Tiêu đề 7.ALTERNATING CURRENT - Questions.pdf ✅

7.ALTERNATING CURRENT (1.)Choke coil works on the principle of (a.) Transient current (b.) Self induction (c.) Mutual induction (d.) Wattless current (2.)The impedance of a R-C circuit is Z1 for frequency f and Z2 for frequency 2f. Then, Z1 Z2 is (a.) Between 1 and 2 (b.) 2 (c.) Between 1 2 and 1 (d.) 1 2 (3.)The voltage of domestic ac is 220 volt. What does the represent (a.) Mean voltage (b.) Peak voltage (c.) Root mean voltage (d.) Root mean square voltage (4.)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 (5.)The phase difference between the current and voltage of LCR circuit in series combination at resonance is (a.) 0 (b.) π/2 (c.) π (d.) −π (6.)What is the r. m. s. value of an alternating current which when passed through a resistor produces heat which is thrice of that produced by a direct current of 2 amperes in the same resistor (a.) 6 amp (b.) 2 amp (c.) 3.46 amp (d.) 0.66 amp (7.)In an AC circuit the emf(e) and the current (i) at any instant are given respectively by e = E0 sin ωt i = I0 sin(ωt − φ) The average power in the circuit over one cycle of AC is (a.) E0I0 2 (b.) E0I0 2 sinφ (c.) E0I0 2 cos φ (d.) E0I0 (8.)In an AC series circuit, the instantaneous current is maximum when the instantaneous voltage is maximum. The circuit element connected to the source will be (a.) Pure inductor (b.) Pure capacitor (c.) Pure resistor (d.) Combination of capacitor and an inductor (9.)If a current of 3 A flowing in the primary coil is reduced to zero in 0.001 s, the induced emf in between the two coils is 15000 V, the coefficient of mutual induction is (a.) 0.5 H (b.) 5 H (c.) 1.5 H (d.) 10 H (10.)The r. m. s. voltage of the wave form shown is (a.) 10 V (b.) 7 V (c.) 6.37 V (d.) None of these (11.)In the non-resonant circuit, what will be the nature of the circuit for frequencies higher than the resonant frequency (a.) Resistive (b.) Capacitive (c.) Inductive (d.) None of the above (12.)An AC voltage source of variable angular frequency ω and fixed amplitude V0 is connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When ω is increased (a.) The bulb glows dimmer (b.) The bulb glows brighter (c.) Total impedance of the circuit is unchanged (d.) Total impedance of the circuit increases (13.)If a current I given by I0 sin(ωt − π 2 ) flows in an ac circuit across, which an ac potential of E = E0 sin ωt has been applied, then the power consumption P in the circuit will be (a.) P = E0I0 √2 (b.) P = √2E0I0 0 t + 10 – 10 Y
(c.) P = E0I0 2 (d.) P = 0 (14.)An inductor L and a capacitor C are connected in the circuit as shown in the figure. The frequency of the power supply is equal to the resonant frequency of the circuit. Which ammeter will read zero ampere (a.) A1 (b.) A2 (c.) A3 (d.) None of these (15.)An inductor of 2 H and a resistance of 10 Ω are connected in series with a battery of 5 V. the initial rate of change of current is (a.) 0.5 As −1 (b.) 2.0 As −1 (c.) 2.5 As −1 (d.) 0.25 As −1 (16.)In a region of uniform magnetic induction B = 10−2 tesla, a circular coil of radius 30 cm and resistance π 2 ohm is rotated about an axis which is perpendicular to the direction of B and which forms a diameter of the coil. If the rotates at 200 rpm the amplitude of the alternating current induced in the coil is (a.) 4π 2 mA (b.) 30 mA (c.) 6 mA (d.) 200 mA (17.)An alternating current source of frequency 100 Hz is joined to a combination of a resistance, a capacitance and a coil in series. The potential difference across the coil, the resistance and the capacitor is 46, 8 and 40 volt respectively. The electromotive force of alternating current source in volt is (a.) 94 (b.) 14 (c.) 10 (d.) 76 (18.)A circuit consists of an inductance of 0.5 mH and a capacitor of 20μF. The frequency of the L – C oscillations is approximately (a.) 400 Hz (b.) 88 Hz (c.) 1600 Hz (d.) 2400 Hz (19.)A resistance R, inductance L and capacitor C are connected in series to an oscillator of frequency f. If resonant frequency is f, then current will lag the voltage when (a.) f = 0 (b.) f < fr (c.) f = fr (d.) f > fr (20.)The maximum value of AC voltage in a circuit is 707 V. Its rms value is (a.) 70.7 V (b.) 100 V (c.) 500 V (d.) 707 V (21.)Voltage and current in an ac circuit are given by V = 5 sin(100πt − π 6 ) and I = 4 sin (100 πt + π 6 ) (a.) Voltage leads the current by 30° (b.) Current leads the voltage by 30° (c.) Current leads the voltage by 60° (d.) Voltage leads the current by 60° (22.)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 π/2 (b.) Current is ahead of e.m.f. by π/2 (c.) Current lags behind e.m.f. by π (d.) Current is ahead of e.m.f. by π (23.)In an L − R circuit shown in above figure switch S is closed at time t = 0. If e denotes the induced emf across inductor and i, the current in the circuit at any time t, then which of the following graphs, figure shows the variation of e with i? (a.) (b.) (c.) A1 A2 A3 C E = E0 sint L
(d.) (24.)Which of the following statement is incorrect? (a.) In a L – C – R series AC circuit, as the frequency of the source increases, the impedance of the circuit first decreases and then increases (b.) If the net reactance of an L – C – R series AC circuit is same as its resistance, then the current lags behind the voltage by 45° (c.) At resonance, the impedance of an AC circuit becomes purely resistive. (d.) Below resonance, voltage leads the current while above it, current leads the voltage (25.)In a L – R circuit of 3 mH inductance and 4 Ω resistance, emf E = 4 cos 1000t V is applied. The amplitude of emf is (a.) 0.8 A (b.) 4 7 A (c.) 1.0 A (d.) 4 √7 A (26.)A conducting wire frame is placed in a magnetic field, which is directed into the paper, figure. The magnetic field is increasing at a constant rate. The directions of induced current in wires AB and CD are (a.) A to B and C to D (b.) B to A and C to D (c.) A to B and D to C (d.) B to A and D to C (27.)If the total charge stored in the LC circuit is Q0, then for t ≥ 0 (a.) The charge on the capacitor is Q = Q0 cos ( π 2 + t √LC) (b.) The charge on the capacitor is Q = Q0 cos ( π 2 − t √LC) (c.) The charge on the capacitor is Q = −LC d 2Q dt 2 (d.) The charge on the capacitor is Q = 1 √LC d 2Q dt 2 (28.)An inductor of inductance L = 400 mH and resistors of resistances R1 = 4Ω and R2 = 2Ω are connected to battery of emf 12 V as shown in the figure. The internal resistance of the battery is negligible. The switch S is closed at t = 0. The potential drop across L as a function of time is (a.) 6e −5t V (b.) 12 t e −3t V (c.) 6(1 − e −t/0.2 ) V (d.) 12 e −5t V (29.)At high frequency, the capacitor offer (a.) More reactance (b.) Less reactance (c.) Zero reactance (d.) Infinite reactance (30.)For a series L-C-R circuit at resonance, the statement which is not true is (a.) Peak energy stored by a capacitor = peak energy stored by an inductor (b.) Average power = apparent power (c.) Wattles current is zero (d.) Power factor is zero (31.)What is the approximate peak value of an alternating current producing four times the heat produced per second by a steady current of 2.0 A in a resistor (a.) 2.8 A (b.) 4.0 A (c.) 5.6 A (d.) 8.0 A (32.)Two coils are at fixed locations. When coil 1 has no current and the current in the coil 2 increases at the rate 15.0 As −1 , the emf in coil 1 is 25.0 mV. When coil 2 has no current of 3.6 A. The flux linkage in coil 2 (a.) 4 mWb (b.) 6 mWb (c.) 10 mWb (d.) 16 mWb (33.)An L – C – R circuit of R = 100 Ω is connected to an AC source 100 V, 50 Hz. The magnitude of phase difference between current and voltage is 30°. The power dissipated in the L – C – R circuit is (a.) 50 W (b.) 86.6 W (c.) 100 W (d.) 200 W (34.)The current passing through a choke coil of 5 H is decreasing at the rate of 2 As −1 . The emf developed across the coil is (a.) −10 V (b.) + 10 V (c.) 2.5 V