Tiêu đề 23. Alternating Current Easy Ans.pdf ✅

1. The power is transmitted from a power house on high voltage ac because (a) Electric current travels faster at higher volts (b) It is more economical due to less power wastage (c) It is difficult to generate power at low voltage (d) Chances of stealing transmission lines are minimized 2. The potential difference V and the current i flowing through an instrument in an ac circuit of frequency f are given by V = 5 cos t volts and I = 2 sin t amperes (where  = 2f). The power dissipated in the instrument is (a) Zero (b) 10 W (c) 5 W (d) 2.5 W 3. In an ac circuit, V and I are given by V = 100 sin (100 t) volts, I t  mA      = + 3 100 sin 100  . The power dissipated in circuit is (a) 104 watt (b) 10 watt (c) 2.5 watt (d) 5 watt 4. Alternating current can not be measured by dc ammeter because (a) ac cannot pass through dc ammeter (b) Average value of complete cycle is zero (c) ac is virtual (d) ac changes its direction 5. The resistance of a coil for dc is in ohms. In ac, the resistance (a) Will remain same (b) Will increase (c) Will decrease (d) Will be zero 6. If instantaneous current is given by i = 4 cos( t +) amperes, then the r.m.s. value of current is (a) 4 amperes (b) 2 2 amperes (c) 4 2 amperes (d) Zero amperes 7. In an ac circuit, peak value of voltage is 423 volts. Its effective voltage is (a) 400 volts (b) 323 volts (c) 300 volts (d) 340 volts 8. In an ac circuit I = 100 sin 200 t. The time required for the current to achieve its peak value will be (a) sec 100 1 (b) sec 200 1 (c) sec 300 1 (d) sec 400 1 9. The peak value of an Alternating current is 6 amp, then r.m.s. value of current will be (a) 3 A (b) 3 3 A (c) 3 2 A (d) 2 3 A 10. A generator produces a voltage that is given by V = 240 sin120 t , where t is in seconds. The frequency and r.m.s. voltage are (a) 60 Hz and 240 V (b) 19 Hz and 120 V (c) 19 Hz and 170 V (d) 754 Hz and 70 V 11. If E0 represents the peak value of the voltage in an ac circuit, the r.m.s. value of the voltage will be (a)  E0 (b) 2 E0 (c)  E0 (d) 2 E0 12. The peak value of 220 volts of ac mains is (a) 155.6 volts (b) 220.0 volts (c) 311.0 volts (d) 440 volts 13. A sinusoidal ac current flows through a resistor of resistance R. If the peak current is p I , then the power dissipated is (a) cos 2 I pR (b) I pR 2 2 1 (c) I pR 4 2  (d) I pR 1 2  14. A 40  electric heater is connected to a 200 V, 50 Hz mains supply. The peak value of electric current flowing in the circuit is approximately (a) 2.5 A (b) 5.0 A (c) 7 A (d) 10 A 15. The frequency of ac mains in India is (a) 30 c/s or Hz (b) 50 c/s or Hz (c) 60 c/s or Hz (d) 120 c/s or Hz 16. The r.m.s. value of an ac of 50 Hz is 10 amp. The time taken by the alternating current in reaching from zero to maximum value and the peak value of current will be (a) 2  10–2 sec and 14.14 amp (b) 1  10–2 sec and 7.07 amp (c) 5  10–3 sec and 7.07 amp (d) 5  10–3 sec and 14.14 amp 17. The root mean square value of the alternating current is equal to (a) Twice the peak value (b) Half the peak value
(c) 2 1 times the peak value (d) Equal to the peak value 18. The peak value of an alternating e.m.f. E is given by E E cos t = 0 is 10 volts and its frequency is 50 Hz. At time t sec 600 1 = , the instantaneous e.m.f. is (a) 10 V (b) 5 3 V (c) 5 V (d) 1 V 19. If a current I given by       − 2 0 sin  I  t flows in an ac circuit across which an ac potential of E E sin t = 0 has been applied, then the power consumption P in the circuit will be (a) 2 0 0 E I P = (b) 2 0 0 P = E I (c) 2 0 0 E I P = (d) P = 0 20. In an ac circuit, the instantaneous values of e.m.f. and current are e = 200 sin 314 t volt and       = + 3 sin 314  i t ampere. The average power consumed in watt is (a) 200 (b) 100 (c) 50 (d) 25 21. An ac generator produced an output voltage E = 170 sin 377 t volts , where t is in seconds. The frequency of ac voltage is (a) 50 Hz (b) 110 Hz (c) 60 Hz (d) 230 Hz 22. In general in an alternating current circuit (a) The average value of current is zero (b) The average value of square of the current is zero (c) Average power dissipation is zero (d) The phase difference between voltage and current is zero 23. An alternating current is given by the equation i i cos t i sin t = 1 + 2 . The r.m.s. current is given by (a) ( ) 2 1 1 2 i + i (b) 2 2 ( ) 2 1 i i i + (c) 2 1 / 2 2 2 1 ( ) 2 1 i + i (d) 2 1 / 2 2 2 1 ( ) 2 1 i + i 24. In an ac circuit, the current is given by       = − 2 5 sin 100  i t and the ac potential is V = 200 sin(100 )volt. Then the power consumption is (a) 20 watts (b) 40 watts (c) 1000 watts (d) 0 watt 25. An electric lamp is connected to 220 V, 50 Hz supply. Then the peak value of voltage is (a) 210 V (b) 211 V (c) 311 V (d) 320 V 26. In a circuit, the value of the alternating current is measured by hot wire ammeter as 10 ampere. Its peak value will be (a) 10 A (b) 20 A (c) 14.14 A (d) 7.07 A 27. The voltage of domestic ac is 220 volt. What does this represent (a) Mean voltage (b) Peak voltage (c) Root mean voltage (d) Root mean square voltage 28. The r.m.s. voltage of domestic electricity supply is 220 volt . Electrical appliances should be designed to withstand an instantaneous voltage of (a) 220 V (b) 310 V (c) 330 V (d) 440 V 29. The process by which ac is converted into dc is known as (b) Purification (b) Amplification (c) Rectification (d) Current amplification 30. In an ac circuit with voltage V and current I, the power dissipated is (a) VI (b) VI 2 1 (c) VI 2 1 (d) Depends on the phase between V and I 31. For an ac circuit V = 15 sint and I = 20 cost the average power consumed in this circuit is (a) 300 Watt (b) 150 Watt (c) 75 Watt (d) zero 32. A bulb is connected first with dc and then ac of same voltage then it will shine brightly with (a) AC (b) DC (c) Brightness will be in ratio 1/1.4 (d) Equally with both 33. An ac supply gives 30 V r.m.s. which passes through a 10  resistance. The power dissipated in it is
(a) 90 2 W (b) 90 W (c) 45 2 W (d) 45 W 34. The frequency of an alternating voltage is 50 cycles/sec and its amplitude is 120V. Then the r.m.s. value of voltage is (a) 101.3V (b) 84.8V (c) 70.7V (d) 56.5V 35. A resistance of 20 ohms is connected to a source of an alternating potential V = 220 sin(100 t) . The time taken by the current to change from its peak value to r.m.s value is (a) 0.2 sec (b) 0.25 sec (c) 3 25 10 −  sec (d) 3 2.5 10 −  sec 36. Voltage and current in an ac circuit are given by       = − 6 5 sin 100  V t and       = + 6 4 sin 100  I t (a) Voltage leads the current by o 30 (b) Current leads the voltage by o 30 (c) Current leads the voltage by o 60 (d) Voltage leads the current by o 60 37. If an ac main supply is given to be 220 V. What would be the average e.m.f. during a positive half cycle (a) 198V (b) 386V (c) 256V (d) None of these 38. In an ac circuit, the r.m.s. value of current, Irms is related to the peak current, I0 by the relation (a) 0 1 I I rms  = (b) 0 2 1 I I rms = (c) 2 0 I I rms = (d) 0 I I rms =  39. An alternating voltage is represented as E = 20 sin300 t. The average value of voltage over one cycle will be (a) Zero (b) 10 volt (c) 20 2 volt (d) 2 20 volt 40. The ratio of peak value and r.m.s value of an alternating current is (a) 1 (b) 2 1 (c) 2 (d) 1 / 2 41. A 280 ohm electric bulb is connected to 200V electric line. The peak value of current in the bulb will be (a) About one ampere (b) Zero (c) About two ampere (d) About four ampere 42. An ac source is rated at 220V, 50 Hz. The time taken for voltage to change from its peak value to zero is (a) 50 sec (b) 0.02 sec (c) 5 sec (d) 3 5 10 −  sec 43. If the value of potential in an ac, circuit is 10V, then the peak value of potential is (a) 2 10 (b) 10 2 (c) 20 2 (d) 2 20 44. A lamp consumes only 50% of peak power in an a.c. circuit. What is the phase difference between the applied voltage and the circuit current (a) 6  (b) 3  (c) 4  (d) 2  45. If an alternating voltage is represented as E = 141 sin(628 t), then the rms value of the voltage and the frequency are respectively (a) 141V, 628 Hz (b) 100 V, 50 Hz (c) 100 V,100 Hz (d) 141V, 100 Hz 46. The maximum value of a.c. voltage in a circuit is 707V. Its rms value is (a) 70.7 V (b) 100 V (c) 500 V (d) 707 V 47. Choke coil works on the principle of (a) Transient current (b) Self induction (c) Mutual induction (d) Wattless current 48. A choke coil has (a) High inductance and low resistance (b) Low inductance and high resistance (c) High inductance and high resistance (d) Low inductance and low resistance 49. Choke coil is used to control (a) ac (b) dc (c) Both ac and dc (d) Neither ac nor dc 50. Current in the circuit is wattless, if (a) Inductance in the circuit is zero (b) Resistance in the circuit is zero (c) Current is alternating (d) Resistance and inductance both are zero 51. The phase angle between e.m.f. and current in LCR series ac circuit is
(a) 0 to  / 2 (b)  / 4 (c)  / 2 (d)  52. A choke coil is preferred to a rheostat in ac circuit as (a) It consumes almost zero power (b) It increases current (c) It increases power (d) It increases voltage 53. An alternating e.m.f. is applied to purely capacitive circuit. The phase relation between e.m.f. and current flowing in the circuit is or In a circuit containing capacitance only (a) e.m.f. is ahead of current by  / 2 (b) Current is ahead of e.m.f. by  / 2 (c) Current lags behind e.m.f. by  (d) Current is ahead of e.m.f. by  54. An ac source is connected to a resistive circuits. Which of the following is true (a) Current leads the voltage and both are in same phase (b) Current lags behind the voltage and both are in same phase (c) Current and voltage are in same phase (d) Any of the above may be true depending upon the value of resistance 55. The average power dissipated in a pure inductor of inductance L when an ac current is passing through it, is (a) 2 2 1 LI (b) 2 4 1 LI (c) 2 2 Li (d) Zero 56. An alternating current of frequency ' f ' is flowing in a circuit containing a resistance R and a choke L in series. The impedance of this circuit is (a) R + 2fL (b) 2 2 2 2 R + 4 f L (c) 2 2 R + L (d) R 2fL 2 + 57. A resonant ac circuit contains a capacitor of capacitance F 6 10 − and an inductor of 10 . 4 H − The frequency of electrical oscillations will be (a) Hz 5 10 (b) 10 Hz (c) Hz 2 10 5 (d) Hz 2 10 58. Power delivered by the source of the circuit becomes maximum, when (a) L = C (b) C L   1 = (c) 2 1       = − C L   (d) L = C 59. An alternating voltage is connected in series with a resistance R and an inductance L If the potential drop across the resistance is 200 V and across the inductance is 150 V, then the applied voltage is (a) 350 V (b) 250 V (c) 500 V (d) 300 V 60. An inductive circuit contains resistance of 10  and an inductance of 20 H. If an ac voltage of 120 V and frequency 60 Hz is applied to this circuit, the current would be nearly (a) 0.32 amp (b) 0.016 amp (c) 0.48 amp (d) 0.80 amp 61. Same current is flowing in two alternating circuits. The first circuit contains only inductance and the other contains only a capacitor. If the frequency of the e.m.f. of ac is increased, the effect on the value of the current will be (a) Increases in the first circuit and decreases in the other (b) Increases in both the circuits (c) Decreases in both the circuits (d) Decreases in the first circuit and increases in the other 62. A capacitor is a perfect insulator for (a) Alternating currents (b) Direct currents (c) Both ac and dc (d) None of these 63. In a circuit containing an inductance of zero resistance, the e.m.f. of the applied ac voltage leads the current by (a) 90o (b) 45o (c) 30o (d) 0 o 64. In a pure inductive circuit or In an ac circuit containing inductance only, the current (a) Leads the e.m.f. by 90o (b) Lags behind the e.m.f. by 90o (c) Sometimes leads and sometime lags behind the e.m.f. (d) Is in phase with the e.m.f. 65. A 20 volts ac is applied to a circuit consisting of a resistance and a coil with negligible resistance. If the voltage across the resistance is 12 V, the voltage across the coil is (a) 16 volts (b) 10 volts (c) 8 volts (d) 6 volts 66. A resistance of 300  and an inductance of  1 henry are connected in series to a ac voltage of 20 volts and 200 Hz frequency. The phase angle between the voltage and current is