Multiple Choice Questions Choose the correct alternative. Each question is
carrying one mark.
1. The power factor for a purely capacitive circuit is :
- (a) 1
- (b) √2
- (c) 1/√2
- (d) Zero
2. Phase difference between voltage and current when an a.c. source is connected to an inductor :
- (a) 0°
- (b) 90°
- (c) 45°
- (d) 180°
3. Phase difference between voltage and current when an a.c. source is connected to a capacitor :
- (a) 90°
- (b) 45°
- (C) 180°
- (d) 0°
4. Which quantity is increased in step-down transformer ?
- (a) Resistance
- (b) Power
- (c) Current
- (d) Charge
5. The power in series RLC-circuit with AC source is given by :
- (a) zero
- (b) VI sinɸ
- (c) VI
- (d) VI cosɸ
6. The r.m.s. value of an a.c. is 5 A. Its maximum value will be :
- (a) 10 A
- (b) 5√2A
- (c) 5/√2A
- (d) 5√3A
7. In LCR-circuit, the capacitance is changed from C to 4C. For the same resonance frequency, the inductance should be changed from L to :
- (a) 2L
- (b) 4L
- (c) L/2
- (d) L/4
8. The ratio of primary to secondary turns in an ideal transformer are 2 : 1. If the power input is P, then power output is :
- (a) 2P
- (b) P
- (C) P/2
- (d) none of these.
9. Power dissipated in a series LCR-circuit at resonance is :
- (a) `I^{2}`R
- (b) zero
- (c) `I^{2}`⍵L
- (d) `\frac{I^{2}}{⍵C}`
10. Phase difference between the voltage drop across R and C in a series LCR-circuit connected to an a.c. source is :
- (a) zero
- (b) 90°
- (c) 180°
- (d) 45°
11. Power factor is one for :
- (a) pure inductor
- (b) pure capacitor
- (c) pure resistor
- (d) either an inductor or a capacitor.
12. The square root of the product of inductance and capacitance has dimensions of:
- (a) length
- (b) mass
- (c) time
- (d) no dimensions.
13. In an a.c, circuit the current lags behind the voltage by the circuit contains :
- (a) only R
- (b) only L
- (c) only C
- (d) R and L both.
14. The power factor of LCR circuit at resonance is :
- (a) zero
- (b) 1/2
- (c) 1/√2
- (d) 1
15. If ɸ is the phase difference between the instantaneous values of voltage V and current I in an a.c. circuit, then the average power loss over a complete cycle is:
- (a) VI sinɸ
- (b) VI cosɸ
- (c) VI
- (d) VI/2
16. The values of L, C and R for a circuit are 1H, 9F and 3Ω. What is the quality factor for the circuit at resonance ?
- (a) 1
- (b) 9
- (c) 1/9
- (d) 1/3
17. In a transformer, loss due to eddy currents is minimised by :
- (a) laminating the core
- (b) laminating the core of steel
- (c) increasing the number of turns of primary coil
- (d) lubricating the core.
18. The core of a transformer is laminated to reduce:
- (a) flux leakage
- (b) hysteresis loss
- (c) output power
- (d) eddy current
19. An A.C. source of voltage E = 20 sin100t in connected cross a resistance 20Ω. The rms value of the current in circuit is:
- (a) 1A
- (b) 1/2A
- (c) √2A
- (d) 1/ √2A
20. The rate of change of current of 10 A/s in a coil produces an e.m.f. of 5 V. Then the self
inductance of the coil in henry is :
- (a) 0.5
- (b) 0.25
- (c) 1
- (d) 1.25.
21. The instantaneous voltage through a device of impedance 20Ω is e = 80 sin1001t. The effective value of the current is:
- (a) 3A
- (b) 2.828 A
- (c) 1.732 A
- (d) √2A
22. A pure inductance coil of 30 mH is connected to an a.c. source of 220 V. The r.m.s current in
the coil is :
- (a) 50.35 A
- (b) 23.4 A
- (C) 30.5 A
- (d) 12.3 A.
23. An LCR series circuit is at resonance. Then :
- (a) the phase difference between current and voltage is 90°
- (b) the phase difference between current and voltage is 45°
- (c) its impedance is purely resistive
- (d) its impedance is zero.
24. An LCR series a.c. circuit is at resonance with 10 V each across L, C and R. If the resistance is halved, the respective voltages across L, C and R are :
- (a) 10 V, 10 V and 5 V
- (b) 10 V, 10 V and 10 V
- (c) 20V, 20 V and 5 V
- (d) 20 V, 20 V and 10 V
25. In which of the following circuits the maximum power dissipation is observed?
- (a) Pure capacitive circuit
- (b) Pure inductive circuit
- (c) Pure resistive circuit
- (d) None of these
26. A capacitor :
- (a) blocks AC but provides an easy path for DC
- (b) blocks DC but provides an easy path for AC
- (c) blocks both AC and DC
- (d) offers easy path for AC and DC
27. When AC source is connected across series LCR combination, maximum power loss will occur when :
- (a) current and voltage are in phase
- (b) current from source is minimum
- (c) inductance is minimum
- (d) capacitance is maximum
28. A transformer works on the principle of:
- (a) self induction
- (b) electrical inertia
- (c) mutual induction
- (d) magnetic effect of the electrical current
