Title 6-ac-circuits-.pdf ✅

AC Circuits 1. If the rms current in a 50 Hz ac-circuit is 5 A, the value of the current 1/300 seconds after its value becomes zero is. (A) 5√2A (B) 5√3/2A (C) 5/6 A (D) 5/√2A 2. A resistance is connected to a capacitor in AC and the phase difference is π 4 between current and voltage. When the same resistance is connected to an inductor, phase difference becomes tan−1 (2). Power factor of the circuit when both capacitor and inductor are connected to the resistance will be : (A) 1 (B) 1 √2 (C) 1 √3 (D) 1 2 3. Which of the following combinations should be selected for better tuning of an LCR circuit used for communication? (A) R = 20Ω, L = 1.5H, C = 35μF (B) R = 25Ω, L = 2.5H, C = 45μF (C) R = 15Ω, L = 3.5H, C = 30μF (D) R = 25Ω, L = 1.5H, C = 45μF 4. The output of a step-down transformer is measured to be 24V when connected to a 12 watt light bulb. The value of the peak current is. (A) 1/√2A (B) √2A (C) 2A (D) 2√2A 5. The figure shows an experimental plot discharging of a capacitor in an R-C circuit. The time constant τ of this circuit lies between : (A) 150 s and 200 s (B) 0 and 50 s (C) 50 s and 100 s (D) 100 s and 150 s 6. As the frequency of an ac circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit? (A) Inductor and capacitor (B) Resistor and inductor (C) Resistor and capacitor (D) Resistor, inductor and capacitor 7. Alternating current can not be measured by DC ammeter because : (A) AC cannot pass through DC ammeter (B) AC changes direction (C) Average value of current for complete cycle is zero (D) DC ammeter will get damaged 8. In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit? (A) Only resistor (B) Resistor and an inductor (C) Resistor and a capacitor (D) Only a capacitor 9. The core of any transformer is laminated so as to : (A) Reduce the energy loss due to eddy currents (B) Make it light weight (C) Make it robust and strong (D) Increase the secondary voltage 10. Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is(are) correct? (A) For a given power level, there is a lower current (B) Lower current implies less power loss (C) Transmission lines can be made thinner (D) It is easy to reduce the voltage at the receiving end using step-down transformers 11. In an AC generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B. The maximum value of emf generated in the coil is : (A) NABRω (B) NAB (C) A BR (D) NABω
12. For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I 2Zcos φ. (A) Here, the power factor cos φ ≥ 0, P ≥ 0 (B) The driving force an give no energy to the oscillator (P = 0) in some cases (C) The driving force cannot syphon out (P < 0) the energy out of oscillator (D) The driving force can take away energy out of oscillator 13. Assertion : Choke coil is preferred to a capacitor to reduce AC in a circuit. Reason : Inductor acts as a leak-proof wattless resistor. (A) A (B) B (C) C (D) D (E) E 14. In an LCR circuit, capacitance is changed form C to 2C. For the resonant frequency to remain unchanged, the inductance should be changed from L to ? (A) 4L (B) 2L (C) L/2 (D) L/4 15. Assertion : At series resonance, LCR circuit have a minimum current. Reason : At resonance, in LCR circuit, the current and emf remain in phase with each other. (A) A (B) B (C) C (D) D (E) E 16. The phase difference between the alternating current and emf is π/2. Which of the following cannot be the constituent of the circuit? (A) C alone (B) L, C (C) R, L (D) L alone 17. The line that draws power supply to your house from street has (A) Zero average current (B) 220 V average voltage (C) Voltage and current out of phase by 90∘ (D) Voltage and current possibly differing in phase φ such that |φ| < π 2 18. A resistance ' R ' draws power ' P ' when connected to an AC source. If an inductance is now placed in series with the resistance, such that the impedance of the circuit becomes ' Z ', the power drawn will be : (A) P ( R Z ) (B) P (C) P ( R Z ) 2 (D) P√ R Z 19. A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil 6 A, the voltage across the secondary coil and the current in the primary coil respectively are : (A) 300 V, 15 A (B) 450 V, 15 A (C) 450 V, 13.5 A (D) 600 V, 15 A 20. In an electrical circuit R, L, C and ac voltage source are all connected in series. When L is removed from the circuit. The phase difference between the voltage and the current in the circuit is π/3 if instead, C is removed from the circuit, the phase difference is again π/3 the power factor of the circuit is : (A) 1 2 (B) 1 √2 (C) 1 (D) √3 2 21. The transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produce oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be : (A) f/2 (B) f/4 (C) 8f (D) f/2√2 22. In a circuit L, C and R are connected in series with a alternating voltage source of frequency f. the current leads the voltage by 45∘ . The value of C is : (A) 1 πf(2πfL−R) (B) 1 2πf(2πfL−R) (C) 1 πf(2πfL+R) (D) 1 2πf(2πfL+R) 23. The power factor of an AC circuit with having resistance R and inductance L (connected in series) and an angular frequency ω is : (A) R ωL (B) R (R2−ω2L 2) 1/2
(C) ωL R (D) R (R2+ω2L 2) 1/2 24. The current l in an A.C. circuit inductance coil varies with time according to the graph given below. Which one of the following graphs gives the variation of voltage with time? (A) (B) (C) (D) 25. A condenser of capacity C is charged to a potential difference of V1. the plates of the condenser are then connected to an ideal inductor of inductance L. the current through the inductor when the potential difference across the condenser reduced to V2 is : (A) ( C(V1−V2 ) 2 L ) 1 2 (B) C(V1 2−V2 2 ) L (c) C(V1 2+V2 2 ) L (D) ( C(V1 2−V2 2 ) L ) 1 2 26. An ac is given by equation I = I1cos ωt + I2sin ωt. The rms value of current is given by : (A) I1+I2 2 (B) (I1+I2 ) 2 √2 (C) 1 √2 √I1 2 + I2 2 (D) I1 2+I2 2 2 27. A series LR circuit is connected to a voltage source with V(t) = V0sin ωt. After very large time, current I(t) behaves as (t0 ≫ L R ) (A) (B) (C)
(D) 28. An LCR circuit is equivalent to a damped pendulum. In an LCR circuit the capacitor is charged to Q0 and then connected to the L and R as shown here. If a student plots graphs of the square of maximum charge (QMax 2 ) on capacitor with time (t) for two different value L1 and L2 (L1 > L2 ) of L then which of the following represents this graph correctly? (plots are schematic and not drawn to scale) (A) (B) (C) (D) 29. For the LCR circuit, shown here, the current is observed to lead the applied voltage. An additional capacitor C ′ , when joined with the capacitor C present in the circuit, makes the power factor of the circuit unity. The capacitor C ′ , must have been connected in : (A) series with C and has a magnitude 1−ω2LC ω2L (B) series with C and has a magnitude C (ω2LC−1) (C) parallel with C and has a magnitude C (ω2LC−1) V = V0sin ωt (D) parallel with C and has a magnitude 1−ω2LC ω2L INTEGER ANSWER TYPE QUESTIONS The Answers to the following questions are positive integers of 1/2/3 digits or zero. 30. In an AC circuit the voltage applied is E = E0sin ωt. The resulting current in the circuit is I = I0sin (ωt − π 2 ). The power consumption in the circuit is given by xE0I. 31. The self-inductance of the motor of an electric fans is 10H. In order to impart maximum power at 50 Hz, it should be connected to a capacitance of : (in μF )