AC BASICS AND NETWORKS 1

AC BASICS AND NETWORKS

11. Unit of reactive power is
a) VA
b) watt
c) VAR
d) ohm
ans: c

12. An ac source of 200V RMS supplies active power of 600 W and reactive power of 800 VAR. The RMS current drawn from the source is
a) 10 A
b) 5 A
c) 3.75 A
d) 2.5 A
ans: b

13. With the increase in applied frequency, the dielectric loss in the material will
a) increase
b) decrease
c) remains constant
d) becomes zero
ans: a

14. In an ac series RLC circuit, the voltage across R and L is 20 V, the voltage across L and C is 9 V and voltage and voltage across RLC is 15 V, What is the voltage across C?
a) 7V
b) 12 V
c) 16 V
d) 21 V
ans: a

15. The quantity (2 + j9) represents the resistive and reactive components of current drawn by an ac circuit. The phase angle between the applied voltage and current is
a) 41.4'
b) 53.13'
c) 36.87'
d) 46.86'
ans: c

16. Under the condition resonance in the R-L-C series circuit, the pf of the circuit is
a) 0.5 lagging
b) 0.5 leading
c) unity
d) zero
ans: c

17. In a series RLC circuit at resonance, the magnitude of voltage developed across the capacitor
a) is always zero
b) can never be greater than the input voltage
c) can be greater than the input voltage however,  it is 90 degrees out of phase with the input voltage
d) can be greater than the input voltage and is in phase with the input voltage.
ans: c

18. In A series R-L-C circuit, the maximum voltage across the capacitor occurs at a frequency
a) double the resonant frequency
b) equal to the resonant frequency
c) below the resonant frequency
d) none of the above
ans: c

19. In a series R-L-C circuit, the voltage across inductance will be maximum
a) at the resonant frequency
b) just after resonant frequency
c) just before resonant frequency
d) just before and after resonant frequency
ans: b

20. For a series R-L-C circuit, the power factor at the lower power frequency is
a) 0.5 lagging
b) 0.5 leading
c) unity
d) 0.707 leading
ans: d

AC BASICS AND NETWORKS 2

AC BASICS AND NETWORKS

Multiple-choice questions

21. Q-factor of as series R-L-C circuit possessing resonant frequency of 10 Hz and bandwidth of 5 Hz is
a) 0.5
b) 2
c) 2.5
d) 50
ans: b

22. When Q-factor of a circuit is high , then
a) power factor of the circuit is high
b) impedance of the circuit is high
c) bandwidth is large
d) none of the above
ans: b

23. A high  Q coil has
a) large bandwidth
b) high losses
c) low losses
d) flat response
ans: c

24. A coil is tuned to resonance at 500 kHz with a resonating capacitor of 36 pf at 250 kHz , the resonance is obtained with a resonating capacitor of 160 pf .What is the self-capacitance of the coil?
a) 2.66 pf
b) 5.33 pf
c) 8 pf
d) 10.66 pf
ans: b

25. A R-L-C resonant circuit has a resonance frequency of 1.5 MHz and a bandwidth of 10kHz . If c= 150pf, then the effective resistance of the circuit will be
a)  29.5  ohm
b) 14.75 ohm
c) 9.4 ohm
d) 4.7ohm
ans: d

AC BASICS AND NETWORKS

1. The rms value of a sine wave is 100 A. Its peak value is
a) 70.7 A
b) 141 A
c) 150 A
d) 282.8 A
ans: 2

2. The rms value of the resultant current in a wire which carries a dc current of 10 A and a sinusoidal alternating current of peak value 20 A is
a) 14.1 A
b) 17.3 A
c) 22.4 A
d) none of the above
ans: b

3. A boiler at home is switched on to the AC mains supplying power at 230 V, 50 Hz. The frequency of instantaneous power consumed is
a) 0 Hz
b) 50 Hz
c) 100 Hz
d) 150 Hz
ans: c

4. Unit of inductive reactance is
a) Henry
b) millihenry
c) Wb
d) Ohm
ans: d

5. A 10 mH inductor carries a sinusoidal current of 1A  rms at a frequency of 50 Hz. The average power dissipated by the indicator is
a) 0 W
b) 0.25 W
c) 0.5 W
d) 1.0 W
ans: a

6. A circuit component that opposes the change in circuit voltage is
a) resistance
b) capacitance
c) inductance
d) all of the above
ans: b

7. The reactance offered by a capacitor to AC of frequency 50 Hz is 10 ohm. If the frequency is increased to 100 Hz , reactance becomes
a) 20 ohm
b) 5 ohm
c) 2.5 ohm
d) 30 ohm
ans: b

8. The pf of a practical inductor is
a) unity
b) zero
c) lagging
d) leading
ans: c

9. The power factor of an ordinary electric bulb is
a) zero
b) unity
c) slightly more than unity
d) slightly less than unity
ans: d

10. A 2-terminal network consists of one of the RLC elements. The element is connected to ans ac supply. The current through the element is I A. When an inductor is inserted in series between the source and the element , the current through the element becomes 2I A . What is the element?
a) A resistor
b) An inductor
c) A capacitor
d) cannot be a single element
ans: c