Tiêu đề ALTERNATING CURRENT-1.pdf ✅

CHAPTER 7 ALTERNATING CURRENT Exercise 1: NCERT Based Topic-wise MCQs 7.1 INTRODUCTION 1. Alternating current cannot be measured by D.C. ammeter, because NCERT Page-233 / N-177 (a) A.C. is virtual (b) A.C. changes its direction (c) A.C. cannot pass through D.C. ammeter (d) average value of A.C for complete cycle is zero 7.2 AC VOLTAGE APPLIED TO A RESISTOR 2. The r.m.s. value of potential difference V shown in the figure is NCERT Page-234 / N-180 (a) V0 (b) V0/√2 (c) V0/2 (d) V0/√3 3. Determine the rms value of the emf given by E( in volt ) = 8sin (ωt) + 6sin (2ωt) NCERT Page-234 / N-180 (a) 5√2V (b) 7√2V (c) 10V (d) 10√2V 4. The alternating current of equivalent value of I0 √2 is NCERT Page-236 / N-179 (a) peak current (b) r.m.s. current (c) D.C. current (d) all of these 5. The peak value of the a.c. current flowing through a resistor is given by NCERT Page-234 / N-179 (a) I0 = e0/R (b) I = e/R
(c) I0 = e0 (d) I0 = R/e0 6. The sinusoidal A.C. current flows through a resistor of resistance R. If the peak current is Ip, then power dissipated is (a) Ip 2Rcos θ NCERT Page-235 / N-179 (b) 1 2 Ip 2R (c) 4 π Ip 2R (d) 1 π2 Ip 2R 7. The r.m.s value of an a.c. of 50Hz is 10amp. The time taken by the alternating current in reaching from zero to maximum value and the peak value of current will be NCERT Page-236 / N-179 (a) 2 × 10−2 sec and 14.14amp (b) 1 × 10−2 sec and 7.07amp (c) 5 × 10−3 sec and 7.07amp (d) 5 × 10−3 sec and 14.14amp 8. An alternating current is given by i = i1cos ωt + i2sin ωt The rms current is given by (a) i1+i2 √2 NCERT Page-236 / N-180 (b) |i1+i2| √2 (c) √ i1 2+i2 2 2 (d) √ i1 2+i2 2 √2 9. The voltage of an ac supply varies with time (t) as V = 120sin 100πtcos 100πt. The maximum voltage and frequency respectively are NCERT Page-236 / N-180 (a) 120 volt, 100Hz (b) 120 √2 volt, 100Hz (c) 60 volt, 200Hz (d) 60 volt, 100Hz 10. A resistance of 40Ω is connected to a source of alternating current rated 220V, 50Hz. Find the time taken by the current to change from its maximum value to rms value : NCERT Page-236 / N-180 (a) 2.5ms (b) 1.25ms (c) 2.5s (d) 0.25s 11. A resistance of 20ohm is connected to a source of an alternating potential V = 200cos (100πt). The time taken by the current to change from its peak value to rms value is NCERT Page-234 / N-178 (a) 2.5 × 10−3 s (b) 25 × 10−3 s (c) 0.25s (d) 0.20s 12. Current in an ac circuit is given by i = 3sin ωt + 4cos ωt then NCERT Page-236 / N-180 (a) rms value of current is 5 A (b) mean value of this current in one half period will be 6/π (c) if voltage applied is V = Vmsin ωt then the circuit must be containing resistance and capacitance. (d) if voltage applied is V = Vm sin ωt, the circuit may contain resistance and inductance. 13. An ac voltage is represented by E = 220√2cos (50π)t NCERT Page-236 / N-180
How many times will the current become zero in 1s ? (a) 50 times (b) 100 times (c) 30 times (d) 25 times 14. A group of electric lamps having a total power rating of 1000 watt is supplied by an AC voltage E = 200sin(310t + 60∘ ). Then the rms value of the circuit current is NCERT Page-236 / N-180 (a) 10A (b) 10√2A (c) 20A (d) 20√2A 15. The equation of alternating current is : NCERT Page-236 / N-180 I = 50√2sin 400πt amp. Then the frequency and root mean square of current are respectively (a) 200Hz, 50amp (b) 400πHz, 50√2amp (c) 200Hz, 50√2amp (d) 50Hz, 200amp 16. The current flowing through an ac circuit is given by I = 5sin (120πt)A How long will the current take to reach the peak value starting from zero? NCERT Page-236 / N-180 (a) 1 60 s (b) 60s (c) 1 120 s (d) 1 240 s 17. Using an A.C. voltmeter the potential difference in the electrical line in a house is read to be 234 volt. If the line frequency is known to be 50 cycles/second, the equation for the line voltage is NCERT Page-234 / N-178 (a) V = 165sin (100πt) (c) V = 220sin (100πt) (b) V = 331sin (100πt) (d) V = 440sin (100πt) 7.3 Representation of AC Current and Voltage by Rotating Vectors-Phasors 18. An A.C. source is connected to a resistive circuit. Which of the following is true? NCERT Page-237 / N-181 (a) Current leads ahead of voltage in phase (b) Current lags behind voltage in phase (c) Current and voltage are in same phase (d) Any of the above may be true depending upon the value of resistance. 7.4 AC VOLTAGE APPLIED TO AN INDUCTOR 19. If the frequency of an A.C. is made 4 times of its initial value, the inductive reactance will (a) be 4 times NCERT Page-239 / N-183 (c) be half (b) be 2 times (d) romain the same 20. In the case of an inductor NCERT Page-239 / N-182
(a) voltage lags the current by π 2 (b) voltage leads the current by π 2 (c) voltage leads the current by π 3 (d) voltage leads the current by π 4 21. The instantaneous voltage through a device of impedance 20Ω is e = 80sin 100πt. The effective value of the current is NCERT Page-238 / N-182 (a) 3A (b) 2.828A (c) 1.732A (d) 4A 22. A sinusoidal voltage V(t) = 100sin (500t) is applied across a pure inductance of L = 0.02H. The current through the coil is: NCERT Page-238 /N − 182 (a) 10cos (500t) (b) −10cos (500t) (c) 10sin (500t) (d) −10sin (500t) 23. With increase in frequency of an A.C. supply, the inductive reactance NCERT / Page-238 / N-182 (a) decreases (b) increases directly with frequency (c) increases as square of frequency (d) decreases inversely with frequency 24. An alternating emf E = 440sin 100πt is applied to a circuit containing an inductance of √2 π .H. If an a.c. ammeter is connected in the circuit, its reading will be : NCERT Page-238 / N-182 (a) 4.4A (b) 1.55A (c) 2.2A (d) 3.11A 25. The equation of current in a purely inductive circuit is 5sin (49πt − 30∘ ). If the inductance is 30mH then the equation for the voltage across the inductor, will be : NCERT Page-238 / N-182 (Let π = 22 7 ) (a) 1.47sin (49πt − 30∘ ) (b) 1.47sin (49πt + 60∘ ) (c) 23.1sin (49πt − 30∘ ) (d) 23.1sin (49πt + 60∘ ) 7.5 AC VOLTAGE APPLIED TO A CAPACITOR 26. The ratio of mean value over half cycle to r.m.s. value of A.C. is NCERT Page-241 / N-184 (a) 2: π (b) 2√2: π (c)√2: π (d) √2: 1 27. When an ac voltage of 220V is applied to the capacitor C, then NCERT Page-241 / N-184, 185 (a) the maximum voltage between plates is 220V. (b) the current is in phase with the applied voltage.