Chapter

16
Alternating current

Assignment 01
1. Peak value of a sinusoidal voltage is „Vo‟. p-p voltage is
(a) Vo /2 (b) Vo
(c) 2Vo (d) None of the above
2. Angle of sinusoidal voltage is
(a) 2 (b) 2
t
T T
(c)  (d) 2T
t
2T
3. If  is the angular frequency of rotation of the coil, then the out put V of A.C
generator at any instant is given by V =…………
(a) Vo cot 2ft (b) Vo tan 2t
(c) Vo sin t (d) Vo sin 2t
4. A sinusoidal current has rms value of 100 A. The peak value is…….
(a) 141.4A (b) 14.14A
(c) 1.414A (d) 0.1414A
5. At resonance impedance of parallel resonance circuit is
(a) Maximum (b) Minimum
(c) Zero (d) Infinity
6. Resonance curve for RLC series circuit is a plot of „f‟ versus
(a) Voltage (b) current
(c) Impedance (d) reactance
7. Current in a dry cell is an example of …..
(a) Super current (b) dry current

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(c) A.C (d) D.C
8. Such a current which is measured at any instant w.r.t some reference point is
called….
(a) peak current (b) peak to peak current
(c) instantaneous current (d) an A.C
9. The value of voltage or current that exists in a circuit at any instant of time t
measured from some reference point is known as its……
(a) Instantaneous value (b) absolute value
(c) Maximum value (d) minimum value
10. Each point on the A.C wave form corresponds to certain……..
(a) Phase (b) dispersion
(c) Dissipation (d) none
11. Phase of A.C at the positive peak is
(a) 0o (b) 90o
(c) 180o (d) 270o
12. The units of reactance are
(a) volts (b) amperes
(c) ohms (d) mho
13. Active power in A.C circuit is
(a) VI (b) VI cos θ
(c) VI sin θ (d) None of the above
14. The range of amplitude modulation is
(a) 54KHz to 160KHz (b) 540KHz to 1600KHz
(c) 88MHz to 108MHz (d) 880Hz to 1080 MHz
15. The combine effective resistance in an a.c. circuit is
(a) An inductance (b) An impedance
(c) A mutual inductance (d) None of these
16. Which of the following waves are more energetic?
(a) Radio waves (b) Infrared waves
(c) Ultra violet (d) gamma – rays
17. Peak value of a sinusoidal voltage is „Vo‟. Average value of a sinusoidal
voltage is
(a) Vo /2 (b) Vo
(c) 2Vo (d) zero
18. An A.C dynamo operates on the principals of- - - - - induction
(a) Mutual (b) Self
(c) Electro magnetic (d) Mechanical

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19. The frequency of electromagnetic waves used for radio transmission as
compared to that of visible region is
(a) Smaller (b) Larger
(c) Same (d) Double
20. There is a sinusoidal current in a resistor. What is the mean power dissipated
in the resistor?
(a) 1/ [2 (maximum current)]1/2 (b) 1/ [2 (minimum power)]1/2
(c) 1/ 2 (maximum current) (d) ½ (maximum power)
21. In electromagnetic waves, the electric and magnetic fields are
(a) parallel to each other (b) anti parallel to each other
(c) perpendicular to each other (d) at 45o to each other
22. The opposition offered by a capacitor to the flow of an A.C is called……
(a) capacitance (b) reactance
(c) inductance (d) none
23. The circuit in which current and voltage are in phase, the power factor is….
(a) zero (b) 1
(c) negative (d) 0.83
24. As a result of change of polarity in A.C., the direction of the current flow in the
circuit…………
(a) does not change (b) also changes
(c) always remains perpendicular to the circuit
(d) none
25. In a D.C circuit, the element which controls the current or voltage is a …….
(a) Transistor (b) Transmitter
(c) Resistor (d) none
26. In a circuit containing capacitor, the alternating voltage
(a) leads the current by 90° (b) is in phase with current
(c) lags the current by 90° (d) none of these
27. D.C current cannot flow through a capacitor continuously because of the
presence of …..medium between the plates of the capacitor.
(a) a conducting (b) an insulating
(c) metallic (d) none
28. In a pure resistive A.C circuit, the instantaneous values of both voltage and
current are……..
(a) out of phase (b) in phase
(c) may and may not be in phase (d) all of the above

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29. The opposition offered by an inductor to the flow of an A.C is called……
(a) resistance (b) reactance of an inductor
(c) inductance of an inductor (d) all may be
30. The true condition of generating a “resonance frequency” by the combination
of inductive reactance XL and capacitive reactance Xc is…….
(a) XL>Xc (b) XL< XC
(c) XL = Xc (d) none
31. At resonance frequency, the current in R-L-C series circuit is…..
(a) maximum (b) minimum
(c) infinite (d) zero
32. The process of combining a low frequency signal with a high frequency radio
wave is called……
(a) rectification (b) amplification
(c) modulation (d) all
33. The highest value reached by the voltage or current in one cycle is called
its……
(a) peak value (b) top value
(c) instantaneous (d) average value
34. Reactance of a capacitor is given by…….
(a) Xc = 21f c (b) Xc= 1c
(c) a&b (d) none of these
35. The reactance of a coil changes directly with
(a) frequency of a.c (b) inductance
(c) a&b (d) none of these
36. A piece of wire along which charges are made to accelerate is known as….
(a) transmitting antenna (b) receiving antenna
(c) modulator (d) none of these
37. A choke coil is a coil with…..inductance and ……resistance.
(a) low , high (b) low , low
(c) high , high (d) high , negligible small
38. The process of mixing sound wave with carrier waves is called……..
(a) modulation (b) demodulation
(c) amplification (d) rectification
39. The peak value Vo and the root mean square value Vrms of an A.C .Voltage are
related by……….
Vo V0
(a) Vrms= 2 (b) Vrms = 2

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(c) Vrms = 1.4V0 (d) Vrms = 2V0
40. In which of the following components, there is no power dissipation in an
alternating current?
(a) pure resistance (b) pure capacitive
(c) both pure inductive & capacitive (d) pure inductive
41. The power factor…
(a) depends on the phase angle between voltage and current
(b) does not depend on the phase angle between voltage and current
(c) has its maximum value 2
(d) none of these
42. The range of the F.M is - - - - - - - A.M:
(a) Less than (b) greater than
(c) equal to (d) None
43. In an R-L-C parallel circuit at XL = XC , the flow of current is
(a) Become infinite (b) zero
(c) Maximum (d) small but not zero
44. The frequency of an alternating current in our homes is 50 Hz. It means that in
one second, a bulb will light
(a) up 50 times (b) off 50 times
(c) up 100 times (d) off 25 times
45. A voltmeter reads 220 V for an alternating source. The root mean square
value of this voltage is
(a) 220 V (b) 314 V
(c) 154 V (d) 110 V
46. An alternating voltage is given by V = 310 sin (100  t) volt. The peak value of
this voltage is
(a) 434 V (b) 310 V
(c) 620 V (d) 0

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Assignment 02
1. The frequency of alternating current is
I
(a) f  (b) f T
T
I I
(c) f  (d) f 
T T
2. The graph between time and voltage is known as
(a) Cosine curve (b) Sine curve
(c) Tangent curve (d) Varying curve
3. If 'f' be the frequency of rotation in time 't' then the angle '  ' through which the
coil is rotated is given by
2f
(a)   (b)   2 ft
t
t t
(c)   (d) 
2f 2f
4. The graph between voltage and time showing the variation between them is
known as waveform of
(a) Direct voltage (b) The rms voltage
(c) Peak voltage (d) Alternating voltage
5. The average value of current and voltage over a complete cycle is
(a) Positive (b) Maximum
(c) Zero (d) Zero nor maximum
6. If V0 is the peak value of A.C. voltage, its root mean square (rms) value is
V0
(a) Vrms  (b) Vrms  2 V0
2
V0 2
(c) Vrms  (d) Vrms 
2 V0
7. If I0 is the peak value of A.C. current, then the root mean square (rms) value of
current will be
I0 2
(a) I rms  (b) I rms 
2 I0
I0
(c) I rms  2 I0 (d) I rms 
2
8. If I0 is the peak value of A.C. current, its average value over the complete
cycle is

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Chapter - 16 Alternating current
I0
(a) 2 I0 (b)
2
I0
(c) (d) Zero
2
9. The basic circuit element in a D.C. circuit which controls the current or voltage
is
(a) Inductor (b) Resistor
(c) Capacitor (d) All of the above
10. In an A.C. circuit with resistor only, the current and voltage have a phase
angle of
(a) 1800 (b) 900
0
(c) 0 (d) 600
11. The rms value of the voltage is given as
(a) Vrms=0.505 V0 (b) Vrms=0.303 V0
(c) Vrms=0.252 V0 (d) Vrms=0.707 V0
12. An A.C. voltmeter reads 220 V, its peak value will be
(a) 225 volts (b) 340 volts
(c) 311.12 volts (d) 300 volts
13. The phase at the positive peak is
3
(a)  (b)
2

(c) 2 (d)
2
14. The device which allows only the continuous flow of an A.C. through a circuit
is
(a) Capacitor (b) Inductor
(c) D.C. motor (d) Battery
15. In an A.C. voltage, the power in the resistance is given by
V2
(a) P  VI (b) P
R
(c) P   R
2
(d) All of the above
16. Reactance of a capacitor connected across an A.C. source is also given by
1 1
(a) X c  (b) Xc 
c c
1 
(c) X c  2 2 (d) Xc 
c c
17. At higher frequency, the value of reactance of a capacitor in A.C. circuit is

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SMART PHYSICS Smart M.C.Q
(a) Small (b) Large
(c) Zero (d) Medium
18. 100 F capacitor is connected to an alternating voltage of 24V and frequency
50Hz. The reactance of the capacitor is
(a) 35 (b) 40
(c) 30.5 (d) 31.8
19. The opposition offered by the inductor to the follow of an A.C. is called
(a) Capacitance (b) Reactive resistance
(c) Inductive reactance (d) Impedance
20. The inductive reactance 'XL' of coil of inductance 'L' across an A.C. source is
given by
2f
(a)  L  2fL (b) L 
L
1 1
(c)  L  (d)
2fL 2fL
21. The inductive reactance 'XL' of inductor across an A.C. source is given by
(a) c (b) L

(c) (d) L
L
22. At low frequency of the alternating current, the current will be
(a) Small (b) Large
(c) Zero (d) Infinite
23. Power dissipated in a pure inductor is
(a) Large (b) Small
(c) Infinite (d) Zero
24. The ratio of the rms value of the applied voltage to the rms value of resulting
A.C. is
(a) Reactance (b) Resistance
(c) Impedance (d) Conductance
25. The impedance 'Z' can be expressed by

(a)   (b)   V
V
V
(c)    V
2
(d)

26. When 10V are applied to an A.C. circuit, the current flowing in it is 100 mA, its
impedance is
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(a) 50 (b) 75
(c) 100 (d) 90
27. The power dissipated in A.C. circuit is given by
(a)    
2
(b)   V sin 
(c)    L
2
(d)   V cos 
28. In the equation   V cos  , cos  is called
(a) Phase angle (b) Limiting angle
(c) Critical angle (d) Power factor
29. The A.C. circuit in which current and voltage are in phase, the power factor is
(a) 1 (b) Zero
(c) Infinity (d) 0.5
30. In R-L-C series circuit, resonance takes when
(a)  L   c (b) XL  Xc
(c) X L  X c (d) Xc  XL
31. The impedance of R-L-C series circuit resonance circuit at resonant frequency
is
(a) Less than R
(b) Greater than R
(c) Equal to R
(d) Sometimes smaller and sometimes greater than R
32. In R-L-C series circuit, the current at resonance frequency is
(a) Minimum (b) Zero
(c) Maximum (d) Infinite
33. If the value of C in a series RLC circuit is increased, the resonance frequency
(a) Increases (b) Decreases
(c) Remains the same (d) Becomes infinite
34. The resonant frequency 'fr' of parallel L – C circuit is
(a) f r  LC (b) f r  2 LC
 2
(c) f r  (d) fr 
2 LC LC
35. For L – C parallel resonance circuits, the current at resonance frequency is
(a) Maximum (b) Minimum
(c) Infinite (d) Zero
36. In a resonance circuit of frequency 1000 KHz with a inductor of 5 mH, the
capacitance will be required

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(a) 10 pF (b) 8 pF
(c) 3 pF (d) 5.09 pF
37. Voltage across only two lines of a three phase A.C. supply is about
(a) 220 V (b) 240 V
(c) 500 V (d) 400 V
38. In a three phase A.C. generator, if the first coil has phase 0, then the other two
coils will have phases
(a) 200 and 1400 (b) 1200 and 2400
(c) 1200 and 1600 (d) 200 and 1600
39. The electric field, magnetic field and the direction of propagation of
electromagnetic waves are mutually
(a) Parallel (b) Anti-parallel
(c) Orthogonal (d) Rectangular
40. In free space, the speed of electromagnetic waves is
(a) 3106 ms-1 (b) 4107 ms-1
(c) 3108 ms-1 (d) 31010 ms-1
41. Which of the following has greater frequency?
(a)   rays (b) X – rays
(c) Ultraviolet rays (d) Infrared rays
42. Electromagnetic waves transmit energy 'E' equal to
hf 1 2
(a) E  (b) E mv
c 2
(c) E  m0 c E  hf
2
(d)
43. The electromagnetic waves propagated out in space from antenna of a
transmitter are known as
(a)   rays (b) Cosmic rays
(c) Radio waves (d) Light waves
44. When electrons in a transmitting antenna vibrate 94000 times per second,
they produce radio waves having frequency
(a) 100 KHz (b) 120 KHz
(c) 80 KHz (d) 94 KHz
45. The frequency modulation (F.M.) transmission frequencies range from
(a) 540 KHz to 1600 KHz (b) 200 MHz to 320 MHz
(c) 50 MHz to 175 MHz (d) 88 MHz to 108 MHz

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46. Which one of the following represents velocity of carrier waves?
(a) 31010 ms-1 (b) 3108 ms-1
(c) 3106 ms-1 (d) 3109 ms-1
47. A sinusoidal current has rms (effective) value of 10A. Its maximum value is
(a) 7.77 A (b) 14.14 A
(c) 20 A (d) 10 A
48. Which of the following are electromagnetic waves?
(a) Sound waves (b) Water waves
(c) Light waves (d) Waves along a spring
49. Electromagnetic waves transport
(a) Charge (b) Current
(c) Wavelength (d) Energy
50. The minimum phase angle between V and I of R-L series circuit is
(a) 00 (b) 450
(c) 900 (d) 1800

Answers
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SMART PHYSICS Smart M.C.Q

Assignment 01
01 (c) 12 (c) 23 (b) 34 (c) 45 (a)
02 (b) 13 (b) 24 (b) 35 (c) 46 (b)
03 (c) 14 (b) 25 (c) 36 (a)
04 (a) 15 (b) 26 (c) 37 (d)
05 (a) 16 (d) 27 (b) 38 (a)
06 (b) 17 (a) 28 (b) 39 (b)

07 (d) 18 (c) 29 (b) 40 (c)

09 (a) 20 (d) 31 (a) 42 (a)

11 (b) 22 (b) 33 (a) 44 (c)

Assignment 02
01 c 12 c 23 d 34 c 45 d

02 b 13 d 24 b 35 b 46 b
03 b 14 a 25 c 36 d 47 b

04 d 15 d 26 c 37 d 48 c
05 c 16 b 27 d 38 b 49 d
06 a 17 a 28 d 39 c 50 c
07 d 18 d 29 a 40 c

08 d 19 c 30 c 41 a
09 b 20 a 31 c 42 d

10 c 21 b 32 c 43 c
11 d 22 a 33 b 44 d

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