

Pdel = 100 W

[d] The interconnection is valid, but in this circuit the voltage drop across the 5 A current source is 35 V, positive at the top; 20 V source is developing (delivering), the 15 V source is developing (delivering), and the 5 A source is absorbing: P20V

= −(20)(5) = −100 W (dev/del)

P15V

= −(15)(5) = −75 W (dev/del)

P5A

= (35)(5) = 175 W

P 2.2

Pabs =

(abs)

Pdel = 175 W

The interconnect is valid since the voltage sources can all carry 5 A of current supplied by the current source, and the current source can carry the voltage drop required by the interconnection. Note that the branch containing the 10 V, 40 V, and 5 A sources must have the same voltage drop as the branch containing the 50 V source, so the 5 A current source must have a voltage drop of 20 V, positive at the right. The voltages and currents are summarize in the circuit below:

2–12

CHAPTER 2. Circuit Elements

P50V

(50)(5) = 250 W (abs)

P10V

(10)(5) = 50 W

P40V

−(40)(5) = −200 W (dev)

P5A

−(20)(5) = −100 W

P 2.3

(abs)

(dev)

Pdev = 300 W

The interconnection is valid. The 10 A current source has a voltage drop of 100 V, positive at the top, because the 100 V source supplies its voltage drop across a pair of terminals shared by the 10 A current source. The right hand branch of the circuit must also have a voltage drop of 100 V from the left terminal of the 40 V source to the bottom terminal of the 5 A current source, because this branch shares the same terminals as the 100 V source. This means that the voltage drop across the 5 A current source is 140 V, positive at the top. Also, the two voltage sources can carry the current required of the interconnection. This is summarized in the figure below:

From the values of voltage and current in the figure, the power supplied by the current sources is calculated as follows: P10A = −(100)(10) = −1000 W (1000 W supplied) P5A X

−(140)(5) = −700 W

(700 W supplied)

Pdev = 1700 W

P 2.4

The interconnection is not valid. Note that the 3 A and 4 A sources are both connected in the same branch of the circuit. A valid interconnection would require these two current sources to supply the same current in the same direction, which they do not.

P 2.5

The interconnection is valid, since the voltage sources can carry the currents supplied by the 2 A and 3 A current sources, and the current sources can carry whatever voltage drop from the top node to the bottom node is required by the interconnection. In particular, note the the voltage drop between the top and bottom nodes in the right hand branch must be the same as the voltage drop between the top and bottom nodes in the left hand branch. In particular, this means that −v1 + 8 V = 12 V + v2

Problems

2–13

Hence any combination of v1 and v2 such that v1 + v2 = −4 V is a valid solution. P 2.6

[a] Because both current sources are in the same branch of the circuit, their values must be the same. Therefore, v1 = 0.4 → v1 = 0.4(50) = 20 V 50 [b] p = v1(0.4) = (20)(0.4) = 8 W (absorbed)

P 2.7

[a] The voltage drop from the top node to the bottom node in this circuit must be the same for every path from the top to the bottom. Therefore, the voltages of the two voltage sources are equal: −αi∆ = 6 Also, the current i∆ is in the same branch as the 15 mA current source, but in the opposite direction, so i∆ = −0.015 Substituting, 6 = 400 0.015 The interconnection is valid if α = 400 V/A. −α(−0.015) = 6

→

α=

[b] The voltage across the current source must equal the voltage across the 6 V source, since both are connected between the top and bottom nodes. Using the passive sign convention, p = vi = (6)(0.015) = 0.09 = 90 mW [c] Since the power is positive, the current source is absorbing power. P 2.8

[a] Yes, each of the voltage sources can carry the current required by the interconnection, and each of the current sources can carry the voltage drop required by the interconnection. (Note that i1 = 50 mA.) [b] No, because the voltage drop between the top terminal and the bottom terminal cannot be determined. For example, define v1, v2 , and v3 as shown:

2–14

CHAPTER 2. Circuit Elements The voltage drop across the left branch, the center branch, and the right branch must be the same, since these branches are connected at the same two terminals. This requires that v1 − 20 = v2 = v3 + 30 But this equation has three unknown voltages, so the individual voltages cannot be determined, and thus the power of the sources cannot be determined.

P 2.9

The interconnection is invalid. In the middle branch, the value of the current ix must be 50 mA, since the 50 mA current source supplies current in this branch in the same direction as the current ix. Therefore, the voltage supplied by the dependent voltage source in the right hand branch is 1800(0.05) = 90 V. This gives a voltage drop from the top terminal to the bottom terminal in the right hand branch of 90 + 60 = 150 V. But the voltage drop between these same terminals in the left hand branch is 30 V, due to the voltage source in that branch. Therefore, the interconnection is invalid.

P 2.10

First, 10va = 5 V, so va = 0.5 V. Then recognize that each of the three branches is connected between the same two nodes, so each of these branches must have the same voltage drop. The voltage drop across the middle branch is 5 V, and since va = 0.5 V, vg = 0.5 − 5 = −4.5 V. Also, the voltage drop across the left branch is 5 V, so 20 + v9A = 5 V, and v9A = −15 V, where v9A is positive at the top. Note that the current through the 20 V source must be 9 A, flowing from top to bottom, and the current through the vg is 6 A flowing from top to bottom. Let’s find the power associated with the left and middle branches: p9A = (9)(−15) = −135 W p20V = (9)(20) = 180 W pvg = −(6)(−4.5) = 27 W p6A = (6)(0.5) = 3 W Since there is only one component left, we can find the total power: ptotal = −135 + 180 + 27 + 3 + pds = 75 + pds = 0 so pds must equal −75 W. Therefore, X

Pdev =

Pabs = 210 W

Problems P 2.11

2–15

[a] Using the passive sign convention and Ohm’s law, v = Ri = (3000)(0.015) = 45 V v2 452 = = 0.675 = 675 mW R 3000 [c] Using the passive sign convention with the current direction reversed,

[b] PR =

v = −Ri = −(3000)(0.015) = −45 V PR = P 2.12

v2 −452 = = 0.675 = 675 mW R 3000

[a] Using the passive sign convention and Ohm’s law, i=−

40 v =− = −0.016 = −16 mA R 2500

[b] PR = Ri2 = (2500)(−0.016)2 = 0.64 = 640 mW [c] Using the passive sign convention with the voltage polarity reversed, i=

40 v = = 0.016 = 16 mA R 2500

PR = Ri2 = (2500)(0.016)2 = 0.64 = 640 mW P 2.13

[a]

[b]

Vbb

no-load voltage of battery

Rbb

internal resistance of battery

resistance of wire between battery and switch

resistance of wire between switch and lamp A

resistance of lamp A

resistance of lamp B

resistance of wire between lamp A and lamp B

Rg1

resistance of frame between battery and lamp A

Rg2

resistance of frame between lamp A and lamp B

switch

2–16 P 2.14

CHAPTER 2. Circuit Elements Since we know the device is a resistor, we can use Ohm’s law to calculate the resistance. From Fig. P2.14(a), v = Ri

v i

Using the values in the table of Fig. P2.14(b), R=

−7200 −3600 3600 7200 10,800 = = = = = 1.2 kΩ −6 −3 3 6 9

Note that this value is found in Appendix H. P 2.15

Since we know the device is a resistor, we can use the power equation. From Fig. P2.15(a), p = vi =

v2 R

v2 p

Using the values in the table of Fig. P2.13(b) R=

(−8)2 (−4)2 (4)2 (8)2 = = = 640 × 10−3 160 × 10−3 160 × 10−3 640 × 10−3 =

(12)2 (16)2 = = 100 Ω 1440 × 10−3 2560 × 10−3

Note that this value is found in Appendix H. P 2.16

The resistor value is the ratio of the power to the square of the current: p R = 2 . Using the values for power and current in Fig. P2.16(b), i 33 × 10−3 74.25 × 10−3 132 × 10−3 8.25 × 10−3 = = = (0.5 × 10−3 )2 (1 × 10−3 )2 (1.5 × 10−3 )2 (2 × 10−3 )2 =

206.25 × 10−3 297 × 10−3 = = 33 kΩ (2.5 × 10−3 )2 (3 × 10−3 )2

Note that this is a value from Appendix H. P 2.17

Label the unknown resistor currents and voltages:

Problems

2–17

[a] KCL at the top node: 0.02 = i1 + i2 KVL around the right loop: −vo + v2 − 5 = 0 Use Ohm’s law to write the resistor voltages in the previous equation in terms of the resistor currents: −5000i1 + 2000i2 − 5 = 0

→

−5000i1 + 2000i2 = 5

Multiply the KCL equation by −2000 and add it to the KVL equation to eliminate i2: −2000(i1 + i2) + (−5000i1 + 2000i2 ) = −2000(0.02) + 5

→

−7000i1 = −35

Solving, −35 = 0.005 = 5 mA −7000 Therefore, i1 =

vo = Ri1 = (5000)(0.005) = 25 V [b] p20mA = −(0.02)vo = −(0.02)(25) = −0.5 W i2 = 0.02 − i1 = 0.02 − 0.005 = 0.015 A p5V = −(5)i2 = −(5)(0.015) = −0.075 W p5k = 5000i21 = 5000(0.005)2 = 0.125 W p2k = 2000i22 = 2000(0.015)2 = 0.45 W ptotal = p20mA + p5V + p5k + p2k = −0.5 − 0.075 + 0.125 + 0.45 = 0 Thus the power in the circuit balances. P 2.18

[a]

20ia

80ib

4ib

ig = ia + ib = 5ib

50 =

4ig + 80ib = 20ib + 80ib = 100ib

0.5 A, therefore, ia = 2 A

and

ig = 2.5 A

[b] ib = 0.5 A [c] vo = 80ib = 40 V

2–18

CHAPTER 2. Circuit Elements

[d]

p4Ω

= i2g (4) = 6.25(4) = 25 W

p20Ω

= i2a (20) = (4)(20) = 80 W

p80Ω

= i2b (80) = 0.25(80) = 20 W

[e] p50V (delivered) = 50ig = 125 W Check: X

Pdis = 25 + 80 + 20 = 125 W

Pdel = 125 W

P 2.19

[a] Write a KCL equation at the top node: −1.5 + i1 + i2 = 0

i1 + i2 = 1.5

Write a KVL equation around the right loop: −v1 + v2 + v3 = 0

From Ohm’s law, v1 = 100i1 ,

v2 = 150i2 ,

v3 = 250i2

Substituting, −100i1 + 150i2 + 250i2 = 0

− 100i1 + 400i2 = 0

Solving the two equations for i1 and i2 simultaneously, i1 = 1.2 A

and

i2 = 0.3 A

[b] Write a KVL equation clockwise around the left loop: −vo + v1 = 0 So

but

v1 = 100i1 = 100(1.2) = 120 V

vo = v1 = 120 V

[c] Calculate power using p = vi for the source and p = Ri2 for the resistors: psource = −vo(1.5) = −(120)(1.5) = −180 W p100Ω = 1.22 (100) = 144 W

p150Ω = 0.32 (150) = 13.5 W p250Ω = 0.32 (250) = 22.5 W X

Pdev = 180 W

Pabs = 144 + 13.5 + 22.5 = 180 W

Problems P 2.20

2–19

Label the unknown resistor voltages and currents:

[a] ia =

3.5 = 0.02 A 175

i1 = ia = 0.02 A

(Ohm’s law) (KCL)

[b] vb = 200i1 = 200(0.02) = 4 V (Ohm’s law) −v1 + vb + 3.5 = 0

v1 = 3.5 + vb = 3.5 + 4 = 7.5 V (KVL)

[c] va = 0.05(50) = 2.5 V (Ohm’s law) −vg + va + v1 = 0

vg = va + v1 = 2.5 + 7.5 = 10 V

(KVL)

[d] pg = vg (0.05) = 10(0.05) = 0.5 W P 2.21

[a] Use KVL for the right loop to calculate the voltage drop across the right-hand branch vo . This is also the voltage drop across the middle branch, so once vo is known, use Ohm’s law to calculate io : vo

= 1000ia + 4000ia + 3000ia = 8000ia = 8000(0.002) = 16 V

= 2000io 16 io = = 8 mA 2000 [b] KCL at the top node: ig = ia + io = 0.002 + 0.008 = 0.010 A = 10 mA. [c] The voltage drop across the source is v0, seen by writing a KVL equation for the left loop. Thus, pg = −vo ig = −(16)(0.01) = −0.160 W = −160 mW. Thus the source delivers 160 mW. P 2.22

[a]

v2 = 150 − 50(1) = 100V © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

2–20

CHAPTER 2. Circuit Elements i2 =

v2 = 4A 25

i3 + 1 = i2 ,

i3 = 4 − 1 = 3A

v1 = 10i3 + 25i2 = 10(3) + 25(4) = 130V i1 =

v1 130 = = 2A 65 65

Note also that i4 = i1 + i3 = 2 + 3 = 5 A ig = i4 + io = 5 + 1 = 6 A [b]

[c]

p4Ω

= 52 (4) = 100 W

p50Ω

= 12 (50) = 50 W

p65Ω

= 22 (65) = 260 W

p10Ω

= 32 (10) = 90 W

p25Ω

= 42 (25) = 400 W

Pdis = 100 + 50 + 260 + 90 + 400 = 900 W

Pdev = 150ig = 150(6) = 900 W P 2.23

Label all unknown resistor voltages and currents:

Ohms’ law for 5 kΩ resistor: v1 = (0.01)(5000) = 50 V KVL for lower left loop: −80 + v2 + 50 = 0 → v2 = 80 − 50 = 30 V Ohm’s law for 1.5 kΩ resistor: i2 = v2/1500 = 30/1500 = 20 mA KCL at center node: i2 = i3 + 0.01 → i3 = i2 − 0.01 = 0.02 − 0.01 = 0.01 = 10 mA Ohm’s law for 3 kΩ resistor v3 = 3000i3 = 3000(0.01) = 30 V KVL for lower right loop: −v1 + v3 + v4 = 0 → v4 = v1 − v3 = 50 − 30 = 20 V © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Problems

2–21

Ohm’s law for 500 Ω resistor: i4 = v4/500 = 20/500 = 0.04 = 40 mA KCL for right node: i3 + iR = i4 → iR = i4 − i3 = 0.04 − 0.01 = 0.03 = 30 mA KVL for outer loop: −80 + vR + v4 = 0 → vR = 80 − v4 = 80 − 20 = 60 V Therefore, R= P 2.24

vR 60 = = 2000 = 2 kΩ iR 0.03

[a]

va = (5 + 10)(4) = 60 V −240 + va + vb = 0 so vb = 240 − va = 240 − 60 = 180 V ie = vb /(14 + 6) = 180/20 = 9 A id = ie − 4 = 9 − 4 = 5 A vc = 4id + vb = 4(5) + 180 = 200 V ic = vc /10 = 200/10 = 20 A vd = 240 − vc = 240 − 200 = 40 V ia = id + ic = 5 + 20 = 25 A R = vd /ia = 40/25 = 1.6 Ω [b] ig = ia + 4 = 25 + 4 = 29 A pg (supplied) = (240)(29) = 6960 W P 2.25

[a]

icd = 80/16 = 5 A vac = 125 − 80 = 45

iac = 45/15 = 3 A

2–22

CHAPTER 2. Circuit Elements iac + ibc = icd

ibc = 5 − 3 = 2 A

vab = 15iac − 5ibc = 15(3) − 5(2) = 35 V

iab = 35/7 = 5 A

ibd = iab − ibc = 5 − 2 = 3 A Calculate the power dissipated by the resistors using the equation pR = Ri2R : p7Ω = (7)(5)2 = 175 W p15Ω = (15)(3)2 = 135 W

p30Ω = (30)(3)2 = 270 W p16Ω = (16)(5)2 = 400 W

p5Ω = (5)(2)2 = 20 W [b] Calculate the current through the voltage source: iad = −iab − iac = −5 − 3 = −8 A Now that we have both the voltage and the current for the source, we can calculate the power supplied by the source: pg = 125(−8) = −1000 W [c]

pg (supplied) = 1000 W

Pdis = 175 + 270 + 135 + 400 + 20 = 1000 W Therefore, X

P 2.26

thus

Psupp =

Pdis

[a]

v2 = 100 + 4(15) = 160 V; i1 =

v1 100 = = 5 A; 4 + 16 20

v1 = 160 − (9 + 11 + 10)(2) = 100 V i3 = i1 − 2 = 5 − 2 = 3 A

vg = v1 + 30i3 = 100 + 30(3) = 190 V i4 = 2 + 4 = 6 A ig = −i4 − i3 = −6 − 3 = −9 A

Problems

2–23

[b] Calculate power using the formula p = Ri2 : p9 Ω = (9)(2)2 = 36 W;

p11 Ω = (11)(2)2 = 44 W

p10 Ω = (10)(2)2 = 40 W;

p5 Ω = (5)(6)2 = 180 W

p30 Ω = (30)(3)2 = 270 W;

p4 Ω = (4)(5)2 = 100 W

p16 Ω = (16)(5)2 = 400 W;

p15 Ω = (15)(4)2 = 240 W

[c] vg = 190 V [d] Sum the power dissipated by the resistors: X

pdiss = 36 + 44 + 40 + 180 + 270 + 100 + 400 + 240 = 1310 W

The power associated with the sources is pvolt−source = (100)(4) = 400 W pcurr−source = vg ig = (190)(−9) = −1710 W Thus the total power dissipated is 1310 + 400 = 1710 W and the total power developed is 1710 W, so the power balances. P 2.27

[a] Start by calculating the voltage drops due to the currents i1 and i2. Then use KVL to calculate the voltage drop across and 35 Ω resistor, and Ohm’s law to find the current in the 35 Ω resistor. Finally, KCL at each of the middle three nodes yields the currents in the two sources and the current in the middle 2 Ω resistor. These calculations are summarized in the figure below:

p147(top)

= −(147)(28) = −4116 W

p147(bottom) = −(147)(21) = −3087 W Therefore the top source supplies 4116 W of power and the bottom source supplies 3087 W of power. [b] X X

Pdis

Psup

Therefore,

= (28)2 (1) + (7)2 (2) + (21)2 (1) + (21)2 (5) + (14)2 (10) + (7)2 (35) = 784 + 98 + 441 + 2205 + 1960 + 1715 = 7203 W = 4116 + 3087 = 7203 W X

Pdis =

Psup = 7203 W

2–24 P 2.28

CHAPTER 2. Circuit Elements [a] Plot the v − i characteristic

From the plot: R=

∆v (130 − 50) = = 8Ω ∆i (10 − 0)

When it = 0, vt = 50 V; therefore the ideal voltage source has a voltage of 50 V.

[b]

−50 = −6.25A 8 Note that this result can also be obtained by extrapolating the v − i characteristic to vt = 0.

When vt = 0,

P 2.29

it =

[a] Plot the v—i characteristic:

Problems

2–25

From the plot: R=

∆v (180 − 100) = 5Ω = ∆i (16 − 0)

When it = 0, vt = 100 V; therefore the ideal current source must have a current of 100/5 = 20 A

[b] We attach a 20 Ω resistor to the device model developed in part (a):

Write a KCL equation at the top node: 20 + it = i1 Write a KVL equation for the right loop, in the direction of the two currents, using Ohm’s law: 5i1 + 20it = 0 Combining the two equations and solving, 5(20 + it ) + 20it = 0

25it = −100;

thus

it = −4 A

Now calculate the power dissipated by the resistor: p20 Ω = 20i2t = 20(−4)2 = 320 W P 2.30

[a]

2–26

CHAPTER 2. Circuit Elements

[b] ∆v = 25V;

∆i = 2.5 mA;

[c] 10,000i1 = 2500is ,

∆v = 10 kΩ ∆i

i1 = 0.25is

0.02 = i1 + is = 1.25is ,

is = 16 mA

[d] vs (open circuit) = (20 × 10−3 )(10 × 103 ) = 200 V

[e] The open circuit voltage can be found in the table of values (or from the plot) as the value of the voltage vs when the current is = 0. Thus, vs (open circuit) = 140 V (from the table) [f] Linear model cannot predict the nonlinear behavior of the practical current source. P 2.31

[a] Begin by constructing a plot of voltage versus current:

[b] Since the plot is linear for 0 ≤ is ≤ 24 mA amd since R = ∆v/∆i, we can calculate R from the plotted values as follows: ∆v 24 − 18 6 = = = 250 Ω ∆i 0.024 − 0 0.024 We can determine the value of the ideal voltage source by considering the value of vs when is = 0. When there is no current, there is no voltage drop across the resistor, so all of the voltage drop at the output is due to the voltage source. Thus the value of the voltage source must be 24 V. The model, valid for 0 ≤ is ≤ 24 mA, is shown below: R=

Problems

2–27

[c] The circuit is shown below:

Write a KVL equation in the clockwise direction, starting below the voltage source. Use Ohm’s law to express the voltage drop across the resistors in terms of the current i: −24 V + 250i + 1000i = 0 Thus,

1250i = 24 V

24 V = 19.2 mA 1250 Ω

[d] The circuit is shown below:

250i = 24 V

24 V = 96 mA 250 Ω

[e] The short circuit current can be found in the table of values (or from the plot) as the value of the current is when the voltage vs = 0. Thus, isc = 48 mA

(from table)

[f] The plot of voltage versus current constructed in part (a) is not linear (it is piecewise linear, but not linear for all values of is ). Since the proposed circuit model is a linear model, it cannot be used to predict the nonlinear behavior exhibited by the plotted data.

2–28

CHAPTER 2. Circuit Elements

P 2.32

Label unknown voltage and current:

−vx + vo + 2ix = 0 vx = 6ix

(KVL)

(Ohm’s law)

Therefore −6ix + vo + 2ix = 0 so vo = 4ix Thus vo 4

ix = Also,

vo 2

(Ohm’s law)

45 = ix + i1

(KCL)

i1 =

Substituting for the currents ix and i1: 45 =

vo vo 3vo + = 4 2 4

Thus vo = 45

4 = 60 V 3

The only two circuit elements that could supply power are the two sources, so calculate the power for each source: vx = 6ix = 6

vo = 6(60/4) = 90 V 4

p45V = −45vx = −45(90) = −4050 W pd.s. = (2ix )i1 = 2(vo /4)(vo /2) = 2(60/4)(60/2) = 900 W Only the independent voltage source is supplying power, so the total power supplied is 4050 W.

Problems P 2.33

2–29

Label unknown current:

−20 + 450i + 150i = 0 so

600i = 20

→

(KVL and Ohm’s law) i = 33.33 mA

vx = 150i = 150(0.0333) = 5 V

(Ohm’s law)

vx vo = 300 = 300(5/100) = 15 V 100

(Ohm’s law)

Calculate the power for all components: p20V = −20i = −20(0.0333) = −0.667 W pd.s. = −vo

vx 100

= −(15)(5/100) = −0.75 W

p450 = 450i2 = 450(0.033)2 = 0.5 W p150 = 150i2 = 150(0.033)2 = 0.1667 W p300 =

vo2 152 = = 0.75 W 300 300

Thus the total power absorbed is pabs = 0.5 + 0.1667 + 0.75 = 1.4167 W P 2.34

The circuit:

v1 = (4000)(0.01) = 40 V

(Ohm’s law)

2–30

CHAPTER 2. Circuit Elements v1 = 2000io + 6000io = 8000io 2

(KVL)

Thus, io =

v1/2 40/2 = = 0.0025 = 2.5 mA 8000 8000

Calculate the power for all components: p10mA = −(0.01)v1 = −(0.01)(40) = −0.4 W pd.s. = −(v1/2)io = −(40/2)(2.5 × 10−3 ) = −0.05 W p4k =

v12 402 = = 0.4 W 4000 4000

p2k = 2000i2o = 2000(2.5 × 10−3 )2 = 0.0125 W p6k = 6000i2o = 6000(2.5 × 10−3 )2 = 0.0375 W Therefore, ptotal = −0.4 − 0.05 + 0.4 + 0.0125 + 0.0375 = 0 Thus the power in the circuit balances. P 2.35

[a] io = 0 because no current can exist in a single conductor connecting two parts of a circuit. [b]

18 = (12 + 6)ig

ig = 1 A

v∆ = 6ig = 6V

v∆ /2 = 3 A

10i1 = 5i2 , so i1 + 2i1 = −3 A; therefore, i1 = −1 A [c] i2 = 2i1 = −2 A.

Problems P 2.36

2–31

[a] −50 − 20iσ + 18i∆ = 0 −18i∆ + 5iσ + 40iσ = 0 Therefore,

18i∆ = 45iσ

− 50 − 20iσ + 45iσ = 0,

iσ = 2 A

18i∆ = 45iσ = 90; so i∆ = 5 A vo = 40iσ = 80 V [b] ig = current out of the positive terminal of the 50 V source vd = voltage drop across the 8i∆ source ig = i∆ + iσ + 8i∆ = 9i∆ + iσ = 47 A vd = 80 − 20 = 60 V X

Pgen

= 50ig + 20iσ ig = 50(47) + 20(2)(47) = 4230 W

Pdiss

= 18i2∆ + 5iσ (ig − i∆ ) + 40i2σ + 8i∆ vd + 8i∆ (20) = (18)(25) + 10(47 − 5) + 4(40) + 40(60) + 40(20) = 4230 W; Therefore,

P 2.37

40i2 +

Pgen

5 5 + = 0; 40 10

Pdiss = 4230 W i2 = −15.625 mA

v1 = 80i2 = −1.25 V 25i1 +

(−1.25) + (−0.015625) = 0; 20

i1 = 3.125 mA

vg = 60i1 + 260i1 = 320i1 Therefore, vg = 1 V. P 2.38

iE − iB − iC = 0 iC = βiB

therefore iE = (1 + β)iB

i2 = −iB + i1 Vo + iE RE − (i1 − iB )R2 = 0 −i1R1 + VCC − (i1 − iB )R2 = 0

i1 =

VCC + iB R2 R1 + R2

2–32

CHAPTER 2. Circuit Elements

Vo + iE RE + iB R2 −

VCC + iB R2 R2 = 0 R1 + R2

Now replace iE by (1 + β)iB and solve for iB . Thus iB = P 2.39

[VCC R2 /(R1 + R2 )] − Vo (1 + β)RE + R1 R2 /(R1 + R2 )

Here is Equation 2.25: iB =

(VCC R2 )/(R1 + R2 ) − V0 (R1 R2 )/(R1 + R2 ) + (1 + β)RE

VCC R2 (10)(60,000) = 6V = R1 + R2 100,000 R1 R2 (40,000)(60,000) = 24 kΩ = R1 + R2 100,000 iB =

5.4 6 − 0.6 = = 0.18 mA 24,000 + 50(120) 30,000

iC = βiB = (49)(0.18) = 8.82 mA iE = iC + iB = 8.82 + 0.18 = 9 mA v3d = (0.009)(120) = 1.08V vbd = Vo + v3d = 1.68V i2 =

vbd 1.68 = = 28 µA R2 60,000

i1 = i2 + iB = 28 + 180 = 208 µA vab = 40,000(208 × 10−6 ) = 8.32 V iCC = iC + i1 = 8.82 + 0.208 = 9.028 mA v13 + (8.82 × 10−3 )(750) + 1.08 = 10 V v13 = 2.305 V

Problems P 2.40

2–33

[a]

[b]

P 2.41

Each radiator is modeled as a 48 Ω resistor:

Write a KVL equation for each of the three loops: 240 = 5A 48

−240 + 48i1 = 0

→

i1 =

−48i1 + 48i2 = 0

→

i2 = i1 = 5 A

−48i2 + 48i3 = 0

→

i3 = i2 = 5 A

Therefore, the current through each radiator is 5 A and the power for each radiator is prad = Ri2 = 48(5)2 = 1200 W There are three radiators, so the total power for this heating system is ptotal = 3prad = 3(1200) = 3600 W

2–34 P 2.42

CHAPTER 2. Circuit Elements Each radiator is modeled as a 48 Ω resistor:

Write a KVL equation for the left and right loops: −240 + 48i1 = 0

→

−48i1 + 48i2 + 48i2 = 0

i1 =

240 = 5A 48

→

i2 =

5 i1 = = 2.5 A 2 2

The power for the center radiator is pcen = 48i21 = 48(5)2 = 1200 W The power for each of the radiators on the right is pright = 48i22 = 48(2.5)2 = 300 W Thus the total power for this heating system is ptotal = pcen + 2pright = 1200 + 2(300) = 1800 W The center radiator produces 1200 W, just like the three radiators in Problem 2.41. But the other two radiators produce only 300 W each, which is 1/4th of the power of the radiators in Problem 2.41. The total power of this configuration is 1/2 of the total power in Fig. 2.41. P 2.43

Each radiator is modeled as a 48 Ω resistor:

Write a KVL equation for the left and right loops: −240 + 48i1 + 48i2 = 0 © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

Problems −48i2 + 48i3 = 0

→

2–35

i2 = i3

Write a KCL equation at the top node: i1 = i2 + i3

→

i1 = i2 + i2 = 2i2

Substituting into the first KVL equation gives −240 + 48(2i2 ) + 48i2 = 0

→

i2 =

240 = 1.67 A 3(48)

Solve for the currents i1 and i3: i3 = i2 = 1.67 A;

i1 = 2i2 = 2(1.67) = 3.33 A

Calculate the power for each radiator using the current for each radiator: pleft = 48i21 = 48(3.33)2 = 533.33 W pmiddle = pright = 48i22 = 48(1.67)2 = 133.33 W Thus the total power for this heating system is ptotal = pleft + pmiddle + pright = 533.33 + 133.33 + 133.33 = 800 W All radiators in this configuration have much less power than their counterparts in Fig. 2.41. The total power for this configuration is only 22.2% of the total power for the heating system in Fig. 2.41. P 2.44

Each radiator is modeled as a 48 Ω resistor:

Write a KVL equation for this loop: −240 + 48i + 48i + 48i = 0

→

240 = 1.67 A 3(48)

Calculate the power for each radiator: prad = 48i2 = 48(1.67)2 = 133.33 W Calculate the total power for this heating system: ptotal = 3prad = 3(133.33) = 400 W Each radiator has much less power than the radiators in Fig. 2.41, and the total power of this configuration is just 1/9th of the total power in Fig. 2.41.

3 Simple Resistive Circuits Assessment Problems AP 3.1

Start from the right hand side of the circuit and make series and parallel combinations of the resistors until one equivalent resistor remains. Begin by combining the 6 Ω resistor and the 10 Ω resistor in series: 6 Ω + 10 Ω = 16 Ω Now combine this 16 Ω resistor in parallel with the 64 Ω resistor: 16 Ωk64 Ω =

(16)(64) 1024 = = 12.8 Ω 16 + 64 80

This equivalent 12.8 Ω resistor is in series with the 7.2 Ω resistor: 12.8 Ω + 7.2 Ω = 20 Ω Finally, this equivalent 20 Ω resistor is in parallel with the 30 Ω resistor: 20 Ωk30 Ω =

(20)(30) 600 = = 12 Ω 20 + 30 50

Thus, the simplified circuit is as shown:

3–1 © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

3–2

CHAPTER 3. Simple Resistive Circuits [a] With the simplified circuit we can use Ohm’s law to find the voltage across both the current source and the 12 Ω equivalent resistor: v = (12 Ω)(5 A) = 60 V [b] Now that we know the value of the voltage drop across the current source, we can use the formula p = −vi to find the power associated with the source: p = −(60 V)(5 A) = −300 W Thus, the source delivers 300 W of power to the circuit. [c] We now can return to the original circuit, shown in the first figure. In this circuit, v = 60 V, as calculated in part (a). This is also the voltage drop across the 30 Ω resistor, so we can use Ohm’s law to calculate the current through this resistor: 60 V =2A 30 Ω Now write a KCL equation at the upper left node to find the current iB : iA =

−5 A + iA + iB = 0

iB = 5 A − iA = 5 A − 2 A = 3 A

Next, write a KVL equation around the outer loop of the circuit, using Ohm’s law to express the voltage drop across the resistors in terms of the current through the resistors: −v + 7.2iB + 6iC + 10iC = 0 So Thus

16iC = v − 7.2iB = 60 V − (7.2)(3) = 38.4 V iC =

38.4 = 2.4 A 16

Now that we have the current through the 10 Ω resistor we can use the formula p = Ri2 to find the power: p10 Ω = (10)(2.4)2 = 57.6 W AP 3.2

Problems

3–3

[a] We can use voltage division to calculate the voltage vo across the 75 kΩ resistor: 75,000 vo (no load) = (200 V) = 150 V 75,000 + 25,000 [b] When we have a load resistance of 150 kΩ then the voltage vo is across the parallel combination of the 75 kΩ resistor and the 150 kΩ resistor. First, calculate the equivalent resistance of the parallel combination: (75,000)(150,000) = 50,000 Ω = 50 kΩ 75,000 + 150,000 Now use voltage division to find vo across this equivalent resistance: 50,000 vo = (200 V) = 133.3 V 50,000 + 25,000

75 kΩk150 kΩ =

[c] If the load terminals are short-circuited, the 75 kΩ resistor is effectively removed from the circuit, leaving only the voltage source and the 25 kΩ resistor. We can calculate the current in the resistor using Ohm’s law: 200 V i= = 8 mA 25 kΩ Now we can use the formula p = Ri2 to find the power dissipated in the 25 kΩ resistor: p25k = (25,000)(0.008)2 = 1.6 W [d] The power dissipated in the 75 kΩ resistor will be maximum at no load since vo is maximum. In part (a) we determined that the no-load voltage is 150 V, so be can use the formula p = v 2/R to calculate the power: p75k (max) =

(150)2 = 0.3 W 75,000

AP 3.3

[a] We will write a current division equation for the current throught the 80Ω resistor and use this equation to solve for R: i80Ω = Thus

R (20 A) = 4 A R + 40 Ω + 80 Ω 16R = 480

and

20R = 4(R + 120)

480 = 30 Ω 16

3–4

CHAPTER 3. Simple Resistive Circuits [b] With R = 30 Ω we can calculate the current through R using current division, and then use this current to find the power dissipated by R, using the formula p = Ri2 : 40 + 80 iR = so pR = (30)(16)2 = 7680 W (20 A) = 16 A 40 + 80 + 30 [c] Write a KVL equation around the outer loop to solve for the voltage v, and then use the formula p = −vi to calculate the power delivered by the current source: −v + (60 Ω)(20 A) + (30 Ω)(16 A) = 0 Thus,

v = 1200 + 480 = 1680 V

psource = −(1680 V)(20 A) = −33,600 W

Thus, the current source generates 33,600 W of power. AP 3.4

[a] First we need to determine the equivalent resistance to the right of the 40 Ω and 70 Ω resistors: 1 1 1 1 1 Req = 20 Ωk30 Ωk(50 Ω + 10 Ω) so + + = = Req 20 Ω 30 Ω 60 Ω 10 Ω Thus,

Req = 10 Ω

Now we can use voltage division to find the voltage vo : 40 (60 V) = 20 V vo = 40 + 10 + 70 [b] The current through the 40 Ω resistor can be found using Ohm’s law: vo 20 V = = 0.5 A 40 40 Ω This current flows from left to right through the 40 Ω resistor. To use current division, we need to find the equivalent resistance of the two parallel branches containing the 20 Ω resistor and the 50 Ω and 10 Ω resistors: (20)(60) 20 Ωk(50 Ω + 10 Ω) = = 15 Ω 20 + 60 Now we use current division to find the current in the 30 Ω branch: 15 i30Ω = (0.5 A) = 0.16667 A = 166.67 mA 15 + 30 i40Ω =

Problems

3–5

[c] We can find the power dissipated by the 50 Ω resistor if we can find the current in this resistor. We can use current division to find this current from the current in the 40 Ω resistor, but first we need to calculate the equivalent resistance of the 20 Ω branch and the 30 Ω branch: (20)(30) = 12 Ω 20 + 30 Current division gives:

20 Ωk30 Ω =

i50Ω = Thus,

12 (0.5 A) = 0.08333 A 12 + 50 + 10 p50Ω = (50)(0.08333)2 = 0.34722 W = 347.22 mW

AP 3.5 [a]

We can find the current i using Ohm’s law: i=

1V = 0.01 A = 10 mA 100 Ω

[b]

Rm = 50 Ωk5.555 Ω = 5 Ω We can use the meter resistance to find the current using Ohm’s law: imeas =

1V = 0.009524 = 9.524 mA 100 Ω + 5 Ω

AP 3.6 [a]

3–6

CHAPTER 3. Simple Resistive Circuits Use voltage division to find the voltage v: v=

75,000 (60 V) = 50 V 75,000 + 15,000

[b]

The meter resistance is a series combination of resistances: Rm = 149,950 + 50 = 150,000 Ω We can use voltage division to find v, but first we must calculate the equivalent resistance of the parallel combination of the 75 kΩ resistor and the voltmeter: (75,000)(150,000) 75,000 Ωk150,000 Ω = = 50 kΩ 75,000 + 150,000 Thus,

vmeas =

50,000 (60 V) = 46.15 V 50,000 + 15,000

AP 3.7 [a] Using the condition for a balanced bridge, the products of the opposite resistors must be equal. Therefore, (1000)(150) = 1500 Ω = 1.5 kΩ 100 [b] When the bridge is balanced, there is no current flowing through the meter, so the meter acts like an open circuit. This places the following branches in parallel: The branch with the voltage source, the branch with the series combination R1 and R3 and the branch with the series combination of R2 and Rx . We can find the current in the latter two branches using Ohm’s law: 100Rx = (1000)(150)

Rx =

5V 5V = 20 mA; iR2 ,Rx = = 2 mA 100 Ω + 150 Ω 1000 + 1500 We can calculate the power dissipated by each resistor using the formula p = Ri2 : iR1 ,R3 =

p100Ω = (100 Ω)(0.02 A)2 = 40 mW p150Ω = (150 Ω)(0.02 A)2 = 60 mW

Problems

3–7

p1000Ω = (1000 Ω)(0.002 A)2 = 4 mW p1500Ω = (1500 Ω)(0.002 A)2 = 6 mW Since none of the power dissipation values exceeds 250 mW, the bridge can be left in the balanced state without exceeding the power-dissipating capacity of the resistors. AP 3.8 Convert the three Y-connected resistors, 20 Ω, 10 Ω, and 5 Ω to three ∆-connected resistors Ra , Rb, and Rc . To assist you the figure below has both the Y-connected resistors and the ∆-connected resistors

(5)(10) + (5)(20) + (10)(20) = 17.5 Ω 20 (5)(10) + (5)(20) + (10)(20) = 35 Ω Rb = 10 (5)(10) + (5)(20) + (10)(20) Rc = = 70 Ω 5 Ra =

The circuit with these new ∆-connected resistors is shown below:

From this circuit we see that the 70 Ω resistor is parallel to the 28 Ω resistor: 70 Ωk28 Ω =

(70)(28) = 20 Ω 70 + 28

Also, the 17.5 Ω resistor is parallel to the 105 Ω resistor: 17.5 Ωk105 Ω =

(17.5)(105) = 15 Ω 17.5 + 105

3–8

CHAPTER 3. Simple Resistive Circuits Once the parallel combinations are made, we can see that the equivalent 20 Ω resistor is in series with the equivalent 15 Ω resistor, giving an equivalent resistance of 20 Ω + 15 Ω = 35 Ω. Finally, this equivalent 35 Ω resistor is in parallel with the other 35 Ω resistor: 35 Ωk35 Ω =

(35)(35) = 17.5 Ω 35 + 35

Thus, the resistance seen by the 2 A source is 17.5 Ω, and the voltage can be calculated using Ohm’s law: v = (17.5 Ω)(2 A) = 35 V

Problems

3–9

Problems P 3.1

[a] From Ex. 3-1: i1 = 4 A, i2 = 8 A, is = 12 A at node b: −12 + 4 + 8 = 0, at node d: 12 − 4 − 8 = 0

[b] v1

= 4is = 48 V

v3 = 3i2 = 24 V

v2 = 18i1 = 72 V v4 = 6i2 = 48 V loop abda: −120 + 48 + 72 = 0, loop bcdb: −72 + 24 + 48 = 0, loop abcda: −120 + 48 + 24 + 48 = 0 P 3.2

[a] p4Ω

i2s 4 = (12)2 4 = 576 W

p18Ω = (4)2 18 = 288 W

p3Ω

(8)2 3 = 192 W

p6Ω = (8)2 6 = 384 W

[b] p120V (delivered) = 120is = 120(12) = 1440 W [c] pdiss = 576 + 288 + 192 + 384 = 1440 W P 3.3

[a] The 5 kΩ and 7 kΩ resistors are in series. The simplified circuit is shown below:

[b] The 800 Ω and 1200 Ω resistors are in series, as are the 300 Ω and 200 Ω resistors. The simplified circuit is shown below:

3–10

CHAPTER 3. Simple Resistive Circuits

[c] The 35 Ω, 15 Ω, and 25 Ω resistors are in series. as are the 10 Ω and 40 Ω resistors. The simplified circuit is shown below:

[d] The 50 Ω and 90 Ω resistors are in series, as are the 80 Ω and 70 Ω resistors. The simplified circuit is shown below:

P 3.4

[a] The 36 Ω and 18 Ω resistors are in parallel. The simplified circuit is shown below:

[b] The 200 Ω and 120 Ω resistors are in parallel, as are the 210 Ω and 280 Ω resistors. The simplified circuit is shown below:

[c] The 100 kΩ, 150 kΩ, and 60 kΩ resistors are in parallel, as are the 75 kΩ and 50 kΩ resistors. The simplified circuit is shown below:

Problems

3–11

[d] The 750 Ω and 500 Ω resistors are in parallel, as are the 1.5 kΩ and 3 kΩ resistors. The simplified circuit is shown below:

P 3.5

Always work from the side of the circuit furthest from the source. Remember that the current in all series-connected circuits is the same, and that the voltage drop across all parallel-connected resistors is the same. [a] Circuit in Fig. P3.3(a): Req = [(7000 + 5000)k6000] + 8000 = 12,000k6000 + 8000 = 4000 + 8000 = 12 kΩ Circuit in Fig. P3.3(b): Req = [500k(800 + 1200)] + 300 + 200 = (500k2000) + 300 + 200 = 400 + 300 + 200 = 900 Ω Circuit in Fig. P3.3(c): Req = (35 + 15 + 25)k(10 + 40) = 75k50 = 30 Ω Circuit in Fig. P3.3(d): Req = ([(70 + 80)k100] + 50 + 90)k300 = [(150k100) + 50 + 90]k300 = (60 + 50 + 90)k300 = 200k300 = 120 Ω [b] Note that in every case, the power delivered by the source must equal the power absorbed by the equivalent resistance in the circuit. For the circuit in Fig. P3.3(a): P =

Vs2 182 = = 0.027 = 27 mW Req 12,000

3–12

CHAPTER 3. Simple Resistive Circuits For the circuit in Fig. P3.3(b): P =

Vs2 272 = 0.81 = 810 mW = Req 900

For the circuit in Fig. P3.3(c): P =

Vs2 902 = 270 W = Req 30

For the circuit in Fig. P3.3(d): P = Is2(Req ) = (0.03)2 (120) = 0.108 = 108 mW P 3.6

Always work from the side of the circuit furthest from the source. Remember that the current in all series-connected circuits is the same, and that the voltage drop across all parallel-connected resistors is the same. [a] Circuit in Fig. P3.4(a): Req = (36k18) + 24 = 12 + 24 = 36 Ω Circuit in Fig. P3.4(b): Req = 200k120k[(210k280) + 180] = 200k120k(120 + 180) = 200k120k300 = 60 Ω Circuit in Fig. P3.4(c): Req = (75 kk50 k) + (100 kk150 kk60 k) + 90 k = 30 k + 30 k + 90 k = 150 kΩ Circuit in Fig. P3.4(d): Req = [(600 + 900)k750k500] + (1500k3000) + 2000 = (1500k750k500) + 1000 + 2000 = 250 + 1000 + 2000 = 3250 = 3.25 kΩ [b] Note that in every case, the power delivered by the source must equal the power absorbed by the equivalent resistance in the circuit. For the circuit in Fig. P3.4(a): P =

182 Vs2 = =9W Req 36

For the circuit in Fig. P3.4(b): P = Is2(Req ) = (0.03)2 (60) = 0.054 = 54 mW For the circuit in Fig. P3.4(c): Vs2 602 P = = = 0.024 = 24 mW Req 150,000 For the circuit in Fig. P3.4(d): P =

Vs2 652 = = 1.3 W Req 3250

Problems P 3.7

3–13

[a] Circuit in Fig. P3.7(a): Req = ([(15k60) + (30k45) + 20]k50) + 25 + 10 = [(12 + 18 + 20)k50] + 25 + 10 = (50k50) + 25 + 10 = 25 + 25 + 10 = 60 Ω Circuit in Fig. P3.7(b) – begin by simplifying the 75 Ω resistor and all resistors to its right: [(18 + 12)k60 + 30]k75 = (30k60 + 30)k75 = (20 + 30)k75 = 50k75 = 30 Ω Now simplify the remainder of the circuit: Req = ([(30 + 20)k50] + (20k60))k40 = [(50k50) + 15]k40 = (25 + 15)k40 = 40k40 = 20 Ω Circuit in Fig. P3.7(c) – begin by simplifying the left and right sides of the circuit: Rleft = [(1800 + 1200)k2000] + 300 = (3000k2000) + 300 = 1200 + 300 = 1500 Ω Rright = [(500 + 2500)k1000] + 750 = (3000k1000) + 750 = 750 + 750 = 1500 Ω Now find the equivalent resistance seen by the source: Req = (Rleft kRright) + 250 + 3000 = (1500k1500) + 250 + 3000 = 750 + 250 + 3000 = 4000 = 4 kΩ Circuit in Fig. P3.7(d): Req = ([(750 + 250)k1000] + 100)k([(150 + 600)k500] + 300) = [(1000k1000) + 100]k[(750k500) + 300] = (500 + 100)k(300 + 300) = 600k600 = 300 Ω [b] Note that in every case, the power delivered by the source must equal the power absorbed by the equivalent resistance in the circuit. For the circuit in Fig. P3.7(a): P =

Vs2 302 = = 15 W Req 60

For the circuit in Fig. P3.7(b): P = Is2(Req ) = (0.08)2 (20) = 0.128 = 128 mW For the circuit in Fig. P3.7(c): P =

Vs2 202 = = 0.1 = 100 mW Req 4000

For the circuit in Fig. P3.7(d): P = Is2(Req ) = (0.05)2 (300) = 0.75 = 750 mW

3–14 P 3.8

CHAPTER 3. Simple Resistive Circuits [a] Rab = 24 + (90k60) + 12 = 24 + 36 + 12 = 72 Ω [b] Rab = [(4 k + 6 k + 2 k)k8 k] + 5.2 k = (12 kk8 k) + 5.2 k = 4.8 k + 5.2 k = 10 kΩ [c] Rab = 1200k720k(320 + 480) = 1200k720k800 = 288 Ω

P 3.9

Write an expression for the resistors in series and parallel from the right side of the circuit to the left. Then simplify the resulting expression from left to right to find the equivalent resistance. [a] Rab = [(26 + 10)k18 + 6]k36 = (36k18 + 6)k36 = (12 + 6)k36 = 18k36 = 12 Ω [b] Rab = [(12 + 18)k10k15k20 + 16]k30 + 4 + 14 = (30k10k15k20 + 16)k30 + 4 + 14 = (4 + 16)k30 + 4 + 14 = 20k30 + 4 + 14 = 12 + 4 + 14 = 30 Ω [c] Rab = (500k1500k750 + 250)k2000 + 1000 = (250 + 250)k2000 + 1000 = 500k2000 + 1000 = 400 + 1000 = 1400 Ω [d] Note that the wire on the far right of the circuit effectively removes the 60 Ω resistor! Rab = [([(30 + 18)k16 + 28]k40 + 20)k24 + 25 + 10]k50 = ([(48k16 + 28)k40 + 20]k24 + 25 + 10)k50 = ([(12 + 28)k40 + 20]k24 + 25 + 10)k50 = [(40k40 + 20)k24 + 25 + 10]k50 = [(20 + 20)k24 + 25 + 10]k50 = (40k24 + 25 + 10)k50 = (15 + 25 + 10)k50 = 50k50 = 25 Ω

P 3.10

[a] R + R = 2R [b] R + R + R + · · · + R = nR [c] R + R = 2R = 3000 so R = 1500 = 1.5 kΩ This is a resistor from Appendix H. [d] nR = 4000; so if n = 4, R = 1 kΩ This is a resistor from Appendix H.

P 3.11

[a] Req = RkR = [b] Req

= = =

[c]

R2 R = 2R 2

RkRkRk · · · kR (n R’s) R Rk n−1 R2 /(n − 1) R2 R = = R + R/(n − 1) nR n

R = 5000 so R = 10 kΩ 2 This is a resistor from Appendix H.

Problems [d]

P 3.12

3–15

R = 4000 so R = 4000n n If n = 3 r = 4000(3) = 12 kΩ This is a resistor from Appendix H. So put three 12k resistors in parallel to get 4kΩ.

160(3300) = 66 V (4700 + 3300) [b] i = 160/8000 = 20 mA

[a] vo =

PR1 = (400 × 10−6 )(4.7 × 103 ) = 1.88 W PR2 = (400 × 10−6 )(3.3 × 103 ) = 1.32 W [c] Since R1 and R2 carry the same current and R1 > R2 to satisfy the voltage requirement, first pick R1 to meet the 0.5 W specification iR1 =

160 − 66 , R1

Thus, R1 ≥

942 0.5

Therefore, or

94 R1

R1 ≤ 0.5

R1 ≥ 17,672 Ω

Now use the voltage specification: R2 (160) = 66 R2 + 17,672 Thus, R2 = 12,408 Ω P 3.13

P 3.14

20R2 R2 + 40

R2 = 10 Ω

20Re 40 + Re

Re =

Thus,

120 10RL = 17 10 + RL

[a] vo =

40R2 =8 R1 + R2

Let Re = R2 kRL = vo =

120 Ω 17 so

R1 = 4R2

R2 RL R2 + RL

40Re = 7.5 R1 + Re

Then, 4R2 = 4.33Re = Thus, R2 = 300 Ω

RL = 24 Ω

R1 = 4.33Re

4.33(3600R2 ) 3600 + R2

and

R1 = 4(300) = 1200 Ω

3–16

CHAPTER 3. Simple Resistive Circuits [b] The resistor that must dissipate the most power is R1 , as it has the largest resistance and carries the same current as the parallel combination of R2 and the load resistor. The power dissipated in R1 will be maximum when the voltage across R1 is maximum. This will occur when the voltage divider has a resistive load. Thus, vR1 = 40 − 7.5 = 32.5 V pR1 =

32.52 = 880.2 m W 1200

Thus the minimum power rating for all resistors should be 1 W. P 3.15

Refer to the solution to Problem 3.16. The voltage divider will reach the maximum power it can safely dissipate when the power dissipated in R1 equals 1 W. Thus, vR2 1 =1 1200

vR1 = 34.64 V

vo = 40 − 34.64 = 5.36 V So,

40Re = 5.36 1200 + Re

Thus,

and

(300)RL = 185.68 300 + RL

Re = 185.68 Ω

and

RL = 487.26 Ω

The minimum value for RL from Appendix H is 560 Ω. P 3.16

Req = 10k[6 + 5k(8 + 12)] = 10k(6 + 5k20) = 10k(6 + 4) = 5 Ω v10A = v10Ω = (10 A)(5 Ω) = 50 V Using voltage division: v5Ω =

5k(8 + 12) 4 (50) = (50) = 20 V 6 + 5k(8 + 12) 6+4

Thus, p5Ω =

2 v5Ω 202 = = 80 W 5 5

Problems P 3.17

3–17

[a]

Req = (10 + 20)k[12 + (90k10)] = 30k15 = 10 Ω v2.4A = 10(2.4) = 24 V vo = v20Ω = v90Ω = io =

20 (24) = 16 V 10 + 20

9 90k10 (24) = (24) = 14.4 V 6 + (90k10) 15

14.4 = 0.16 A 90

(v2.4A − v90Ω)2 (24 − 14.4)2 = = 15.36 W 6 6 [c] p2.4A = −(2.4)(24) = −57.6 W Thus the power developed by the current source is 57.6 W.

[b] p6Ω =

P 3.18

Begin by using KCL at the top node to relate the branch currents to the current supplied by the source. Then use the relationships among the branch currents to express every term in the KCL equation using just i2: 0.05 = i1 + i2 + i3 + i4 = 0.6i2 + i2 + 2i2 + 4i1 = 0.6i2 + i2 + 2i2 + 4(0.6i2 ) = 6i2 Therefore, i2 = 0.05/6 = 0.00833 = 8.33 mA Find the remaining currents using the value of i2 : i1 = 0.6i2 = 0.6(0.00833) = 0.005 = 5 mA i3 = 2i2 = 2(0.00833) = 0.01667 = 16.67 mA i4 = 4i1 = 4(0.005) = 0.02 = 20 mA Since the resistors are in parallel, the same voltage, 25 V, appears across each of them. We know the current and the voltage for every resistor so we can use Ohm’s law to calculate the values of the resistors: R1 = 25/i1 = 25/0.005 = 5000 = 5 kΩ

3–18

CHAPTER 3. Simple Resistive Circuits R2 = 25/i2 = 25/0.00833 = 3000 = 3 kΩ R3 = 25/i3 = 25/0.01667 = 1500 = 1.5 kΩ R4 = 25/i4 = 25/0.02 = 1250 = 1.25 kΩ The resulting circuit is shown below:

P 3.19

(24)2 = 80, R1 + R2 + R3

Therefore, R1 + R2 + R3 = 7.2 Ω

(R1 + R2 )24 = 12 (R1 + R2 + R3 ) Therefore, 2(R1 + R2) = R1 + R2 + R3 Thus, R1 + R2 = R3 ;

2R3 = 7.2;

R3 = 3.6 Ω

R2 (24) =5 R1 + R2 + R3 4.8R2 = R1 + R2 + 3.6 = 7.2 Thus, R2 = 1.5 Ω; P 3.20

R1 = 7.2 − R2 − R3 = 2.1 Ω

[a]

20 kΩ + 40 kΩ = 60 kΩ 30 kΩk60 kΩ = 20 kΩ vo1 =

20,000 (180) = 120 V (10,000 + 20,000)

vo =

40,000 (vo1) = 80 V 60,000

Problems

3–19

[b]

180 = 4.5 mA 40,000

30,000i = 135 V vo =

40,000 (135) = 90 V 60,000

[c] It removes the loading effect of the second voltage divider on the first voltage divider. Observe that the open circuit voltage of the first divider is 30,000 0 vo1 = (180) = 135 V 40,000 Now note this is the input voltage to the second voltage divider when the current-controlled voltage source is used. P 3.21

[a] At no load:

vo = kvs =

R2 vs . R1 + R2

At full load:

vo = αvs =

Therefore k

1−α α

Thus

R2 R1 + R2 Re R1 + Re

R1 =

[b] R1

and and

where Re =

Ro R2 Ro + R2

(1 − k) R2 k (1 − α) Re R1 = α

R1 =

R2 Ro (1 − k) = R2 Ro + R2 k

Solving for R2 yields Also,

Re vs , R1 + Re

R2 =

(1 − k) R2 k

(k − α) Ro α(1 − k)

.·.

R1 =

(k − α) Ro αk

0.05 Ro = 2.5 kΩ 0.68 0.05 Ro = 14.167 kΩ 0.12

3–20

CHAPTER 3. Simple Resistive Circuits [c ]

Maximum dissipation in R2 occurs at no load, therefore, PR2 (max) =

[(60)(0.85)]2 = 183.6 mW 14,167

Maximum dissipation in R1 occurs at full load. PR1 (max) =

[60 − 0.80(60)]2 = 57.60 mW 2500

[d ]

P 3.22

PR1

PR2

(60)2 = 1.44 W = 1440 mW 2500 (0)2 =0W 14,167

[a] Let vo be the voltage across the parallel branches, positive at the upper terminal, then ig = voG1 + voG2 + · · · + voGN = vo (G1 + G2 + · · · + GN ) It follows that

vo =

ig (G1 + G2 + · · · + GN )

The current in the k th branch is ik = [b] i5 = P 3.23

ik = vo Gk ;

Thus,

ig Gk [G1 + G2 + · · · + GN ]

40(0.2) = 3.2 A 2 + 0.2 + 0.125 + 0.1 + 0.05 + 0.025

[a] The equivalent resistance of the 6 kΩ resistor and the resistors to its right is 6 kk(5 k + 7 k) = 6 kk12 k = 4 kΩ

Problems

3–21

Using voltage division, 4000 (18) = 6 V 8000 + 4000 5000 [b] v5k = (6) = 2.5 V 5000 + 7000 v6k =

P 3.24

[a] The equivalent resistance of the 100 Ω resistor and the resistors to its right is 100k(80 + 70) = 100k150 = 60 Ω Using current division, (50 + 90 + 60)k300 120 (0.03) = (0.03) = 0.018 = 18 mA 50 + 90 + 60 200

i50 = [b] v70 = P 3.25

60 (80 + 70)k100 (0.018) = (0.018) = 0.0072 = 7.2 mA 80 + 70 150

[a] The equivalent resistance of the circuit to the right of, and including, the 50 Ω resistor is [(60k15) + (45k30) + 20]k50 = 25 Ω Thus by voltage division, v25 =

25 (30) = 12.5 V 25 + 25 + 10

[b] The current in the 25 Ω resistor can be found from its voltage using Ohm’s law: 12.5 i25 = = 0.5 A 25 [c] The current in the 25 Ω resistor divides between two branches – one containing 50 Ω and one containing (45k30) + (15k60) + 20 = 50 Ω. Using current division, 50k50 25 (i25) = (0.5) = 0.25 A 50 50 [d] The voltage drop across the 50 Ω resistor can be found using Ohm’s law: i50 =

v50 = 50i50 = 50(0.25) = 12.5 V [e] The voltage v50 divides across the equivalent resistance (45k30) Ω, the equivalent resistance (15k60) Ω, and the 20 Ω resistor. Using voltage division, v60 = v15k60 =

12 15k60 (12.5) = (12.5) = 3 V (15k60) + (30k45) + 20 12 + 18 + 20

3–22 P 3.26

CHAPTER 3. Simple Resistive Circuits [a] The equivalent resistance to the right of the 36 Ω resistor is 6 + [18k(26 + 10)] = 18 Ω By current division, 36k18 (0.45) = 0.15 = 150 mA 36 [b] Using Ohm’s law, i36 =

v36 = 36i36 = 36(0.15) = 5.4 V [c] Before using voltage division, find the equivalent resistance of the 18 Ω resistor and the resistors to its right: 18k(26 + 10) = 12 Ω Now use voltage division: 12 (5.4) = 3.6 V 12 + 6 10 [d] v10 = (3.6) = 1 V 10 + 26 v18 =

P 3.27

[a] Begin by finding the equivalent resistance of the 30 Ω resistor and all resistors to its right: ([(12 + 18)k10k15k20] + 16)k30 = 12 Ω Now use voltage division to find the voltage across the 4 Ω resistor: v4 =

4 (6) = 0.8 V 4 + 12 + 14

[b] Use Ohm’s law to find the current in the 4 Ω resistor: i4 = v4 /4 = 0.8/4 = 0.2 A [c] Begin by finding the equivalent resistance of all resistors to the right of the 30 Ω resistor: [(12 + 18)k10k15k20] + 16 = 20 Ω Now use current division: 30k20 i16 = (0.2) = 0.12 = 120 mA 20 [d] Note that the current in the 16 Ω resistor divides among four branches – 20 Ω, 15 Ω, 10 Ω, and (12 + 18) Ω: 20k15k10k(12 + 18) (0.12) = 0.048 = 48 mA 10 [e] Use Ohm’s law to find the voltage across the 10 Ω resistor: i10 =

v10 = 10i10 = 10(0.048) = 0.48 V

Problems

P 3.28

[f] v18 =

18 (0.48) = 0.288 = 288 mV 12 + 18

[a] v6k =

6 (18) = 13.5 V 6+2

v3k =

3–23

3 (18) = 4.5 V 3+9

vx = v6k − v3k = 13.5 − 4.5 = 9 V 6 [b] v6k = (Vs ) = 0.75Vs 8 v3k =

3 (Vs ) = 0.25Vs 12

vx = (0.75Vs ) − (0.25Vs ) = 0.5Vs P 3.29

Use current division to find the current in the branch containing the 10 k and 15 k resistors, from bottom to top i10k+15k =

(10 k + 15 k)k(3 k + 12 k) (18) = 6.75 mA 10 k + 15 k

Use Ohm’s law to find the voltage drop across the 15 k resistor, positive at the top: v15k = −(6.75 m)(15 k) = −101.25 V Find the current in the branch containing the 3 k and 12 k resistors, from bottom to top i10k+15k =

(10 k + 15 k)k(3 k + 12 k) (18) = 11.25 mA 3 k + 12 k

Use Ohm’s law to find the voltage drop across the 12 k resistor, positive at the top: v12k = −(12 k)(11.25 m) = −135 V vo = v15k − v12k = −101.25 − (−135) = 33.75 V P 3.30

The equivalent resistance of the circuit to the right of the 90 Ω resistor is Req = [(150k75) + 40]k(30 + 60) = 90k90 = 45 Ω

3–24

CHAPTER 3. Simple Resistive Circuits Use voltage division to find the voltage drop between the top and bottom nodes: vReq =

45 (3) = 1 V 45 + 90

Use voltage division again to find v1 from vReq: 150k75 50 5 (1) = (1) = V 150k75 + 40 90 9

v1 =

Use voltage division one more time to find v2 from vReq: 30 1 (1) = V 30 + 60 3

v2 = P 3.31

Find the equivalent resistance of all the resistors except the 2 Ω: 5 Ωk20 Ω = 4 Ω;

4 Ω + 6 Ω = 10 Ω;

10k(15 + 12 + 13) = 8 Ω = Req

Use Ohm’s law to find the current ig : ig =

125 125 = 12.5 A = 2 + Req 2+8

Use current division to find the current in the 6 Ω resistor: i6Ω =

8 (12.5) = 10 A 6+4

Use current division again to find io : io = P 3.32

5k20 5k20 i6Ω = (10) = 2 A 20 20

Use current division to find the current in the 8 Ω resistor. Begin by finding the equivalent resistance of the 8 Ω resistor and all resistors to its right: Req = ([(20k80) + 4]k30) + 8 = 20 Ω i8 =

60kReq 60k20 (0.25) = (0.25) = 0.1875 = 187.5 mA Req 20

Use current division to find i1 from i8 : i1 =

30k[4 + (80k20)] 30k20 (i8) = (0.1875) = 0.075 = 75 mA 30 30

Problems

3–25

Use current division to find i4Ω from i8: i4Ω =

30k[4 + (80k20)] 30k20 (0.1875) = 0.1125 = 112.5 mA (i8) = 4 + (80k20) 20

Finally, use current division to find i2 from i4Ω : i2 = P 3.33

80k20 80k20 (i4Ω ) = (0.1125) = 0.09 = 90 mA 20 20

The current in the shunt resistor at full-scale deflection is iA = ifullscale − 3 × 10−3 A. The voltage across RA at full-scale deflection is always 150 mV; therefore, RA =

150 × 10−3 150 = ifullscale − 3 × 10−3 1000ifullscale − 3

150 = 30.018 mΩ 5000 − 3 [b] Let Rm be the equivalent ammeter resistance:

[a] RA =

0.15 = 0.03 = 30 mΩ 5 150 [c] RA = = 1.546 Ω 100 − 3 0.15 [d] Rm = = 1.5 Ω 0.1 Rm =

P 3.34

Original meter:

Re =

50 × 10−3 = 0.01 Ω 5

Modified meter:

Re =

(0.02)(0.01) = 0.00667 Ω 0.03

.·. (Ifs )(0.00667) = 50 × 10−3 .·. Ifs = 7.5 A

3–26 P 3.35

CHAPTER 3. Simple Resistive Circuits At full scale the voltage across the shunt resistor will be 200 mV; therefore the power dissipated will be PA =

(200 × 10−3 )2 RA

Therefore RA ≥

(200 × 10−3 )2 = 40 mΩ 1.0

Otherwise the power dissipated in RA will exceed its power rating of 1 W When RA = 40 mΩ, the shunt current will be iA =

200 × 10−3 =5A 40 × 10−3

The measured current will be imeas = 5 + 0.002 = 5.002 A .·. Full-scale reading for practical purposes is 5 A. P 3.36

[a] The model of the ammeter is an ideal ammeter in parallel with a resistor whose resistance is given by 100 mV = 50 Ω. 2 mA We can calculate the current through the real meter using current division: (25/12) 25 1 im = (imeas ) = (imeas ) = imeas 50 + (25/12) 625 25

Rm =

[b] At full scale, imeas = 5 A and im = 2 mA so 5 − 0.002 = 4998 mA flows throught the resistor RA : RA =

100 mV 100 = Ω 4998 mA 4998

im =

(100/4998) 1 (imeas) = (imeas) 50 + (100/4998) 2500

[c] Yes P 3.37

For all full-scale readings the total resistance is RV + Rmovement =

full-scale reading 10−3

We can calculate the resistance of the movement as follows: Rmovement = Therefore,

20 mV = 20 Ω 1 mA RV = 1000 (full-scale reading) − 20

Problems

3–27

[a] RV = 1000(50) − 20 = 49, 980 Ω [b] RV = 1000(5) − 20 = 4980 Ω [c] RV = 1000(0.25) − 20 = 230 Ω [d] RV = 1000(0.025) − 20 = 5 Ω P 3.38

[a] vmeas = (50 × 10−3 )[15k45k(4980 + 20)] = 0.5612 V [b] vtrue = (50 × 10−3 )(15k45) = 0.5625 V 0.5612 % error = − 1 × 100 = −0.224% 0.5625

P 3.39

The measured value is ig =

50 = 1.995526 A; (15.05618 + 10)

The true value is ig =

imeas =

60 (1.996) = 1.494768 A 80.1

60k20 = 15 Ω.

50 = 2 A; (15 + 10)

%error = P 3.40

60k20.1 = 15.05618 Ω.

itrue =

60 (2) = 1.5 A 80

1.494768 − 1 × 100 = −0.34878% ≈ −0.35% 1.5

Begin by using current division to find the actual value of the current io : itrue =

15 (50 mA) = 12.5 mA 15 + 45

imeas =

15 (50 mA) = 12.4792 mA 15 + 45 + 0.1

12.4792 − 1 100 = −0.166389% ≈ −0.17% % error = 12.5

P 3.41

[a ]

20 × 103 i1 + 80 × 103 (i1 − iB ) = 7.5

3–28

CHAPTER 3. Simple Resistive Circuits 80 × 103 (i1 − iB) = 0.6 + 40iB (0.2 × 103 ) . ·.

100i1 − 80iB = 7.5 × 10−3

80i1 − 88iB = 0.6 × 10−3 Calculator solution yields iB = 225 µA [b] With the insertion of the ammeter the equations become 100i1 − 80iB = 7.5 × 10−3

(no change)

80 × 103 (i1 − iB) = 103 iB + 0.6 + 40iB (200) 80i1 − 89iB = 0.6 × 10−3 Calculator solution yields iB = 216 µA 216 [c] % error = − 1 100 = −4% 225 P 3.42

[a] Since the unknown voltage is greater than either voltmeter’s maximum reading, the only possible way to use the voltmeters would be to connect them in series. [b ]

Rm1 = (300)(900) = 270 kΩ;

Rm2 = (150)(1200) = 180 kΩ

.·. Rm1 + Rm2 = 450 kΩ i1 max =

300 × 10−3 = 1.11 mA; 270

i2 max =

150 × 10−3 = 0.833 mA 180

.·. imax = 0.833 mA since meters are in series vmax = (0.833 × 10−3 )(270 + 180)103 = 375 V Thus the meters can be used to measure the voltage. 320 [c] im = = 0.711 mA 450 × 103 vm1 = (0.711)(270) = 192 V;

vm2 = (0.711)(180) = 128 V

4 Techniques of Circuit Analysis Assessment Problems AP 4.1 [a] Redraw the circuit, labeling the reference node and the two node voltages:

The two node voltage equations are v1 v1 v1 − v2 −15 + + + = 0 60 15 5 v2 v2 − v1 + = 0 5+ 2 5 Place in standard these equations form: 1 1 1 1 v1 + + + v2 − = 15 60 15 5 5 1 1 1 v1 − + v2 + = −5 5 2 5 Solving, v1 = 60 V and v2 = 10 V; Therefore, i1 = (v1 − v2)/5 = 10 A [b] p15A = −(15 A)v1 = −(15 A)(60 V) = −900 W = 900 W(delivered) [c] p5A = (5 A)v2 = (5 A)(10 V) = 50 W= −50 W(delivered)

4–1 © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.

4–2

CHAPTER 4. Techniques of Circuit Analysis

AP 4.2 Redraw the circuit, choosing the node voltages and reference node as shown:

The two node voltage equations are: v1 v1 − v2 + = 0 −4.5 + 1 6+2 v2 v2 − v1 v2 − 30 + + = 0 12 6+2 4 Place form: these equations in standard 1 1 + v2 − = 4.5 v1 1 + 8 8 1 1 1 1 v1 − + v2 + + = 7.5 8 12 8 4 Solving, v1 = 6 V v2 = 18 V To find the voltage v, first find the current i through the series-connected 6 Ω and 2 Ω resistors: i=

v1 − v2 6 − 18 = = −1.5 A 6+2 8

Using a KVL equation, calculate v: v = 2i + v2 = 2(−1.5) + 18 = 15 V AP 4.3 [a] Redraw the circuit, choosing the node voltages and reference node as shown:

The node voltage equations are: v1 − 50 v1 v1 − v2 + + − 3i1 = 6 8 2 v2 v2 − v1 −5 + + + 3i1 = 4 2

0 0