2nd Year Physics Chapter 16 Alternating Current Notes MCQs Solutions

physics 12th notes mcqs chapter 16

1. A changing magnetic field creates

 
 
 
 

2. In three phase voltage across any two five lines is about

 
 
 
 

3. In a purely capacitive A.C circuit, the current is:

 
 
 
 

4. Alternating current can be produced by:

 
 
 
 

5. In electromagnetic waves, the electric and magnetic fields are:

 
 
 
 

6. Resonance frequency fr =

 
 
 
 

7. In an RLC circuit, the impedance is:

 
 
 
 

8. The waveform of alternating voltage is a:          

 
 
 
 

9. A capacitor is perfect insulator for

 
 
 
 

10. The purpose of choke in a fluorescent lamp is:

 
 
 
 

11. The effective value of any sinusoidal alternating current or voltage is defined as

 
 
 
 

12. The instantaneous value of the A.C. voltage is given by the relation

 
 
 
 

13. The inductive reactance of an inductor is given by

 
 
 
 

14. At resonance frequency the current in RLC series circuit is

 
 
 
 

15. An A.C. varies as a function of

 
 
 
 

16. The expression P = VI holds only when current and voltage are:

 
 
 
 

17. Energy in an inductance coil is stored in the form of:

 
 
 
 

18. The basic circuit element in a D.C circuit is:     

 
 
 
 

19. Maxwell’s equations were discovered by James Clark Maxwell in

 
 
 
 

20. The waveform of alternating voltage is the graph between:   

 
 
 
 

21. The root mean square value of the current is given as

 
 
 
 

22. If the peak value of A.C. voltage is 10√2 , then its root mean square value will be

 
 
 
 

23. Impedance is the combined effect of:

 
 
 
 

24. Alternating current is converted to direct current by:

 
 
 
 

25. In pure resistive A.C circuit, instantaneous voltage or current

 
 
 
 

26. The basic circuit elements in A.C circuits are:

 
 
 
 

27. current is opposed by

 
 
 
 

28. The sum of positive and negative peak values are usually written

 
 
 
 

29. When A.C voltage source is connected to a capacitor:

 
 
 
 

30. If I₀ is the peak value of A.C. current, its root mean square value

 
 
 
 

31. Power dissipated in pure conductor is:

 
 
 
 

32. A pure inductive coil is that which has:             

 
 
 
 

33. At resonance the value of current in RLC series circuit is

 
 
 
 

34. The time during which the voltage sources changes its polarity once is called:

 
 
 
 

35. Ohm is the unit of

 
 
 
 

36. The slope of q-t curve at any instant of time gives:

 
 
 
 

37. Impedance is composed of

 
 
 
 

38. In three phase A.C supply coils are inclined at an angle of

 
 
 
 

39. When XL= XCthis condition is called

 
 
 
 

40. As series resonance in L.C.R circuit, the impedance is equal to:

 
 
 
 

41. Choke consumes extremely small:

 
 
 
 

42. In which of the following, the loss of energy is less:

 
 
 
 

43. With increase of frequency of A.C supply, the inductive reactance is:

 
 
 
 

44. In L.C parallel circuit, the coil draws:                            

 
 
 
 

45. The inductive reactance of an inductor is given by

 
 
 
 

46. The power dissipated in a pure inductive or capacitive circuit is

 
 
 
 

47. If Vᵣₘₛ=10√2 then peak voltage V₀ is

 
 
 
 

48. The reactance of an inductor at 50 Hz is 10Ω its reactance at 100 Hz becomes

 
 
 
 

49. If the frequency of AC supplied is doubled then the capacitor reactance becomes

 
 
 
 

50. To maintain the current in an inductor, the applied alternating voltage must be:

 
 
 
 

51. In case of capacitor, S.I. unit of reactance is

 
 
 
 

52. The low frequency signal used for modulation is called:

 
 
 
 

53. The velocity of an oscillating charge as it moves to and fro along a wire is:

 
 
 
 

54. Main reason for worldwide use of A.C is:

 
 
 
 

55. The current which keeps on reversing its direction with time is

 
 
 
 

56. Who proved that light waves are electromagnetic?

 
 
 
 

57. The frequency of A.C used in Pakistan is

 
 
 
 

58. Electromagnetic waves consist of

 
 
 
 

59. In RLC series circuit the condition for resonance is

 
 
 
 

60. The frequency of the range 20-20000 Hz is

 
 
 
 

61. The most common source of A.C voltage is:    

 
 
 
 

62. The unit of impedance is:

 
 
 
 

63. The inductive reactance of an inductor is given by

 
 
 
 

64. A.C can be measure with the help of:

 
 
 
 

65. The measure of the opposition offered by a capacitor to the flow of A.C is called:

 
 
 
 

66. Modulation is the process of:

 
 
 
 

67. The waves which do not require any material medium for their
propagation are called

 
 
 
 

68. In comparison to D.C transmission losses in A.C are:

 
 
 
 

69. The highest value reached by the voltage or current in one cycle is known as:

 
 
 
 

70. A device which opposes the flow of A.C only is 

 
 
 
 

71. The wave form of alternating voltage is a:

 
 
 
 

72. Modulation is achieved by changing the:

 
 
 
 

73. The phase angle at +ve (positive) peak is 

 
 
 
 

74. The combined effect of resistance and reactance in a circuit is

 
 
 
 

75. A device which allows only the continuous flow of an A.C., through a circuit is:

 
 
 
 

76. At high frequency RLC series circuit shows the behavior of

 
 
 
 

77. At high frequency the value of reactance of capacitor will be

 
 
 
 

78. Electromagnetic waves transmitted form an antenna are

 
 
 
 

79. An inductor may store energy in its:                  

 
 
 
 

80. When A.C voltage is applied to an inductor, the:

 
 
 
 

81. The phase difference between current and voltage in an Inductive circuit is

 
 
 
 

82. The average value of alternating voltage over a complete cycle is:

 
 
 
 

83. In A.C circuits, current and voltage is controlled by:

 
 
 
 

84. The capacitive reactance to pure DC is 

 
 
 
 

85. When an inductor comes close to a metallic object, its inductance is:

 
 
 
 

86. Which current can pass through a capacitor continuously:

 
 
 
 

87. In pure capacitor Ac circuit, the current I and q are

 
 
 
 

88. In tuning circuit if capacitance is doubled and inductance is halved then its frequency

 
 
 
 

89. The electromagnetic waves propagated out in space from antenna of a transmitter are known as

 
 
 
 

90. Alternating current is produced by a voltage source which polarity:

 
 
 
 

91. Electromagnetic weaves transport

 
 
 
 

92. When voltage V and current I are in phase the power is expressed as:

 
 
 
 

93. In an A.C. circuit with capacitor only the current

 
 
 
 

94. In an electromagnetic wave, the electric and magnetic fields are

 
 
 
 

95. During each cycle AC voltage reaches a peak value

 
 
 
 

96. In a three phase A.C. supply the phase difference in voltage of any two phases

 
 
 
 

97. Resistance of choke is:

 
 
 
 

98. A choke is preferred to a capacitor to decrease the A.C in a circuit because the capacitor:

 
 
 
 

99. The electromagnetic spectrum contains:

 
 
 
 

100. Metal detector consist of

 
 
 
 

101. The alternating voltage or current is actually measured by:

 
 
 
 

102. With high frequencies, capacitive reactance:    

 
 
 
 

103. Three phase supply also provides:                     

 
 
 
 

104. Which of the following statement is correct for an A.C circuit:

 
 
 
 

105. A.C varies as function of:

 
 
 
 

106. Resonance frequency of parallel resonance circuit is fr =

 
 
 
 

107. For q – t graph, slope shows:                   

 
 
 
 

108. The instantaneous value of the A.C. voltage is given by the relation

 
 
 
 

109. When resistance is increased in a series LCR circuit:

 
 
 
 

110. Component of circuit which controls current

 
 
 
 

111. Electrical resonance in an LCR A.C circuit is very sharp if:

 
 
 
 

112. Alternating current is one which changes in a:

 
 
 
 

113. The process of combining low frequency signal with high frequency radio waves is called:

 
 
 
 

114. A sinusoidal current has rms value of 10A. its maximum value is

 
 
 
 

115. At resonance frequency the impedance of RLC parallel circuit is

 
 
 
 

116. Which of the following is not electromagnetic wave in nature

 
 
 
 

117. The reactance is the ratio of:

 
 
 
 

118. through inductor, the applied voltage:

 
 
 
 

119. The root mean square value of the current is given as

 
 
 
 

120. In A.C circuits, the A.C instruments indicate:   

 
 
 
 

121. The SI unit of reactance is:

 
 
 
 

122. Pure choke consumes:                 

 
 
 
 

123. A device that allows only the continuous flow of AC through a circuit is:

 
 
 
 

124. The instantaneous voltage across a pure inductance is:

 
 
 
 

125. The direction of propagation of an electromagnetic waves is

 
 
 
 

126. The reciprocal of impedance is called:               

 
 
 
 

127. Alternating current or emf measuring instruments measures its:

 
 
 
 

128. If frequency fis in Hz and inductance L is in milli henry, then XL is in:

 
 
 
 

129. Power factor is defined by:                      

 
 
 
 

130. An A.C Voltmeter reads 220V, its peak value will be

 
 
 
 

131. At resonance RLC series circuit shows the behavior of

 
 
 
 

132. The magnitude of alternating voltage is:

 
 
 
 

133. Alternating current can be measured by D.C ammeter because if:

 
 
 
 

134. In an A.C. circuit with resistor only, the current and voltage have a phase angle of

 
 
 
 

135. Voltage drop in A.C circuit is the product of current and:

 
 
 
 

136. The amplitude modulation A.M transmission frequencies range from:

 
 
 
 

137. 100 F capacitor is connects to an AC-voltage of 24 V and frequency 50Hz. The reactance of the capacitor is

 
 
 
 

138. If frequency f is in Hz and capacitance C is in F then the unit of Xc is:

 
 
 
 

139. 100µF capacitor is connected to an alternating voltage of 24V and
frequency 50Hz. The reactance of the capacitor is

 
 
 
 

140. The graph between time and voltage is:            

 
 
 
 

141. At resonance frequency, the impedance of an R.L.C series circuit is:

 
 
 
 

142. The amplitude of the effective current is:

 
 
 
 

143. A.C voltmeter measures:              

 
 
 
 

144. In capacitor

 
 
 
 

145. In modulation, low frequency signal is known as:

 
 
 
 

146. Average value of current and voltage over a complete cycle is

 
 
 
 

147. The reactance of I farad capacitance when connected to D.C circuit is:

 
 
 
 

148. The electromagnetic waves travel in space with speed of

 
 
 
 

149. Power factor is equal to:

 
 
 
 

150. Resonance frequency of series resonance circuit is fr =

 
 
 
 

151. Alternating voltage is:

 
 
 
 

152. In Pakistan, the frequency of A.C supply is

 
 
 
 

153. For parallel resonant circuit, the resonance current is:

 
 
 
 

154. In a resistive A.C circuit, instantaneous values of voltage and current are:

 
 
 
 

155. If V₀is the peak value of A.C. voltage, its root mean square value

 
 
 
 

156. The reactance of a coil changes directly with

 
 
 
 

157. When we accelerate the charges, which type of waves are produced

 
 
 
 

158. AC source used in Pakistan is:

 
 
 
 

159. The circuit in which current and voltage are in phase, the power factor is:

 
 
 
 

160. If the capacitive reactance of AC circuit is made four times then the frequency of the circuit becomes:

 
 
 
 

161. With increase in frequency of an A.C supply, the impedance of LCR series circuit:

 
 
 
 

162. When electrons in a transmitting antenna vibrate 94000 times
each second, they produce radio waves having frequency

 
 
 
 

163. Which of the following are electromagnetic waves

 
 
 
 

164. Inductive reactance (DL of a coil is expressed in:        

 
 
 
 

165. A capacitor is a perfect insulator for:                 

 
 
 
 

166. Which consumes small power?

 
 
 
 

167. Power dissipation in a pure inductive or in a pure capacitance circuit is:

 
 
 
 

168. S.I. unit of impedance is

 
 
 
 

169. A changing electric flux creates a

 
 
 
 

170. At high frequency, the current through a capacitor is:

 
 
 
 

171. In frequency modulation, the amplitude of carrier waves is:

 
 
 
 

172. If I₀is the peak value of A.C. current, then average value of current

 
 
 
 

173. The condition of resonance in an R.L.C series circuit is that:

 
 
 
 

174. At what frequency will an inductor of 0 H have a reactance of 500?

 
 
 
 

175. Capacitor will have a large reactance at

 
 
 
 

176. Radio frequency choke is:           

 
 
 
 

177. The peak value of alternating voltage is 423 volts, its rms value is:

 
 
 
 

178. At what frequency I henry inductance offer same impendance as IF capacitor:

 
 
 
 

179. Electromagnetic waves do not transport

 
 
 
 

180. The minimum phase angle between V and I of RL-series circuit is

 
 
 
 

181. The reactance of a coil depends upon:  

 
 
 
 

182. As a result of modulation, the resultant wave is known as:

 
 
 
 

183. One complete set of positive and negative value of alternating quantities is called:

 
 
 
 

184. The highest value reached by voltage or current in one cycle is called

 
 
 
 

185. The positive value of current and voltage over a cycle is:

 
 
 
 

186. At resonance frequency in an R.L.C series circuit, VL and Vc:

 
 
 
 

187. The current which changes its direction through the circuit for a complete cycle is called

 
 
 
 

188. The opposition offered by the inductor to the flow of A.C. is called

 
 
 
 

189. Phase difference between V and I of an A C through resistor is:

 
 
 
 

190. In free space the speed of electromagnetic waves is

 
 
 
 

191. Direct current cannot flow through

 
 
 
 

192. Which of the following waves do not travel with speed of light

 
 
 
 

193. For modulation purpose, high frequency radio waves are called:

 
 
 
 

194. The RMS value of alternating voltage is:

 
 
 
 

195. The most common source of alternating current is

 
 
 
 

196. The alternating current can be transmitted:                

 
 
 
 

Short Questions

1. A circuit an iron – cored inductor a switch and a D.C source arranged in series.The switch is closed and after an interval reopened.Explain why a spark jumps across the switch contacts?
2. A sinusoidal current has rms value of 10 A.What is the maximum or peak value?
3. An inductor draws 30 ampers when connected across the terminals of generator A and 6 ampers when connected to B.Assuming the same impressed voltage can you tell which of the two generators A or B is an A.C machine?Give reasons for your answer?
4. Can electrolysis proceed with the help of an A.C source ?
5. Define alternating current (A.C) ?
6. Define direct current (D.C) ?
7. Define impedance and write its unit ?
8. Define impedence of a circuit and give its unit?
9. Define inductive reactance ?
10. Define peak value of voltage or current ?
11. Define power ?
12. Define power factor in A.C?
13. Define root mean square value (i.e. r.m.s) ?
14. Define Root Mean square value of A.C. Voltage?
15. Define time period of an alternating voltage ?
16. Describe some advantages of 3- phase A.C.supply?
17. Describe some important properties of a series resonant circuit?
18. Describe some important properties of parallel resonant circuit?
19. Describe the condition under which the reactance of a capacitor will be small ?
20. Describe the condition which will make the reactance small?
21. Describe the principle upon which the electromagnetic waves can be generated?
22. Describe the types of modulation?
23. Differentiate between Peak value and Peak to Peak value?
24. Explain the conditions under which electromagnetic waves are produced from source?
25. Explain the term peak to peak value ?
26. How an A.C flows through a capacitor ?
27. How are the radio waves received?
28. How current and voltage behave in a R-L series circuit ?
29. How current and voltage behave in an inductive circuit ?
30. How does doubling the frequency affect the reactance of (a) an inductance (b) a capacitor?
31. How many time per second will an inductance lamp reach maximum brilliance when connected to a 50Hz source?
32. How series circuit ?
33. How the out put voltage of an alternating current generator varies with time ?
34. How the radio waves are received ?
35. How the receptian of a particular radio station is selected on your radio set?
36. If ‘T’ is the time period of an A.C then find the frequency of an A.C ?
37. In a R- L circuit will the current lag or lead the voltage?IIIustrate your answer by a vector diagram?
38. Name a device that will (a) permit the flow of direct current but not the alternating current (b) permit alternatig current but not the direct current?
39. Name the device that will (a) permit flow of direct current but oppose the flow of alternating current.(b) permit flow of alternating current but not the direct current?
40. Name the type of modulation ?
41. Show that resonance at the resonance frequency is given as fr= 1/2πLC?
42. What are conditions for representing A.C voltage or current by vector diagram?
43. What are electromagnetic waves ?
44. What are metal detectors ?
45. What are the basic controlling elements of an A.C circuit ?
46. What are the frequencies range for an amplitude modulation transmission ?
47. What are the frequencies range for frequency modulation transmission ?
48. What are the magnitude of Vᵣₘₛ and Iᵣₘₛ respectively ?
49. What do you mean by instantaneous value ?
50. What do you mean by phase of an A.C ?
51. What do you understand about three phases A.C supply?
52. What do you understand by electromagnetic waves?
53. What do you understand by inductive reactance?
54. What do you understand by modulation and modulated carrier wave?
55. What is a choke?
56. What is a choke?Why it is used in A.C circuits?
57. What is a modulation signal ?
58. What is a power factor in an A.C circuit ?
59. What is a R-C series circuit ?
60. What is a resonance circuit ?
61. What is a R-L series circuit ?
62. What is an inductor ?
63. What is choke ?
64. What is impedance?Write its unit?
65. What is meant by A.M and F. M?
66. What is meant by electrical inertia ?
67. What is meant by frequency modulation (F.M.) ?
68. What is meant by impedence?Explain briefly.
69. What is modulation ?
70. What is peak value?
71. What is reactance of a capacitor ?
72. What is reactance?
73. What is resonance frequency of a R-L-C series circuit ?
74. What is the circuit impedance at the resonance frequency ?
75. What is the condition of resonance in a parallel resonance circuit?
76. What is the most common source of alternating voltage ?
77. What is the phase on the A.C wave form at the negative peak ?
78. What is the phase on the A.C waveform at the positive peak ?
79. What is the power through a resistor when both the current and voltage are in phase ?
80. What is the quantity of charge stored on the plates of a capacitor at any istant ?
81. What is the shape of graph for alternating voltage and time ?
82. What is the source of transmitting electromagnetic waves?
83. What is the speed of electromagnetic waves in free space ?
84. What is the type of a circuit used in metal détector ?
85. What is the use of inductor coil ?
86. What the net potential difference alternating voltage of an R-L-C series circuit ?
87. When electromagnetic discovered ?
88. Whether electromagnetic waves require a medium for transmission ?
89. Which apparatus electromagnetic waves ?
90. Why D.C can not flow through a capacitor ?
91. Write some important properties of a series resonance circuit ?
92. Write some important properties of parallel resonant circuit ?
93. Write the equation of impedance of a R-L series circuit ?

Long Questions

1. Define alternating current. Also explain time period and waveform of alternating current.
2. What is the root mean square value of alternating current?
3. Define phase of A.C. Define phase lag and phase head.
4. Describe A.C. through a capacitor.
5. Describe A.C. through an inductor.
6. Explain R.L series circuit.
7. Explain series resonance circuit also discuss its properties.
8. What is parallel resonance circuit? Describe its properties. Also find the resonance frequency for the circuit.
9. Describe the principle of metal detectors.
10. What are the electromagnetic waves?
“11. Explain the principle of generation, transmission and reception of electromagnetic waves with an example. “
12. Explain modulation with its types.
13. An alternating current is represented by the equation I = 20 sin 100 πt. Compute frequency and the maximum and r.m.s values of current.
14. A sinusoidal A.C. has a maximum value of 15 A. What are its r.m.s. values? If the time is recorded from the instant the current is zero and is becoming positive, what is the instantaneous value of the current after 1/300 s, given the frequency is 50 Hz.
15. Find the value of current and inductive reactance when A.C voltage of 220 V at 50 Hz is passed through an inductor of 10 H.
16. A circuit has an inductance of 1/π H and resistance of 2000 ohm. A 50 Hz A.C. is supplied to it. Calculate the reactance and impedance offered by the circuit.
17. An inductor of pure inductance 3/π H is connected in series with a resistance of 40 ohm. Find (i) the peak value of the current (ii) the r.m.s value, and (iii) the phase difference between the current and the applied voltage V = 350 sin (100 π t).
18. A 10mH, 20 ohm coil is connected across 240 V and 180/π Hz source. How much power does it dissipate?
19. Find the value of the current flowing through a capacitance 0.5 micro F when connected to a source of 150 V at 50 Hz.
20. An alternating source e.m.f 12 V and frequency 50 Hz is applied to a capacitor of capacitance 3 micro F in series with a resistor of resistance 1 ohm. Calculate the phase angle.
21. What is the resonant frequency of a circuit which included a coil of inductance 2.5 H and a capacitance 40 micro F?
22. An inductor of inductance 150 micro F is parallel with a variable capacitor whose capacitance can be changed from 500 pico F to 20 pico F. Calculate the maximum frequency and minimum frequency for which the circuit can be tuned.

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