Alternating Current( Chapter-7)
Multiple Choice Questions
1. If an LCR circuit have zero resistance. When ac voltage is applied across this circuit, then the current
lags behind the applied volatge by an angle
(a) 30° (b) 45° (c) 90° (d) 0°
2. In a pure capacitive circuit, the current
π
(a) lags behind the applied emf by angle
2
π
(b) leads the applied emf by an angle
2
(c) leads the applied emf by an angle π
(d) and applied emf are in same phase
3. In a pure resistive circuit, the current
(a) lags behind the applied emf by angle π / 2
(b) leads the applied emf by an angle π / 2
(c) leads the applied emf by an angle π
(d) and applied emf are in same phase
4. In a pure inductive circuit, the current
(a) lags behind the applied emf by angle π / 2
(b) leads the applied emf by an angle π / 2
(c) leads the applied emf by an angle π
(d) and applied emf are in same phase
5. An alternating current is given by i = i1 cos ωt + i2 sin ωt . The rms current is given by:
i1 + i2 i −i i2 + i2
(a) (b) 1 2 (c) 1 2 (d) none of these
2 2 2
6. Alternating current cannot 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
7. The peak value of domestic AC supply voltage is 325V. What is the rms value ?
(a) 325V (b) 230V (c) 200V (d) 325V
8. The frequency of AC is given by I = 100sin 314t . What is the frequency ?
(a) 314Hz (b) 100Hz (c) 50Hz (d) 100Hz
9. A capacitor of capacitance C has reactance X. If capacitance and frequency becomes double, then the
capacitive reactance will be:
X X
(a) 2X (b) 4X (c) (d)
2 4
10. The instantaneous values of alternating current and voltages in a circuit are given as
1 1 π
i= sin (100π t ) ampere; e = sin 100π t + Volt. The average power in watts consumed in
2 2 3
the circuit is :
2
1 3 1 1
(a) (b) (c) (d)
4 4 2 8
11. In an AC circuit an alternating voltage 200 2 sin100t volts is connected to a capacitor of capacity
1µF. The r.m.s. value of the current in the circuit is :
(a) 10 mA (b) 100 mA (c) 200 mA (d) 20 mA
12. The r.m.s. value of potential difference V shown in the figure is :
V
V0
t
O T/2 T
V0 V V
(a) (b) V0 (c) 0 (d) 0
3 2 2
13. A coil has resistance 30 ohm and inductive reactance 20 ohm at 50 Hz frequency. If an ac source, of
200 volt, 100 Hz, is connected across the coil, the current in the coil will be :
20
(a) 2.0 A (b) 4.0 A (c) 8.0 A (d) A
3
14. Power dissipated in an LCR series circuit connected to an A.C. source of emf e is :
1
2 1
2
ε 2
R + Lω −
2
ε 2 R 2 + Lω −
Cω Cω
(a) (b)
R R
ε R
2
ε R
2
(c) (d)
1
2 2 1
2
R 2 + Lω − R + Lω −
Cω Cω
15. The core of a transformer is laminated because :
(a) ratio of voltage in primary and secondary may be increased
(b) energy losses due to eddy currents may be minimized
(c) the weight of the transformer may be reduced
(d) rusting of the core may be prevented
16. In a circuit L, C and R are connected in series with an alternating voltage source of frequency f .
The current leads the voltage by 45°. The value of C is :
1 1 1 1
(a) (b) (c) (d)
π f ( 2π f L − R ) 2π f ( 2π f L − R ) π f ( 2π f L + R ) 2π f ( 2π f L + R )
17. For a series LCR circuit the power loss at resonance is :
V2 V2
(a) (b) I 2 Lω (c) I 2 R (d)
1 Cω
ω L − ω L
18. The current I an A.C., circuit inductance coil varies with time according to the graph given below.
I
t
Which one of the following graphs gives the variation of voltage with time?
3
V
V
(a) (b)
t
t
V
V
(c) (d)
t
t
19. In an A.C. circuit, the current flowing is I = 5sin (100t − π / 2 ) ampere and the potential difference is
V = 200sin (100t ) volts. The power consumption is equal to :
(a) 20 W (b) 0 W (c) 1000 W (d) 40 W
20. In an ac circuit, voltage V and current i are is given by
V = 100sin100t volt and i = 100sin (100t + π / 3) mA
The power dissipated in the circuit is
(a) 10 4 W (b) 10W (c) 2.5W (d) 5W
21. If an LCR circuit contains L = 8 H , C = 0.5µF and R = 100Ω in series. Then the resonance frequency
will be:
(a) 600 rad/s (b) 500 rad/s (c) 600 Hz (d) 500 Hz
For question number 22, 23, 24 and 25, two statements are given-one labelled Assertion (A) and
the other labelled Reason (R), select the correct answer to these questions from the code (a), (b),
(c) and (d) as given below.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not correct explanation of A
(c) A is true R is false
(d) A is false and R is also false
22. Assertion (A): In series L-C-R circuit, resonance can take place.
Reason (R): Resonance take place if inductance and capacitive reactance are equal and opposite.
23. Assertion (A): Phasors V and I for the case of resistor are in same direction.
Reason (R): The phase angle between the volatge and the current is zero.
24. Assertion (A): A step-up transformer cannot be used as a step-down transformer.
Reason (R): A transformer works only in one direction.
25. Assertion (A): Step-down transformer increases the current.
Reason (R): Transformer obeys the law of conservation of energy.
