solution or answer :
An AC source of rms voltage 20 V with internal impedance ZS = (1 + 2j) feeds a load of impedance ZL = (7 + 4J) in the figure below. the reactive power consumed by the load is ?
Question and Answers given below (with solution) :
- In the circuit shown, the switch S is open for a long time and is closed at t = 0. the current I(t) for t > 0+ is
(a) I(t) = 0.5 – 0.125 e-1000t A
(b) I(t) = 1.5 – 0.125 e-1000t A
(c) I(t) = 0.5 – 0.5 e-1000t A
(d) I(t) = 0.375 e-1000t A
- The current I in the circuit shown is
(a) -j1A
(b) j1A
(c) zero
(d) 20 A
- In the circuit shown, the power supplied by the voltage source is
(a) zero
(b) 5 W
(c) 10 W
(d) 100 W
- An AC source of rms voltage 20 V with internal impedance ZS = (1 + 2j) feeds a load of impedance ZL = (7 + 4J) in the figure below. the reactive power consumed by the load is
(a) 8 VAR
(b) 16 VAR
(c) 28 VAR
(d) 32 VAR
- The switch in the circuit shown was on position a for a long time and is moved to position b at time t = 0. the current I(t) for t > 0 is given by
(a) 0.2 e-125tu(t) mA
(b) 20 e-1250t u(t) mA
(c) 0.2 e-1250tu(t) mA
(d) 20 e-1000t u(t) mA
- In the circuit shown, what value of RL maximizes the power delivered to RL?
(a) 2.4
(b) 8/3
(c) 4
(d) 6
- The thevenin equivalent impedance Zth between the nodes P and Q in the following circuit is
(a) 1
(b) 1 + s + 1/s
(c) 2 + s + 1/s
(d) s2 + s + 1/s2 + 2s + 1
- The driving point impedance of the following network is given by Z(s) = 0.2s/s2 + 0.1s + 2 the component values are
(a) L = 5 H, R = 0.5, C = 0 1F
(b) L = 0.1 H, R = 0.5, C = 5 F
(c) L = 5 H, R = 2, C = 0.1 F
(d) L = 0.1 H, R = 2, C = 5 F
- In the following circuit the value of open circuit volage and thevenin resistance at terminal ab are
(a) VOC = 100 V, RTH = 1800
(b) VOC = 0 V, RTH = 270
(c) VOC = 100 V, RTH = 90
(d) VOC = 0 V, RTH = 90
- In the following linear circuit it is given that VAB = 4 V for RL 10 K and VAB = 1 V for RL = 2 K.
The values of thevenin resistance and voltage for the network N are
(a) 16 k, 30 V
(b) 30 k, 16 V
(c) 3 k, 6 V
(d) 50 k, 30 V
- Consider the following linear circuit. three sources have fixed value only IS1 can be varied.
It is given that if IS1 = 3 mA then VOUT = 16 V and if IS1 = 1 mA then VOUT = 8 V. if IS1 = 0.5 mA, then the value of VOUT is
(a) 6 V
(b) 8 V
(c) 4 V
(d) 12 V
- For the circuit shown in figure below the value of RTH is
(a) 100
(b) 136.4
(c) 200
(d) 272.8
- Consider the following circuits shown below.
The relation betwwwn IA and IB is
(a) IB = IA + 6
(b) IB = IA + 2
(c) IB = 1.5IA
(d) IB = IA
- The circuit shown in the figure is used to charge the capacitor C alternately from two current sources as indicated. the switches S1 and S2 are mechanically coupled and connected as follows For 2n T < t <(2n + 1) T, (n = 0,1,2,……..) S1 to P1 and S2 to P2
For (2n + 1) T < t < (2n + 2) T, (n = 0, 1, 2,……) S1 to Q1 and S2 to Q2
Assume that the capacitor has zero initial charge. given that u(t) is a unit step function, the voltage VC (t) across the capacitor is given by
(a) n – 0 (-1)n tu (t – nT)
(b) u(t) + 2 n = 0 (-1)n u (t – nT)
(c) tu (t) + 2 n = 0b(-1)n (t – nT) U (t – nt)
(d) n = 0 [0.5 – e-(t – 2nt) + 0.5 e-(t – 2nT – T)]
The following series RLC circuit with zero initial condition is excited by a unit impulse function (t).
- For t > 0, the output voltage VC (t) is
(a) 1/3 (e-2/3t – e3/2t)
(b) 2/3 te-1/2t
(c) 2/3 e-1/2 t cos (3/2t)
(d) 2/3 e-1/2t sin (3/2t)
- For t > 0. the voltage across the resistor is
(a) 1/3 (e-3/2t – e-1/2t)
(b) e-1/2t [cos (3t/2) – 1/3 sin (3/2)]
(c) 2/3 e-1/2t sin (3t/2)
(d) 2/3 e-1/2t cos (3t/2)
- Two series resonant filters are as shown in the figure. let the 3 dB bandwidth of filter 1 be B1 and that of filter 2 be B2 . the value of B1/B2 is
(a) 4
(b) 1
(c) 1/2
(d) 1/4
- For the circuit shown in the figure, the thevenin voltage and resistance looking into X-Y are
(a) 4/3 V, 2
(b) 4 V, 2/3
(c) 4/3 V, 2/3
(d) 4V, 2
- In the circuit shown VC is 0 volt at = 0 s. for t < 0, the capacitor current IC (t) where t is in second, is given by
(a) 0.50 exp (-25t) mA
(b) 0.25 exp (-25t) mA
(c) 0.50 exp (-12.5t) mA
(d) 0.25 exp (-6.25t) mA
- In the AC network shown in the figure, the phasor voltage VAB (in volt) is
(a) zero
(b) 5 < 300
(c) 12.5 <300
(d) 17 <300
- In the following circuit capacitor is initially uncharged. At t = 0+ the value of d2vc/dt2 and d3vc/dt3
(a) 2 V/s2, -8 V/S3
(b) -8 V/S2 , 26 V/S3
(c) -2 V/S2, 8 V/S3
(d) 8 V/S2, -26 V/S3
- A square pulse of 3 V amplitude is applied to CR circuit shown below. the capacitor is initially uncharged. the output voltage V2 at time t = 2 s is
(a) 3 V
(b) -3 V
(c) 4 V
(d) -4 V
- In the given circuit below initially capacitor is uncharged. the VC (t) for t >0 is
(a) (8 – 8e-t) V
(b) (8 + 8e-t) V
(c) 8 V
(d) (-8 + 8et) V
- In the circuit given below VIN (t) = 10 u(t) , the current IL (t) is
(a) (-0.01 + 0.01e-5000t)A
(b) 0.1 A
(c) (-0.1 – 0.1e-5000t) A
(d) 0.2 A
- In the following circuit the initial values are VC1(0–) = 6 V and VC2 (0–) = 24 V. the voltage VO(t) for t > 0 is
(a) 38 – 8e -t/1.2 V
(b) 8 + 22e -t/1.2 V
(c) 8 + 22e – 3.75t V
(d) 38 – 8e -t/1.2 V
Answers with solution
- (a)
at steady state,
inductor becomes short-circuit.
I(t) at steady state
I(t) = 1.5/3 = 0.5
LEQ = 15 mH
REQ = 5 + 10 = 15
L = LEQ/REQ = 15 x 103/15 = 1/1000
I(t) A – (A – B)e-t/10
= 0.5 – (0.5 – B) e-1000t
= 0.5 (0.5 – 0.375) e-1000t
I(t) = 0.5 – 0.125 e-1000t
when switch is closed for a long time then inductor is short and current through inductor 1.5/2 = 0.75 A
current through inductor remains same after closing the switch at t = 0.
so, current through 10
= 0.75/2 = 0.375
- (a)
XEQ = sL + R x 1/sC/ R + 1/sC
= sL + R / 1 + sRC
IO = V/XEQ
I = XC/XC + R IO
= 1/sC/1/sC + R x V/sL (1 + sRC) + R/1 + sRC
= 1/1 + sRC x V/sL (1 + sRC) + R/(1 + sRC)
= V/ sL (1 + sRC) + R
= V/j x 103 x 20 x 10-3 (1 + j x 103 x 50 x 10-6 + 1)
V/20j (1 + j50 x 10-3) + 1
= V/20j – 1 + 1 = 20/20j = – j1A
- (a)
let current supplied by voltage source. the current in different branch is indicated in circuit using Kirchhoff’s current law.
applying KVL in outer loop,
10 – (I + 3) x (1 + 1) – (I + 2) x 2 = 0
10 – 2(I + 3) – 2 (I – 2) = 0
I = 0
VI = 0
- (b)
current I = V/ZL + ZS = 20 <00/8 + 6J
= 20/82 + 62 = <00/<tan-13/4
= 20/10 < – tan-13/4
2 < – tan-13/4
power consumed by load = |I|2ZL
= 4 (7 + 4J)
= 28 + J16
The reactive power = 16 VAR
- (c)
CEQ = 0.8 x 0.2/0.8 + 0.2 = 0.16
when the switch is at position a for a long time then voltage across capacitor
VC(t = 0) = 100 V
as capacitor acts as open-circuit
at t > 0
the discharging current
I(t) = VO/R e-t/rc for t > 0
= 100/5000 e-t/5 x 103 x 0.16 x 10-6
= 0.020 e-125t A
on comparing RL = 0.2
RLC = 1
L = 0.1 H
R = 2
C = 5 F
- (c)
Maximum power delivered to RL when load resistance equals to thevenin resistance of circuit.
RL = RTH = VOC/ISC
due to open-circuit VOC = 100 V
ISC = I1 + I2
Applying KVL in lower loop.
100 – 8I1 = 0
I1 = 100/8 = 25/2
VX = – 4I1 = – 4 x 25/2 = – 50 V
100 + VX – 4I2 = 0
I2 = 100 – 50/4 = 25/2
ISE = I1 + I2
= 25/2 + 25/2 = 25
RTH = VOC/ISC = 100/25 4
RL = RTH = 4
- (a)
to calculate thevenin resistance, all the current sources get open-circuited and voltage sources short-circuited. writing in impedance of inductor and capacitor.
RTH = (1/S + 1) ||(1 + S)
(1 + S) (1/S + 1)/1 + S + 1/S + 1 = (1/S + 1 + 1 + S)/(1/S + 1 + 1 + S)
RTH = 1
- (d)
dividing point impedance = R ||1/sC||sL
= {(R) (1/sC)/R + 1/sC}||sL
(R/1 + sRC) = sRL/S2RLC + sL + R
R/1 + sRC + sL
Z(s) = 0.2s/s2 + 0.1s + 2
- (d)
by writing loop equation for the circuit,
VS = VX,IS = IX
IS = Test source to find out thevenin equivalent
VS = 600(I1 – I2) + 300 (I1 – I2) + 900(I1)
VS = (600 + 300 + 900)I1 – 600 I2 – 300 I3
VS = 1800I1– 600I2 – 300I3
I1 = IS,I2 = 0.3 VS
I3 = 3IS + 0.2VS
VS = 1800IS – 600(0.01VS) -300 (3IS + 0.01VS)
VS = 1800IS – 6VS – 900IS – 3VS
10VS = 900IS
VS = 90I2
For thevenin equivalent
VS = RTH IS + VOC
So, thevenin voltage VOC = 0 V
Resistance RTH = 90
- (b)
the circuit is shown in figure below.
when RL = 10 K and VAB = 4 V,
Current in circuit
I – VAB/RL = 4/10 = 0.4 mA
thevenin voltage is given by
VTH = I(RTH + RL)
VTH = 0.4 (RTH + 10)
VTH = 0.4 RTH + 4…………..(1)
Simiarly,
RL = 2 K, VAB = 1 V
I = 1/2 = 0.5 mA
VTH = 0.5 (RTH + 2)
VTH = 0.5RTH + 1……………(2)
From eq. (1) and (2)
0.1RTH = 3
RTH = 30 K
VTH = (12 + 4) = 16 V
- (a)
for a linear circuit, we can write
VOUT = AIS1 + BVS1 + CIS2 + DIS3
= AIS1 + (BVS1 + CIS3 + DIS3)
VOUT = AIS1 + K
(VS1, IS2 and IS3 are fixed)
16 = A x 3 + K
8 = A x 1 + K
2A = 8
A = 4
4 + k = 8, K = 4 V
so, the equation for VOUT is
VOUT = 4IS1 + 4
IS1 = 0.5 mA
VOUT = 4 x 0.5 + 4 = 6 V
- (a)
The circuit is shown below,
IX = 1 A, VX = VTEST
VTEST = 100(1 – 2IX) + 300 (1 – 2IX – 0.01VX) + 800
VTEST = 1200 – 800IX – 3VTEST
4VTEST = 1200 – 800 = 400
VTEST = 100 V
RTH = VTEST/1 = 100
- (c)
in circuit (b) transforming the 3 A source is to 18 V source all sources are 1.5 times of that in circuit (a). hence, IB = 1.5 IA.
- (c)
for 2nt < t < (2n + 1) t, n = 0, 1, 2, 3…………….
the capacitor gets charge to its peak value. as the constant current is flowing, the voltage across capacitor is ramp function.
for (2n + 1) T < t (2n + 2) T
the capacitor get discharge from peak value with same slope as for charging to zero value.
- (d)
in frequency domain,
VC(S) = t/s/1/s + 1 + s
VI (t) = s(t)
VI(s) = 1
VC(s) = 1/s2 + s + 1
= 2/3 |B/2/(s + 1/2)2 + (3/2)2|
taking inverse laplace transform,
VC(t) = 2/3 sin (3/2 t) .e-1/2t
- (b)
in frequency domain
VR(s) = 1. VI(s)/1/s + 1 + s
s/s2 + s + 1
VR(s) = s + 1/2 – 1/2/(s + 1/2)2 + 3/4
= (s + 1/2)/(s + 1/2)2 + (3/2)2 – 1/2 /(s + 1/2)2 + (3/2)2
taking inverse laplace transform,
VR(t) = e-t/2 cos 3/2 t -1/2 x 2/3 e-t/2 sin 3/2 t
= e-t/2 [cos 3/2 + 1/3 sin 3/2 t]
- (d)
for series resonant circuit, 3 dB bandwidth is R/L
B1 = R/L1
B2 = R/L2 = R/L1/4 = 4R/L1
B1/B2 = 1/4
Note bandwidth of series resonant circuit is independent from value of capacitor.
RTH = VOC/ISC
VTH = VOC
Applying KCL at node A,
2I – VTH/1 + 2 = I + VTH/2 ……………(1)
I = VTH/1
Putting 2VTH – VTH + 2 = VTH + VTH/2
VTH = 4 V
When XY get shorted, 2A current flows through short-circuited path.
RTH = 4/2 = 2
- (a)
the capacitor voltage
VC(T) = VC(00) – (VC(00) – VC(0)) e-t/req c
REQ = 20 K || 20 K
= 10 K
VC (00) = 10 x 20/20 + 20 = 5 V
given VC(0) = 0
VC(t) = 5 – (5 – 0) e-t/10 x 4 x 10-6 x 103
VC(t) = 5 (1 – e-25t)
IC(t) = C dVC(t)/dt = 4 x 10-6 d/dt 5 (1 – e-25t)
= 4 x 10-6 x 5 x 25 e-25t
IL(T) = 0.50 e-2.5t mA
- (d)
equivalent impedance
= (5 + j3) ||(5 – 3)
= 5 + j3) (5 – j3)/5 + j3 + 5 – j3
= 25 + 9/1 = 3.4
VAB = Current x impedance
= 5 <300 x 3.4
= 17 <300
- (b)
at t = 0+, VC(0+) = 0
VC/20 + VC – e-1/10 + 1/20 . dVC/dt = 0 ……………(1)
dVC/dt + 3VC = 2e-t ……………….(2)
at t = 0+ dVC(0+)/dt = 2 v/s
differentiating eq. (1), d2VC/dt2 + 3 dVC/dt = – 2e-t …………..(3)
at t = 0+, d2VC(0+)/dt2 = -2 – 6 = – 8 v/s2
differentiating eq. (ii), d2VC/dt3 + 3 d2VC/dt = 2e-t
at t = 0+, d3vc(0+)/dt2 = 2 + 24 = 26 v/s3
- (b)
RC = 0.1 x 10-6 x 103 = 10-4 s
as RC is very small, so steady state will be reached in 2 s.
thus, VC = 3 V and V2 = -VC = – 3 V
- (d)
by source transformation, we have equivalent circuit shown below.
thevenin equivalent for above circuit can be obtained as (by putting a test source)
by writing KVL,
VS – IS 0.5 – 9VX + 8 – 500IS = 0
VS = 5IS + 9VX – 8 + 500IS …………….(1)
VX = VS + 8 – 500IS
VS = 5IS + 9[VS + 8 – 500IS] – 8 + 500IS
VS = 1000IS + 64 + 9VS
VS = 125 IS – 8
For thevenin’s equivalent circuit
VS = RTHIS + VTH
VTH = – 8 V, RTH = – 125
The circuit is
VC(t) = VC(00) + [V (0) – V(00)]e-1/rc
VC(t) = – 8 + (0 + 8) e-t/-125 x 8 x 10-3
here, time constant is negative.
VC(t) = – 8 + 8et
- (a)
thevenin equivalent across the inductor for t > 0 can be obtained as
by applying node equation,
IS = 0.01VS + (VS – VA + VIN)/100
100IS = 1. VA + VS – VA + VIN
VS = – VIN + 100IS
For thevenin’s equivalent circuit,
VS = VTH + ISRTH
VTH = – VIN = – 10 V.
RTH = 100
t < 0, IL(0–) = 0
IL(00) = -10/100 = – 0.1A
IL(t) = 0.1 [1 – e-t] where = time constant
= L/RTH = 20 x 10-3/100
IL(t) = 0.1 [1 – e-5000t]
= (-0.1 + 0.1 e-5000t) A
- (a)
let 1/sensitivity = 1/20 = 50 uA
for 0 – 10 V scale, RM = 10 x 20 = 200 k
for 0 – 50 V scale, RM = 50 x 20 = 1 m
for 4 V reading I = 4/10 x 50 = 20 uA
VTH = 20 RTH + 20 x 200 = 4 + 20uRTH……………..(1)
For 5 V reading I = 5/50 x 50 = 5uA
VTH = 5 x RTH + 5 x 1m = 5 + 5RTH ……………….(2)
Solving eqs. (1) and (2)
VTH = 16/3 V. RTH = 200/3 K