Chapter 7, Solution 1.
(a)
=RC = 1/200
For the resistor, V=iR= 56e 200 t  8Re 200 t x10 3
C
1
1

 0.7143  F
200 R 200 X7 X10 3


R
56
 7 k
8
(b)
 =1/200= 5 ms
(c) If value of the voltage at = 0 is 56 .
1
x56  56e 200 t
2


200to  ln 2
e 200 t  2


to 
1
ln 2  3.466 ms
200
Chapter 7, Solution 2.
  R th C
where R th is the Thevenin equivalent at the capacitor terminals.
R th  120 || 80  12  60 
  60  200  10 -3  12 s.
Chapter 7, Solution 3.
R = 10 +20//(20+30) =10 + 40x50/(40 + 50)=32.22 k
  RC  3 2 .2 2 X1 0 3 X1 0 0 X1 0 1 2  3 .2 2 2  S
Chapter 7, Solution 4.
For t<0, v(0-)=40 V.
For t >0. we have a source-free RC circuit.
  RC  2 x10 3 x10 x10 6  0.02
v(t)  v(0)e t /   40e 50 t V
Chapter 7, Solution 5.
Using Fig. 7.85, design a problem to help other students to better understand source-free
RC circuits.
Although there are many ways to work this problem, this is an example based on the
same kind of problem asked in the third edition.
Problem
For the circuit shown in Fig. 7.85, find i(t), t>0.
2
t=0
i
5
+
24 V 4 
_
4Ω
1/3 F
Figure 7.85
For Prob. 7.5.
Solution
Let v be the voltage across the capacitor.
For t <0,
4
v(0  ) 
(24)  16 V
24
For t >0, we have a source-free RC circuit as shown below.
i
5
+
v
–
  RC  (4  5)  3 s
1
3
1/3 F
4
v(t)  v(0)e t /   16e  t / 3 V
dv
1 1
i(t)  C
  ( )16e  t / 3  1.778e t / 3 A
dt
3 3
Chapter 7, Solution 6.
v o  v ( 0) 
2
(40)  6.667 V
10  2
v( t )  v o e  t /  ,   RC  40x10 6 x 2x10 3 
v( t )  6.667e 12.5 t V
2
25
Chapter 7, Solution 7.
Assuming that the switch in Fig. 7.87 has been in position A for a long time and is
moved to position B at t=0. Then at t = 1second, the switch moves from B to C.
Find v C (t) for t  0.
10 k
12V
+

A
B
500 
C
1 k
2 mF
Figure 7.87
For Prob. 7.7
Solution
Step 1.
Determine the initial voltage on the capacitor. Clearly it charges to
12 volts when the switch is at position A because the circuit has reached steady state.
This then leaves us with two simple circuits, the first a 500 Ω resistor in series with a
2 mF capacitor and an initial charge on the capacitor of 12 volts. The second circuit
which exists from t = 1 sec to infinity. The initial condition for the second circuit
will be v C (1) from the first circuit. The time constant for the first circuit is
(500)(0.002) = 1 sec and the time constant for the second circuit is (1,000)(0.002) = 2
sec. v C (∞) = 0 for both circuits.
Step 1.
v C (t) = 12e-t volts for 0 < t < 1 sec and = 12e-1e-2(t-1) at t = 1 sec, and
= 4.415e-2(t-1) volts for 1 sec < t < ∞.
12e-t volts for 0 < t < 1 sec, 4.415e-2(t-1) volts for 1 sec < t < ∞.
Chapter 7, Solution 8.
(a)
  RC 
-i  C
(b)
(c)
(d)
1
4
dv
dt
-4t
- 0.2 e  C (10)(-4) e-4t 
 C  5 mF
1
R
 50 
4C
1
  RC   0.25 s
4
1
1
w C (0)  CV02  (5  10 -3 )(100)  250 mJ
2
2
1 1
1
w R   CV02  CV02 1  e -2t 0  
2 2
2
1
 e -8t 0 
0.5  1  e -8t 0 
2
8t 0
e 2
or
1
t 0  ln (2)  86.6 ms
8
Chapter 7, Solution 9.
For t < 0, the switch is closed so that
4
vo (0) 
(6)  4 V
24
For t >0, we have a source-free RC circuit.
  RC  3 x10 3 x4 x10 3  12 s
v o (t) = v o (0)e–t/τ = 4e–t/12 V.
Chapter 7, Solution 10.
3
(36V )  9 V
39
For t>0, we have a source-free RC circuit
  RC  3 x10 3 x20 x10 6  0.06 s
For t<0,
v(0  ) 
v o (t) = 9e–16.667t V
Let the time be t o .
3 = 9e–16.667to or e16.667to = 9/3 = 3
t o = ln(3)/16.667 = 65.92 ms.
Chapter 7, Solution 11.
For t<0, we have the circuit shown below.
4
4H
4
24 V
8
+
_
4H
io
6A
4
4
4||4= 4x4/8=2
i o (0–) = [2/(2+8)]6 = 1.2 A
For t >0, we have a source-free RL circuit.
4
L
 
 1/ 3 thus,
R 48
i o (t) = 1.2e–3t A.
8
Chapter 7, Solution 12.
Using Fig. 7.92, design a problem to help other students better understand source-free RL
circuits.
Although there are many ways to work this problem, this is an example based on the same kind
of problem asked in the third edition.
Problem
The switch in the circuit in Fig. 7.90 has been closed for a long time. At t = 0, the switch is
opened. Calculate i(t) for t > 0.
Figure 7.90
Solution
When t < 0, the switch is closed and the inductor acts like a short circuit to dc. The 4  resistor
is short-circuited so that the resulting circuit is as shown in Fig. (a).
3
12 V
i(0-)
+

4
(a)
i (0  ) 
12
4A
3
Since the current through an inductor cannot change abruptly,
i ( 0)  i ( 0  )  i ( 0  )  4 A
2H
(b)
When t > 0, the voltage source is cut off and we have the RL circuit in Fig. (b).
L 2
    0 .5
R 4
Hence,
i( t )  i(0) e - t   4 e -2t A
Chapter 7, Solution 13.
(a)  
1
 1ms
103
= 1 ms.
v(t) = i(t)R = 80e–1000t V = R5e–1000tx10–3 or R = 80,000/5 = 16 kΩ.
But τ = L/R = 1/103 or L = 16x103/103 = 16 H.
(b) The energy dissipated in the resistor is
= 200(1–e–1)x10–6 = 126.42 µJ.
(a) 16 kΩ, 16 H, 1 ms
(b) 126.42 µJ
Chapter 7, Solution 14.
RTh
 (40  20)/ / (10  30) 
60 x 40
 24k
100
5 x103
  L/ R 
 0.2083  s
24 x103
Chapter 7, Solution 15
(a) R Th  2  10 // 40  10,
(b) RTh  40 // 160  48  40,

L
 5 / 10  0.5s
R Th
L

 (20 x10 3 ) / 80  0.25 ms
RTh
(a) 10 Ω, 500 ms
(b) 40 Ω, 250 µs
Chapter 7, Solution 16.

(a)
L eq
R eq
L eq  L and R eq  R 2 

(b)
R 1R 3
R 2 (R 1  R 3 )  R 1 R 3

R1  R 3
R1  R 3
L( R 1  R 3 )
R 2 (R 1  R 3 )  R 1 R 3
R 3 (R 1  R 2 )  R 1 R 2
L1 L 2
R 1R 2
and R eq  R 3 

L1  L 2
R1  R 2
R1  R 2
L1L 2 (R 1  R 2 )

(L 1  L 2 ) ( R 3 ( R 1  R 2 )  R 1 R 2 )
where L eq 
Chapter 7, Solution 17.
i( t )  i(0) e - t  ,

14 1
L


R eq
4 16
i (t )  6 e -16t
vo (t )  3i  L
di
 18 e -16t  (1 4)(-16) 6 e -16t
dt
v o ( t )  –6e–16tu(t) V
Chapter 7, Solution 18.
If v( t )  0 , the circuit can be redrawn as shown below.
+
0.4 H
R eq
i(t)
R eq  2 || 3 
6
L 2 5 1
,
   
5
R 5 6 3
-t 
-3t
i (t )  i (0) e  5e
di - 2
vo (t )  -L  (-3)5 e -3t  6 e -3t V
dt 5
v o (t)

Chapter 7, Solution 19.
i
10 
1V
 +
i1
i1
i2
i/2
i2
40 
To find R th we replace the inductor by a 1-V voltage source as shown above.
10 i1  1  40 i 2  0
i  i2  i 2
and
i  i1
But
i.e.
i1  2 i 2  i
1
10 i  1  20 i  0 
 i 
30
1
R th   30 
i
L
6


 0.2 s
R th 30
i( t )  6 e -5t u (t ) A
Chapter 7, Solution 20.
(a)
(b)
(c)

L
1


 R  50L
R 50
di
v  L
dt
-50t
90 e   L(30)(-50) e -50t 
 L  60 mH
R  50L  3 Ω
L
1
 
 20 ms
R 50
1
1
w  L i 2 (0)  (0.06)(30) 2  27 J
2
2
The value of the energy remaining at 10 ms is given by:
w 10 = 0.03(30e–0.5)2 = 0.03(18.196)2 = 9.933 J.
So, the fraction of the energy dissipated in the first 10 ms is given by:
(27–9.933)/27 = 0.6321 or 63.21%.
Chapter 7, Solution 21.
The circuit can be replaced by its Thevenin equivalent shown below.
R th
V th
+

2H
Vth 
80
(60)  40 V
80  40
80
R
R th  40 || 80  R 
3
Vth
40
I  i(0)  i() 

R th 80 3  R

 1
3
40
40
1 
 R 
3
R  80 3
1  40
1
w  L I 2  (2)
2  R  80
2
2
R  13.333 Ω
Chapter 7, Solution 22.
i( t )  i(0) e - t  ,

L
R eq
R eq  5 || 20  1  5  ,

2
5
i( t )  10e–2.5t A
Using current division, the current through the 20 ohm resistor is
5
-i
io 
(-i)   -2 e -2.5t
5  20
5
v( t )  20 i o  –40e–2.5t V
Chapter 7, Solution 23.
Since the 2  resistor, 1/3 H inductor, and the (3+1)  resistor are in parallel,
they always have the same voltage.
-i 
10 10

 7 .5 
 i (0)  -7.5
2 3 1
The Thevenin resistance R th at the inductor’s terminals is
13 1
L
4



R th  2 || (3  1)  ,
3
R th 4 3 4
i (t )  i (0) e - t   -7.5 e -4t , t  0
di
v L  vo  L  -7.5(-4)(1/3) e - 4t
dt
-4t
v o  10e V , t  0
1
vx 
v L  2.5 e-4t V , t  0
3 1
Chapter 7, Solution 24.
(a) v( t )  - 5 u(t)
(b) i( t )  -10  u ( t )  u ( t  3)  10 u ( t  3)  u ( t  5)
= - 10 u(t )  20 u(t  3)  10 u(t  5)
(c) x ( t )  ( t  1)  u ( t  1)  u ( t  2)   u ( t  2)  u ( t  3)
 (4  t )  u ( t  3)  u ( t  4)
= ( t  1) u ( t  1)  ( t  2) u ( t  2)  ( t  3) u ( t  3)  ( t  4) u ( t  4)
= r(t  1)  r(t  2)  r(t  3)  r(t  4)
(d) y( t )  2 u (-t )  5  u ( t )  u ( t  1)
= 2 u(-t )  5 u(t )  5 u(t  1)
Chapter 7, Solution 25.
Design a problem to help other students to better understand singularity functions.
Although there are many ways to work this problem, this is an example based on the
same kind of problem asked in the third edition.
Problem
Sketch each of the following waveforms.
(a) i(t) = [u(t–2)+u(t+2)] A
(b) v(t) = [r(t) – r(t–3) + 4u(t–5) – 8u(t–8)] V
Solution
The waveforms are sketched below.
(a)
i(t) (A)
2
1
-2 -1 0
1
2
3
4
t
(b)
v(t) (V)
7
3
00
–1
1
2
3
4
5
6
7
8
t
v1 ( t )  u ( t  1)  u ( t )   u ( t  1)  u ( t )
v1 ( t )  u(t  1)  2 u(t )  u(t  1)
Chapter 7, Solution 26.
(a)
(b)
(c)
(d)
v 2 ( t )  ( 4  t )  u ( t  2)  u ( t  4) 
v 2 ( t )  -( t  4) u ( t  2)  ( t  4) u ( t  4)
v 2 ( t )  2 u(t  2)  r(t  2)  r(t  4)
v 3 ( t )  2  u(t  2)  u(t  4)  4  u(t  4)  u(t  6)
v 3 ( t )  2 u(t  2)  2 u(t  4)  4 u(t  6)
v 4 ( t )  -t  u ( t  1)  u ( t  2)  -t u(t  1)  t u ( t  2)
v 4 ( t )  (-t  1  1) u ( t  1)  ( t  2  2) u ( t  2)
v 4 ( t )  - r(t  1)  u(t  1)  r(t  2)  2 u(t  2)
Chapter 7, Solution 27.
v(t)= [5u(t+1)+10u(t)–25u(t–1)+15u(t-2)] V
Chapter 7, Solution 28.
i(t) is sketched below.
i(t)
1
0
-1
1
2
3
4
t
Chapter 7, Solution 29
x(t)
(a)
3.679
0
(b)
1
t
y(t)
27.18
0
(c)
t
z (t )  cos 4t (t  1)  cos 4 (t  1)  0.6536 (t  1) , which is sketched below.
z(t)
1
0
t
–0.653δ(t–1)
Chapter 7, Solution 30.
(a)
(b)
  4t

- 4t

2
2
( t  1) dt  4t 2 t 1  4
cos(2t ) ( t  0.5) dt  4t 2 cos(2t ) t 0.5  cos   - 1
Chapter 7, Solution 31.
(a)
(b)
 e  112  10
  e (t  2) dt  e
  5 (t )  e (t )  cos 2t (t ) dt   5  e  cos(2t )

-

-
- 4t 2
- 4t 2
-t
t2
-16
-9
-t
t 0
 5 11  7
Chapter 7, Solution 32.
 u ( )d   1d  
t
(a)
t
t
 t 1
t2
(b)  r (t  1)dt   0dt   (t  1)dt   t 14  4.5
2
1
0
0
1
4
 (t  6)
1
1
5
(c )
1
2
1
4
 (t  2)dt  (t  6) 2
t 2
 16
Chapter 7, Solution 33.
i( t ) 
i (t ) 
1 t
 v(t ) dt  i(0)
L 0
t
10 -3
15 (t  2) dt  0
-3 0
10  10
i( t )  1.5 u( t  2) A
Chapter 7, Solution 34.
(a)
(b)
(c)
d
u(t  1) u(t  1)   (t  1)u(t  1) 
dt
u( t  1) ( t  1)   ( t  1)1  0 ( t  1)   ( t  1)
d
r (t  6) u(t  2)  u(t  6)u(t  2) 
dt
r ( t  6) ( t  2)  u( t  6)1  0 ( t  2)  u( t  6)
d
sin 4t u(t  3)  4 cos 4t u(t  3)  sin 4t (t  3)
dt
 4 cos 4t u( t  3)  sin 4 x 3 ( t  3)
 4 cos 4t u( t  3)  0.5366 ( t  3)
Chapter 7, Solution 35.
(a)
v  Ae 2t ,
v(0)  A  1
v(t) = –e–2tu(t) V
(b)
i  Ae 3 t / 2 ,
i(0)  A  2
i(t) = 2e–1.5tu(t) A
Chapter 7, Solution 36.
(a)
(b)
v( t )  A  B e-t , t  0
A  1,
v(0)  0  1  B
v( t ) 
1  e -t V , t  0
v( t )  A  B e t 2 , t  0
A  -3 ,
v(0)  -6  -3  B
v( t )  - 3  1  e
t 2
 V,
t0
or
B  -1
or
B  -3
Chapter 7, Solution 37.
Let v = v h + v p , v p =10.

vh  4 v
1
h
0
v  10  Ae 0.25t
(a)   4 s
v(0)  2  10  A
v  10  8e 0.25t
(b) v ()  10 V


(c ) v  10  8e 0.25 t u( t ) V




v h  Ae t / 4
A  8
Chapter 7, Solution 38.
Let i = i p +i h

i h  3ih  0
Let i p  ku (t ),

ip  0,


3ku (t )  2u (t )
ip 
ih  Ae 3t u (t )


2
u (t )
3
2
i  ( Ae 3t  )u (t )
3
If i(0) =0, then A + 2/3 = 0, i.e. A=-2/3. Thus,
i
2
(1  e  3 t )u( t )
3
k
2
3
Chapter 7, Solution 39.
(a)
Before t = 0,
v( t ) 
1
(20)  4 V
4 1
After t = 0,
v( t )  v()   v(0)  v() e - t 
  RC  (4)(2)  8 , v(0)  4 ,
v( t )  20  (4  20) e -t 8
v()  20
v( t )  20  16 e - t 8 V
Before t = 0, v  v1  v 2 , where v1 is due to the 12-V source and v 2 is
due to the 2-A source.
v1  12 V
To get v 2 , transform the current source as shown in Fig. (a).
v 2  -8 V
Thus,
v  12  8  4 V
After t = 0, the circuit becomes that shown in Fig. (b).
(b)
2F
+
v2

4
2F
+

8V
12 V
+

3
3
(a)
(b)
v( t )  v()   v(0)  v() e - t 
v()  12 ,
v(0)  4 ,
  RC  (2)(3)  6
-t 6
v( t )  12  (4  12) e
v( t )  12  8 e -t 6 V
Chapter 7, Solution 40.
(a)
(b)
Before t = 0, v  12 V .
After t = 0, v( t )  v()   v(0)  v() e - t 
v()  4 ,
v(0)  12 ,
  RC  (2)(3)  6
v( t )  4  (12  4) e - t 6
v( t )  4  8 e - t 6 V
Before t = 0, v  12 V .
After t = 0, v( t )  v()   v(0)  v() e - t 
After transforming the current source, the circuit is shown below.
t=0
2
12 V
v(0)  12 ,
v  12 V
+

v()  12 ,
4
5F
  RC  (2)(5)  10
Chapter 7, Solution 41.
Using Fig. 7.108, design a problem to help other students to better understand the step response
of an RC circuit.
Although there are many ways to work this problem, this is an example based on the same kind
of problem asked in the third edition.
Problem
For the circuit in Fig. 7.108, find v(t) for t > 0.
Figure 7.108
Solution
v(0)  0 ,
v ( ) 
R eq C  (6 || 30)(1) 
30
(12)  10
36
(6)(30)
5
36
v( t )  v()   v(0)  v() e - t 
v( t )  10  (0  10) e - t 5
v( t )  10 (1  e -0.2t ) u ( t )V
Chapter 7, Solution 42.
(a)
v o ( t )  v o ()   v o (0)  v o () e - t 
4
v o () 
(12)  8
v o ( 0)  0 ,
42
4
  R eq C eq , R eq  2 || 4 
3
4
  (3)  4
3
v o ( t )  8  8 e -t 4
v o ( t )  8 ( 1  e -0.25t ) V
(b)
For this case, v o ()  0 so that
v o ( t )  v o ( 0) e - t 
4
(12)  8 ,
v o ( 0) 
42
  RC  (4)(3)  12
v o ( t )  8 e -t 12 V
Chapter 7, Solution 43.
Before t = 0, the circuit has reached steady state so that the capacitor acts like an open
circuit. The circuit is equivalent to that shown in Fig. (a) after transforming the voltage
source.
0.5i
vo
i
40 
2A
80 
0.5i
(a)
0.5i  2 
vo
vo
i
,
40
80
vo
1 vo
320
 2

 v o 
 64
Hence,
2 80
5
40
vo
 0.8 A
i
80
After t = 0, the circuit is as shown in Fig. (b).
0.5i
vC
i
3 mF
80 
0.5i
(b)
v C ( t )  v C (0) e - t  ,
  R th C
To find R th , we replace the capacitor with a 1-V voltage source as shown in Fig. (c).
0.5i
vC
i
1V
+

0.5i
(c)
80 
vC
1
0 .5

,
i o  0 .5 i 
80 80
80
1 80
R th  
 160  ,
  R th C  480
i o 0.5
v C (0)  64 V
i
v C ( t )  64 e - t 480
dv C
 1 
 64 e - t 480
0.5 i  -i C  -C
 -3 
 480 
dt
i( t )  800 e - t 480 u( t ) mA
Chapter 7, Solution 44.
R eq  6 || 3  2  ,
  RC  4
v( t )  v()   v(0)  v() e - t 
Using voltage division,
3
v(0) 
(60)  20 V ,
36
Thus,
v ( ) 
3
(24)  8 V
3 6
v(t )  8  (20  8) e -t 4  8  12 e -t 4
dv
 -1
i (t )  C
 (2)(12)   e -t 4  - 6 e -0.25t A
dt
4
Chapter 7, Solution 45.
To find R Th , consider the circuit shown below.
20 k
10 k
40 k
RTh  10  20 / / 40  10 
  RThC 
R Th
20 x40 70

k
60
3
70
x103 x3 x10 6  0.07
3
To find v o () , consider the circuit below.
20 k
10 k
+
30V
+
_
40 k
vo
–
v o (∞) = [40/(40+20)]30 = 20 V
v o (t) = v o (∞) + [v o (0)– v o (∞)]e–t/0.07 = [20 –15e–14.286t]u(t) V.
Chapter 7, Solution 46.
  RTh C  (2  6) x0.25  2s,
v(0)  0,
v()  6i s  6 x5  30
v ( t )  v ( )  [v (0)  v ( )]e  t /   30(1  e  t / 2 ) u( t ) V
Chapter 7, Solution 47.
For t < 0, u ( t )  0 ,
u ( t  1)  0 ,
v(0)  0
For 0 < t < 1,   RC  (2  8)(0.1)  1
v(0)  0 ,
v()  (8)(3)  24
v( t )  v()   v(0)  v() e - t 
v( t )  24 1  e - t 
For t > 1,
v(1)  24 1  e -1   15.17
- 6  v() - 24  0 
 v()  30
v( t )  30  (15.17  30) e -(t-1)
v( t )  30  14.83 e -(t-1)
Thus,


 24 1  e - t V ,
0t1
v( t )  
-(t -1)
V,
t 1
 30  14.83 e
Chapter 7, Solution 48.
For t < 0,
u (-t)  1 ,
For t > 0,
u (-t)  0 ,
R th  20  10  30 ,
v()  0
  R th C  (30)(0.1)  3
v( t )  v()   v(0)  v() e - t 
v( t )  10 e -t 3 V
i( t )  C
 - 1
dv
 (0.1)  10 e - t 3
3
dt
i( t ) 
- 1 -t 3
e A
3
Chapter 7, Solution 49.
v()  (2)(4)  8
  R eq C  (10)(0.5)  5
For 0 < t < 1, v(0)  0 ,
R eq  4  6  10 ,
v( t )  v()   v(0)  v() e - t 
v( t )  8  1  e - t 5  V
v(1)  8  1  e -0.2   1.45 ,
For t > 1,
v( t )  v()   v(1)  v() e -( t 1) 
v( t )  1.45 e -( t 1) 5 V
Thus,


 8 1  e -t 5 V , 0  t  1
v( t )  
- ( t 1 ) 5
V,
t 1
 1.45 e
v()  0
Chapter 7, Solution 50.
For the capacitor voltage,
v( t )  v()   v(0)  v() e- t 
v(0)  0
For t > 0, we transform the current source to a voltage source as shown in Fig. (a).
1 k
1 k
+
30 V
+

2 k
v

(a)
v() 
2
(30)  15 V
2 11
R th  (1  1) || 2  1 k
1
1
  R th C  10 3   10 -3 
4
4
-4t
v( t )  15  1  e  , t  0
We now obtain i x from v(t). Consider Fig. (b).
i T 1 k
v
ix
30 mA
But
1 k
i x  30 mA  i T
dv
v
C
iT 
dt
R3
i T ( t )  7.5  1  e -4t  mA 
i T ( t )  7.5  1  e -4t  mA
Thus,
1/4 mF
(b)
1
 10 -3 (-15)(-4) e -4t A
4
i x ( t )  30  7.5  7.5 e -4t mA
i x ( t )  7.5  3  e -4t  mA , t  0
2 k
Chapter 7, Solution 51.
Consider the circuit below.
t=0
R
+
VS
+

i
L

v
After the switch is closed, applying KVL gives
di
VS  Ri  L
dt

VS 
di

or
L  -R  i 

R
dt
di
-R

dt
i  VS R
L
Integrating both sides,

V  i( t ) - R
ln  i  S  I 0 
t

R
L
 i  VS R  - t

ln 
 I0  VS R  
or
i  VS R
 e- t 
I0  VS R
i( t ) 
VS 
VS  -t 
e
  I0 
R 
R
which is the same as Eq. (7.60).
Chapter 7, Solution 52.
Using Fig. 7.118, design a problem to help other students to better understand the step response
of an RL circuit.
Although there are many ways to work this problem, this is an example based on the same kind
of problem asked in the third edition.
Problem
For the circuit in Fig. 7.118, find i(t) for t > 0.
Figure 7.118
Solution
20
i()  2 A
 2 A,
10
i( t )  i()   i(0)  i() e- t 
i ( 0) 
i( t )  2 A
Chapter 7, Solution 53.
(a)
(b)
25
5A
3 2
i ( t )  i ( 0) e - t 
After t = 0,
L 4
    2,
i(0)  5
R 2
i( t )  5 e - t 2 u( t ) A
Before t = 0,
i
Before t = 0, the inductor acts as a short circuit so that the 2  and 4 
resistors are short-circuited.
i( t )  6 A
After t = 0, we have an RL circuit.
L 3
i( t )  i(0) e - t  ,
 
R 2
-2t 3
u( t ) A
i( t )  6 e
Chapter 7, Solution 54.
(a)
Before t = 0, i is obtained by current division or
4
( 2)  1 A
i( t ) 
44
After t = 0,
i( t )  i()   i(0)  i() e- t 
L

,
Req  4  (4 || 12)  7 
R eq

3 .5 1

7
2
i(0)  1 ,
i ( ) 
6 
6
  1   e -2 t
7 
7
1
i( t )   6  e - 2t  A
7
10
2A
Before t = 0, i( t ) 
23
Req  3  (6 || 2)  4.5
After t = 0,
i( t ) 
(b)
(4 || 12)
3
6
(2) 
(2) 
4  (4 || 12)
43
7

L
2
4


R eq 4.5 9
i(0)  2
To find i() , consider the circuit below, at t = when the inductor becomes a
short circuit,
v
i
10 V
2
+

24 V
6
+

2H
3
10  v 24  v v
v



 v  9 i()   3 A and
2
6
3
3
i( t )  3  (2  3) e -9 t 4
i( t )  3  e - 9 t 4 A
Chapter 7, Solution 55.
For t < 0, consider the circuit shown in Fig. (a).
0.5 H
io
3
24 V
io
+

0.5 H
i
+

+
4i o
v

2
(a)
3i o  24  4i o  0 
 i o  24
v
v( t )  4i o  96 V
i   48 A
2
For t > 0, consider the circuit in Fig. (b).
i( t )  i()   i(0)  i() e- t 
i(0)  48 ,
i ( )  0
L
0.5 1
R th  2  ,  


R th
2
4
i( t )  (48) e -4t
v( t )  2 i( t )  96 e -4t u( t )V
8
20 V
+
v

+

(b)
2
Chapter 7, Solution 56.
R eq  6  20 || 5  10  ,

L
 0.05
R
i( t )  i()   i(0)  i() e- t 
i(0) is found by applying nodal analysis to the following circuit.
5
vx
12 
2A
20 
20  v x v x v x v x



5
12 20 6
v
i(0)  x  2 A
6
2
i
6
+
0.5 H


 v x  12
Since 20 || 5  4 ,
4
( 4 )  1 .6
i() 
46
i( t )  1.6  (2  1.6) e- t 0.05  1.6  0.4 e-20t
di 1
v( t )  L  (0.4) (-20) e -20t
dt 2
v( t )  - 4 e -20t V
v
+

20 V
Chapter 7, Solution 57.
At t  0  , the circuit has reached steady state so that the inductors act like short circuits.
6
i
i1
30 V
5
+

i2
20 
20
30
30

 3,
i1 
(3)  2.4 ,
25
6  (5 || 20) 10
i 1 ( 0 )  2 .4 A ,
i 2 ( 0 )  0 .6 A
i
i 2  0 .6
For t > 0, the switch is closed so that the energies in L1 and L 2 flow through the closed
switch and become dissipated in the 5  and 20  resistors.
L
2.5 1
1  1 

i1 ( t )  i1 (0) e - t 1 ,
R1
5
2
i1 ( t )  2.4e–2tu(t) A
i 2 ( t )  i 2 ( 0) e - t  2 ,
2 
L2
4 1


R 2 20 5
i 2 ( t )  600e–5tu(t) mA
Chapter 7, Solution 58.
For t < 0,
For t > 0,
v o (t)  0
i(0)  10 ,
R th  1  3  4  ,
i() 
20
5
1 3
L 14 1



R th
4 16
i( t )  i()   i(0)  i() e- t 
i( t )  5  1  e-16t  A
di
1
 15  1  e-16t   (-16)(5) e-16t
dt
4
-16t
v o ( t )  15  5 e V
vo (t )  3i  L
Chapter 7, Solution 59.
Let i(t) be the current through the inductor.
For t < 0,
vs  0 ,
i(0)  0
For t > 0,
R eq  4  (6 || 3)  6 Ω and  
L 1.5

 0.25 sec.
R eq
6
At t = ∞, the inductor becomes a short and the current delivered by the 18 volts
source is I s = 18/[6+(3||4)] = 18/7.714 = 2.333 amps. The voltage across the 4ohm resistor is equal to 18–6(2.333) = 18–14 = 4 volts. Therefore the current
through the inductor is equal to i(∞) = 4/4 = 1 amp.
i( t )  i()   i(0)  i() e- t 
i( t )  1(1  e -4t ) amps.
v o (t)  L
di
 (1.5)(1)(-4)(-e- 4t )
dt
v o ( t )  [6e–4t]u(t) volts.
Chapter 7, Solution 60.
Let I be the inductor current.
For t < 0,
u(t)  0 
 i(0)  0
For t > 0,
R eq  5 || 20  4  ,
i()  4
i( t )  i()   i(0)  i() e- t 
i( t )  4  1  e - t 2 
 - 1
di
 (8)(-4)  e - t 2
2
dt
v( t )  16 e -0.5t V
v( t )  L

L
8
 2
R eq 4
Chapter 7, Solution 61.
The current source is transformed as shown below.
4
20u(-t) + 40u(t)
+

0.5 H
L 12 1
i(0)  5 ,

 ,
R
4
8
i( t )  i()   i(0)  i() e - t 

i()  10
i( t )  (10 – 5e–8t)u(t) A
v( t )  L
di  1 
  (-5)(-8) e -8t
dt  2 
v( t )  20e–8tu(t) V
Chapter 7, Solution 62.

L
2

1
R eq 3 || 6
For 0 < t < 1, u ( t  1)  0 so that
1
i(0)  0 ,
i() 
6
1
i( t )   1  e - t 
6
1
 1  e -1   0.1054
6
1 1 1
i()   
3 6 2
i( t )  0.5  (0.1054  0.5) e-(t -1)
i( t )  0.5  0.3946 e-(t -1)
For t > 1,
Thus,
i(1) 

1
 1  e -t  A
0t1
i( t )  
6
 0.5  0.3946 e -(t -1) A
t1
Chapter 7, Solution 63.
For t < 0,
u (- t )  1 ,
i ( 0) 
For t > 0,
u (-t)  0 ,
i()  0
L
0.5 1



R th
4 8
R th  5 || 20  4  ,
i( t )  i()   i(0)  i() e - t 
v( t )  L
10
2
5
i( t )  2e–8tu(t) A
di  1 
  (-8)(2) e -8t
dt  2 
v( t )  –8e–8tu(t) V
2e–8tu(t) A, –8e–8tu(t) V
Chapter 7, Solution 64
Determine the value of i L (t) and the total energy dissipated by the circuit from t =
0 sec to t = ∞ sec. The value of v in (t) is equal to [40–40u(t)] volts.
40 
v in (t)
+

40 
R eq
v 1 i L (t)
10 H
v Thev (t
+

i L (t)
10 H
Solution
Step 1.
Determine the Thevenin equivalent circuit to the left of the inductor. This
means we need to find v oc (t) and i sc (t) which gives us v Thev (t) = v oc (t) and R eq =
v oc (t)/i sc (t) (note, this only works for resistor networks in the time domain). This leads to
the second circuit shown above.
Now, with this circuit, we can use the generalized solution to a first order differential
equation, i L (t) = Ae–(t–0)/τ+B where, t 0 = 0, τ = L/R, A+B = i L (0) and 0+B = i L (∞).
Finally, we can use w = (1/2)Li L (t)2 to calculate the energy dissipated by the circuit (w =
[(1/2)Li L (∞)2–(1/2)Li L (0)2].
Step 2.
We now determine the Thevenin equivalent circuit. First we need to pick
a reference node and mark the unknown voltages, as seen above. With the inductor out
of the circuit, the node equation is simply [(v 1 –v in (t))/40] + [(v 1 –0)/40] + 0 (since the
inductor is out of the circuit, there is an open circuit where it was) = 0. This leads to
[(1/40)+(1/40)]v 1 = (1/40)v in (t) or 2v 1 = v in (t) or v 1 = 0.5v in (t) = [20–20u(t)] v oc (t) =
v Thev (t). Now to short the open circuit which produces v 1 = 0 and i sc = –[(0–v in (t))/40] =
v in (t)/40 = 0.025v in (t) A.
Step 3.
Now, everything comes together, R eq = v oc (t)/i sc (t) = 0.5v in (t)/[0.025v in (t)]
= 0.5/0.025 = 20 Ω. Next we find τ = L/R eq = 10/20 = (1/2) sec. At t = 0–, v in (0–) = [10–
0] V (note u(t) = 0 until t = 0). Since it has been at this value for a very long time, the
inductor can be considered a short and the value of the current is equal to 20/20 or i L (0–)
= 1 amp. Since you cannot change the current instantaneously, i L (0) = 1 amp = A+B.
Since v Thev (t) = 20–20 = 0 for all t > 0, all the energy in the inductor will be dissipated by
the circuit and i L (∞) = 0 = B which means that A = 1 and i L (t) = [e–2t] u(t) amps. The
total energy dissipated from t = 0 to ∞ sec is equal to [(1/2)Li L (0)2–(1/2)Li L (∞)2] =
(0.5)10(1)2–0 = 5 J.
Since v s  10  u ( t )  u ( t  1) , this is the same as saying that a 10 V source is turned on at
t = 0 and a -10 V source is turned on later at t = 1. This is shown in the figure below.
Chapter 7, Solution 65.
vs
10
1
t
-10
For 0 < t < 1, i(0)  0 ,
R th  5 || 20  4 ,
i() 
10
2
5
L
2 1

 
R th 4 2
i( t )  i()   i(0)  i() e- t 
i( t )  2  1  e -2t  A
i(1)  2  1  e-2   1.729
For t > 1,
i()  0
i( t )  i(1) e- ( t 1) 
i( t )  1.729 e-2( t 1) A
Thus,
since vs  0
 2  1  e - 2t  A 0  t  1
i( t )  
t1
 1.729 e - 2( t 1) A
Chapter 7, Solution 66.
Using Fig. 7.131, design a problem to help other students to better understand first-order
op amp circuits.
Although there are many ways to work this problem, this is an example based on the
same kind of problem asked in the third edition.
Problem
For the op-amp circuit of Fig. 7.131, find v o . Assume that v s changes abruptly from
0 to 1 V at t=0. Find v o .
50 k
0.5 F
20 k
–
+
vs
+
vo
+
_
–
Figure 7.131 For Prob. 7.66.
Solution
For t<0-, v s =0 so that v o (0)=0
:Let v be the capacitor voltage
For t>0, v s =1. At steady state, the capacitor acts like an open circuit so that we
have an inverting amplifier
v o (∞) = –(50k/20k)(1V) = –2.5 V
τ = RC = 50x103x0.5x10–6 = 25 ms
v o (t) = v o (∞) + (v o (0) – v o (∞))e–t/0.025 = 2.5(e–40t – 1) V.
Chapter 7, Solution 67.
The op amp is a voltage follower so that v o  v as shown below.
R
R

+
vo
v1
vo
+
R
vo

C
At node 1,
v o  v1 v1  0 v1  v o


R
R
R

 v1 
2
v
3 o
At the noninverting terminal,
dv
v  v1
C o  o
0
dt
R
dv
2
1
 RC o  v o  v1  v o  v o  v o
dt
3
3
dv o
v
 o
dt
3RC
v o ( t )  VT e - t 3RC
VT  v o (0)  5 V ,
  3RC  (3)(10  103 )(1  10- 6 ) 
v o ( t )  5e–100t/3u(t) V
3
100
Chapter 7, Solution 68.
This is a very interesting problem which has both an ideal solution as well as a realistic solution.
Let us look at the ideal solution first. Just before the switch closes, the value of the voltage
across the capacitor is zero which means that the voltage at both terminals input of the op amp
are each zero. As soon as the switch closes, the output tries to go to a voltage such that both
inputs to the op amp go to 4 volts. The ideal op amp puts out whatever current is necessary to
reach this condition. An infinite (impulse) current is necessary if the voltage across the capacitor
is to go to 8 volts in zero time (8 volts across the capacitor will result in 4 volts appearing at the
negative terminal of the op amp). So v o will be equal to 8 volts for all t > 0.
What happens in a real circuit? Essentially, the output of the amplifier portion of the op amp
goes to whatever its maximum value can be. Then this maximum voltage appears across the
output resistance of the op amp and the capacitor that is in series with it. This results in an
exponential rise in the capacitor voltage to the steady-state value of 8 volts.
vC(t) = V op amp max (1 – e-t/(RoutC)) volts, for all values of vC less than 8 V,
= 8 V when t is large enough so that the 8 V is reached.
Chapter 7, Solution 69.
Let v x be the capacitor voltage.
For t < 0,
v x ( 0)  0
For t > 0, the 20 k and 100 k resistors are in series and together, they are in
parallel with the capacitor since no current enters the op amp terminals.
As t   , the capacitor acts like an open circuit so that
4
(20  100)  48
v o ( ) 
10
R th  20  100  120 k ,
  R th C  (120  103 )(25  10-3 )  3000
v o ( t )  v o ()   v o (0)  v o () e - t 


v o ( t )  48 1  e - t 3000 V = 48(e–t/3000–1)u(t)V
Chapter 7, Solution 70.
Let v = capacitor voltage.
For t < 0, the switch is open and v(0)  0 .
For t > 0, the switch is closed and the circuit becomes as shown below.
1
+

2
vS
+
+

v

vo
C
R
v1  v 2  v s
0  vs
dv
C
R
dt
 v o  v s  v
where v  v s  v o 
From (1),
dv v s

0
dt RC
- t vs
-1
v s dt  v(0) 
v

RC
RC
Since v is constant,
RC  (20  10 3 )(5  10 -6 )  0.1
- 20 t
v
mV  -200 t mV
0.1
From (3),
v o  v s  v  20  200 t
v o  20 ( 1  10t ) mV
(1)
(2)
(3)
Chapter 7, Solution 71.
We temporarily remove the capacitor and find the Thevenin equivalent at its
terminals. To find R Th , we consider the circuit below.
Ro
20 k
R Th
Since we are assuming an ideal op amp, R o = 0 and R Th =20k . The op amp circuit
is a noninverting amplifier. Hence,
VTh  (1
10
)v s  2v s  6V
10
The Thevenin equivalent is shown below.
20 k
+
6V
+
_
v
10 F
–
Thus,
v(t)  6(1 e  t /  ) , t  0
where   RTHC  20 x10 3 x10 x10 6  0.2
v(t)  6(1 e 5t ), t  0 V
Chapter 7, Solution 72.
The op amp acts as an emitter follower so that the Thevenin equivalent circuit is
shown below.
C
+
3u(t)
Hence,
+

v

io
R
v( t )  v()   v(0)  v() e - t 
v(0)  -2 V , v()  3 V ,   RC  (10  10 3 )(10  10 -6 )  0.1
v( t )  3  (-2 - 3) e -10t  3  5 e -10t
io  C
dv
 (10  10 -6 )(-5)(-10) e -10t
dt
i o  0.5 e -10t mA , t  0
Chapter 7, Solution 73.
Consider the circuit below.
Rf
v1
R1
v2
+
v1
+

C
v

v3

+
At node 2,
v1  v 2
dv
C
R1
dt
At node 3,
dv v 3  v o
C

dt
Rf
But v 3  0 and v  v 2  v 3  v 2 . Hence, (1) becomes
v1  v
dv
C
dt
R1
dv
v 1  v  R 1C
dt
v1
dv
v
or


dt R 1C R 1C
which is similar to Eq. (7.42). Hence,

vT
t0
v( t )  
-t 
t0
 v1   v T  v1  e
where v T  v(0)  1 and v1  4
  R 1C  (10  10 3 )(20  10 -6 )  0.2
 1
t0
v( t )  
-5t
t0
4  3 e
From (2),
v o  -R f C
dv
 (20  10 3 )(20  10 -6 )(15 e -5t )
dt
v o  -6 e -5t , t  0
+
vo

(1)
(2)
v o  - 6 e -5t u(t ) V
Chapter 7, Solution 74.
Let v = capacitor voltage. For t < 0, v(0)  0
10 kΩ
2 µF
is
50 kΩ

+
is
For t > 0,
+
vo

i s  10 A .
Since the current through the feedback resistor is i s , then
v o = –i s x104 volts = –10–5x104 = –100 mV.
It is interesting to look at the capacitor voltage.
is  C
dv v

dt R
v( t )  v()   v(0)  v() e - t 
It is evident that
  RC  (2  10 6 )(50  10 3 )  0.1
At steady state, the capacitor acts like an open circuit so that i s passes through R.
Hence,
v()  i s R  (10  10 6 )(50  10 3 )  0.5 V
Then the voltage across the capacitor is,
v(t) = 500(1–e–10t) mV.
Chapter 7, Solution 75.
Let v1 = voltage at the noninverting terminal.
Let v 2 = voltage at the inverting terminal.
For t > 0,
v1  v 2  v s  4
0  vs
 i o , R 1  20 k
R1
vo  -ioR
Also, i o 
i.e.
dv
v
C ,
dt
R2
(1)
R 2  10 k , C  2 F
- vs
v
dv

C
R1 R 2
dt
(2)
This is a step response.
v( t )  v()   v(0)  v() e - t  ,
where   R 2 C  (10  10 3 )(2  10 -6 ) 
v(0)  1
1
50
At steady state, the capacitor acts like an open circuit so that i o passes through
R 2 . Hence, as t  
- vs
v()
 io 
R1
R2
-R2
- 10
(4)  -2
i.e.
vs 
v() 
20
R1
v( t )  -2  (1  2) e -50t
v( t )  -2  3 e -50t
But
or
v  vs  vo
v o  v s  v  4  2  3 e -50 t
v o  6  3 e -50 t u( t )V
io 
or
- vs
-4

 -0.2 mA
R 1 20k
dv
v
C
 - 0.2 mA
io 
dt
R2
Chapter 7, Solution 76.
The schematic is shown below. For the pulse, we use IPWL and enter the corresponding
values as attributes as shown. By selecting Analysis/Setup/Transient, we let Print Step =
25 ms and Final Step = 2 s since the width of the input pulse is 1 s. After saving and
simulating the circuit, we select Trace/Add and display –V(C1:2). The plot of V(t) is
shown below.
Chapter 7, Solution 77.
The schematic is shown below. We click Marker and insert Mark Voltage Differential at
the terminals of the capacitor to display V after simulation. The plot of V is shown
below. Note from the plot that V(0) = 12 V and V() = -24 V which are correct.
Chapter 7, Solution 78.
(a)
When the switch is in position (a), the schematic is shown below. We insert IPROBE to
display i. After simulation, we obtain,
i(0) = 7.714 A
from the display of IPROBE.
(b)
When the switch is in position (b), the schematic is as shown below. For inductor I1, we
let IC = 7.714. By clicking Analysis/Setup/Transient, we let Print Step = 25 ms and Final Step =
2 s. After Simulation, we click Trace/Add in the probe menu and display I(L1) as shown below.
Note that i() = 12A, which is correct.
Chapter 7, Solution 79.
When the switch is in position 1, i o (0) = 12/3 = 4A. When the switch is in position 2,
L
4
 0.5 A,
R Th  (3  5) // 4  8 / 3,

 3 / 80
i o ( )  
R Th
53
i o ( t )  i o ()  [i o (0)  i o ()]e  t /    0.5  4.5e 80 t / 3 u(t)A
Chapter 7, Solution 80.
(a) When the switch is in position A, the 5-ohm and 6-ohm resistors are short-circuited so
that
i 1 ( 0 )  i 2 ( 0)  v o ( 0)  0
but the current through the 4-H inductor is i L (0) =30/10 = 3A.
(b) When the switch is in position B,
R Th  3 // 6  2,

L
 4 / 2  2 sec
R Th
i L ( t )  i L ( )  [i L (0)  i L ( )]e  t /   0  3e  t / 2  3e  t / 2 A
(c) i1 () 
30
 2 A,
10  5
v o (t )  L
3
i 2 ( )   i L ( )  0 A
9
di L
dt


v o ( )  0 V
Chapter 7, Solution 81.
The schematic is shown below. We use VPWL for the pulse and specify the attributes as shown.
In the Analysis/Setup/Transient menu, we select Print Step = 25 ms and final Step = 3 S. By
inserting a current marker at one terminal of LI, we automatically obtain the plot of i after
simulation as shown below.
2.0A
1.5A
1.0A
0.5A
0A
0s
0.5s
1.0s
1.5s
-I(L1)
Time
2.0s
2.5s
3.0s
Chapter 7, Solution 82.
3  10 -3

 30 
  RC 
 R  
C 100  10 -6
Chapter 7, Solution 83.
v()  120,
v(0)  0,
  RC  34 x10 6 x15 x10 6  510s
v(t )  v()  [v(0)  v()]e t / 
Solving for t gives


85.6  120(1  e t / 510 )
t  510 ln 3.488  637.16 s
speed = 4000m/637.16s = 6.278m/s
Chapter 7, Solution 84.
Let I o be the final value of the current. Then
i(t )  I o (1  e  t /  ),
0.6 I o  I o (1  e  50 t )
  R / L  0.16 / 8  1 / 50


t
1
1
ln
 18.33 ms.
50 0.4
Chapter 7, Solution 85.
(a)
The light is on from 75 volts until 30 volts. During that time we essentially have
a 120-ohm resistor in parallel with a 6-µF capacitor.
v(0) = 75, v(∞) = 0, τ = 120x6x10-6 = 0.72 ms
v(t 1 ) = 75 e  t1 /   30 which leads to t 1 = –0.72ln(0.4) ms = 659.7 µs of lamp on
time.
(b)
  RC  (4  106 )(6  10-6 )  24 s
Since v( t )  v()   v(0)  v() e - t 
v( t 1 )  v()   v(0)  v() e - t1 
v( t 2 )  v()   v(0)  v() e- t 2 
Dividing (1) by (2),
v( t1 )  v()
 e( t 2  t1 ) 
v( t 2 )  v()
 v( t )  v() 

t 0  t 2  t1   ln  1
 v( t 2 )  v() 
 75  120 
  24 ln (2)  16.636 s
t 0  24 ln 
 30  120 
(1)
(2)
Chapter 7, Solution 86.
v( t )  v()   v(0)  v() e- t 
v()  12 ,
v(0)  0
-t 
v( t )  12  1  e 
v( t 0 )  8  12  1  e- t 0  
8
1
 1  e- t 0  
 e- t 0  
12
3
t 0   ln (3)
For R  100 k ,
  RC  (100  103 )(2  10-6 )  0.2 s
t 0  0.2 ln (3)  0.2197 s
For R  1 M ,
  RC  (1  106 )(2  10-6 )  2 s
t 0  2 ln (3)  2.197 s
Thus,
0.2197 s  t 0  2.197 s
Chapter 7, Solution 87.
Let i be the inductor current.
For t < 0,
i (0  ) 
120
 1 .2 A
100
For t > 0, we have an RL circuit
L
50
 
 0.1 ,
i()  0
R 100  400
i( t )  i()   i(0)  i() e - t 
i( t )  1.2 e -10t
At t = 100 ms = 0.1 s,
i(0.1)  1.2 e -1  441mA
which is the same as the current through the resistor.
Chapter 7, Solution 88.
(a)
(b)
  RC  (300  10 3 )(200  10 -12 )  60 s
As a differentiator,
T  10   600 s  0.6 ms
i.e.
Tmin  0.6 ms
  RC  60 s
As an integrator,
T  0.1  6 s
i.e.
Tmax  6 s
Chapter 7, Solution 89.
Since   0.1 T  1 s
L
 1 s
R
L  R  10 -6  (200  10 3 )(1  10 -6 )
L  200 mH
Chapter 7, Solution 90.
We determine the Thevenin equivalent circuit for the capacitor C s .
Rs
v th 
v,
R th  R s || R p
Rs  Rp i
R th
V th
+

Cs
The Thevenin equivalent is an RC circuit. Since
Rs
1
1
v th  v i 


10 R s  R p
10
Rs 
6 2
1
R p   M
9 3
9
  R th C s  15 s
6 (2 3)
 0.6 M
where R th  R p || R s 
62 3
Also,

15  10 -6

 25 pF
Cs 
R th 0.6  10 6
Chapter 7, Solution 91.
12
 240 mA ,
i()  0
50
i( t )  i()   i(0)  i() e - t 
i( t )  240 e - t 
i o ( 0) 

L 2

R R
i( t 0 )  10  240 e - t 0

e t 0   24 
 t 0   ln (24)
t0
2
5


 1.573 
R
ln (24) ln (24)
2
 1.271 
R
1.573
Chapter 7, Solution 92.
 10

-3
dv
 4  10 -9   2  10
iC
- 10
dt

 5  10 -6
0  t  tR
tR  t  tD
 20 A
0  t  2 ms
i( t )  
- 8 mA 2 ms  t  2 ms  5 s
which is sketched below.
5 s
i(t)
20 A
t
2 ms
-8 mA
(not to scale)