CHAPTER

Alternating Current

7.2 AC Voltage Applied to a Resistor

1. The r.m.s. value of potential difference V shown in
the figure is
VA
(a)
3 Which one of the following
of voltage with time?
graphs gives the variation
(b) Vo T/2 T
o
d) (Mains 2011)
N2 (a) (b)
2 In an A.C. circuit, IFms and 1, are related as

(a) rms Tlo (b) me =va2

(c)
(c) ms= l/Tt (d) rmsl,/N2 (1994) ()
Two cables of copper are of equal t
lengths. One of
them has a single wire of area of cross-section (1994)
A,
while other has 10 wires of
cross-sectional
each. Give their suitability for
area A/10 7.5 AC Voltage Applied to a Capacitor
transporting A.C. and 6. A 40 uF capacitor
D.C. isconnected to a 200 V, 50 Hz ac
(a) only multiple strands for A.C., either for D.C. supply. The r.m.s value of the current in the
(b) only multiple strands for A.C., only
is, nearly circu
strand for D.C.
single (a) 1.7 A
(c) 2.5 A (b) 2.05 A
(c) only single strand for D.C., either for A.C. (d) 25.1 A (NEET 2020)
. A
(d) only single strand for A.C., either for D.C. small signal voltage V(t)
across an ideal
=
V, sinot is
appie
(a) Current I(t)capacitor
(1994) C
is in
7.4 AC Voltage Applied to an Inductor (b) Current I() leadsphase with voltage V().
A coil of (c) Current I(t), voltage V(t) by 180.
4. self-inductance L is connected in series lags voltage V(t) by 90°.
with a bulb B and an AC (d) Over a full cyclethe capacitor C does not
bulb decreases when
source.
Brightness of the consume any
energy from the voltage urce

(a) sou
a
capacitance of reactance X¢= X; is included in 8.
(NEET-I 2016)
the same circuit. In an ac
circuit an
(b) an iron rod is inserted in the coil. volts is connected alternating voltage 2002 sin100
to a The
(c) frequency of the AC source is decreased. r.m.s. value of capacitor
the current in
of capacity 1 u
capacity 1

(d) number of turns in the coil is reduced. (a) 10 mA the circuit is

(c) 200 mA (b) 100 mA
(NEET 2013) (d) 20 mA (2011)
9. A
5. The current I in an A.C. circuit with
inductance coil capacitor of
capacity C has ance
X.
varies with time according to the graph given below. lf
capacitance and reacla
reactance will be frequency become doube
then
ermating Current 63
(a) 4X (b) X/2 L R = 100 2

(c) X/4 (d) 2X (2001)

Series LCR Circuit

16 ACVoltage Applied to a
is connected to an ac voltage
0 A series LCR circuit 220 V, 50 Hz

source. When L is removed from the circuit, the (b) 220 V, 2.2 A
(a) 150 V, 2.2 A
and voltage is t/3.
phase difference between current
(c) 220 V, 2.0 A (d) 100 V, 2.0 A (2010)
If instead C is removed from the circuit, the phase
difference is again t/3 between current and voltage. 17. What is the value of inductance L for which the
current is maximum in a series LCR circuit
with
The power factor of the circuit is
C= 10 uF and o = 1000 s-l?
(a) zero (b) 0.5
(a) 1 mH
c) 1.0 (d) -1.0 (NEET 2020)
(b) cannot be calculated unless R is known
I1. A circuit when connected to an AC source of (d) 100 mH (2007)
(c) 10 mH
12 V gives a current of 0.2 A. The same circuit when
connected to a DC source of 12 V, gives a current of 18. In a circuit L, C and R are connected in series with

0.4 A. The circuit is an alternating voltage of frequency f. The

source

(a) series LR (b) series RC current leads the voltage by 45°. The
value ofC is
(c) series LC (d) series LCR
(Odisha NEET 2019) (a) (b) 2 f (21f L- R)
tf(27tf L- R)
12. Which of the following combinations should be
L-C-R circuit used (c) (d) (2005)
selected for better tuning of an nf (27tf L + R) 27Tf(21f L+R)
for communication? The value of quality factor is
a) R= 20 2, L = 1.5 H, C = 35 uF 19.
(6) R= 25 Q, L = 2.5 H, C = 45 uF (a) o l (b) (c) vLC (d) L/R
R RC
(c) R= 15 2, L = 3.5 H, C= 30 uF (2000)
(d) R 25 2, L = 1.5 H, C= 45 uF (NEET-II 2016) An series L-C-R circuit is connected to of a source
20.
13. A series R-C circuit is connected to an alternating A.C. current. At resonance, the phase difference
between the applied voltage and the current in the
voltage source. Consider two situations:
) When capacitor is air filled. circuit, isS

i) When capacitor is mica filled. (a) (b) zero (c) t/4 (d) t/2

resistor is i and voltage across (1994)

Current through
capacitor is V then
(a) i i (b) V. = V 7.7 Power in AC Circuit:The Power Factor
(2015) An inductor 20 mH, a capacitor 100 uF and a
() V. < V, (d) V.> V 21.
resistor 50 2 are connected in series across a source
4
is to a resistance R and an
applied
n ac
voltage of emf, V= 10 sin 314t. The power loss in the circuit
inductor L in series. If R and the inductive reactance
between 1s
are both equal to 3 2, the phase difference (b) 0.43 W
circuit is (a) 0.79 W
the applied voltage and the current in the (d) 1.13 W (NEET 2018)
(a) t/6 (b) Tt/4 (c) 2.74 W
c) Tt/2 (d) zero (2011) 22. The potential differences across the resistance.
15. A COil has resistance 30 ohm and inductive reactance capacitance 80 V, 40 V and
and inductance are
20 ohm at 50 Hz 100 V respectively in an L-C-R circuit. The power
frequency. If an ac source,
the
o factor of this circuit is
0 volt, 100 Hz, is connected across the coil, (a) 0.4 (b) 0.5 (c) 0.8 (d) 1.0
Current in the coil will be
(a) 2.0 A (b) 4.0 A
(NEETI 2016)
23. An inductor 20 mH, a capacitor 50 uF and a resistor
c) 8.0 A (d) A (Mains 2011) are connected in series across
a source of emf
40 2
16. In the
13 V= 10 sin 340t. The power loss in A.C. circuit is
given circuit the reading of voltmeter V, and (a) 0.76 W (b) 0.89 W

are 300 volts each. The reading of the voltmeter (d) 0.67 W (NEET-I 2016)
3 and ammeter A are (c) 0.51
respectively
64 mte NEET-AIPMT Chapterwise Topicwise Solutions Physa
24. In an electrical circuit R, L, 29. For a series LCR circuit, the power loss at resonance ic
C and ac voltage source 1
are all
connected in series. When L is removed from (b) Lo
the circuit, the (a)
phase difference between the voltage
and the current in the circuit is Tt/3. If instead, C is (0C.
removed from the circuit, the phase difference is
(c) R (d) (2002
again t/3. The power factor of the circuit is Co
30. In an a.c. circuit with phase voltage V and current
1
(a) (b) (c)1 (d) vs the power dissipated is
2 2 (2012) (a) V.I
25. The instantaneous values of alternating current and (6) depends on phase angle between V and I

voltages in a circuit are given as

(c)xV.I
2
(d)xVI (1997)

v2 sin (100 tt)

I
ampere 31. In an A.C. circuit, the current flowing is
I = 5 sin (100t T/2) ampere and the potential
sin sin 100+vo difference is V = 200 sin (100t) volts. The power
v2 consumption is equal to
The average power in watts consumed in the circuit is (a) 20 WN (b) 0 W
V3 (c) 1000 W (d) 40 W (1995)
(a) (b) () (d)
4 8
(Mains 2012)7.8 LC Oscillations
26. Power dissipated in an LCR series circuit connected 32. A condenser of capacity C is charged to a potential
to an A.C. source of emf e is difference of Vi. The plates of the condenser are then
connected to an ideal inductor of inductance L. The
current through the inductor when the potential
Co) difference across the condenser reduces to V, is
(a)
R

(a)C-V
L
b) CV-V
L
(b)
R
R o) C+V
(c) L
(Mains 2010

33. A transistor-oscillator using a resonant circuit w

eR inductor L (of negligible resistance) and a capacto
(d) Cin series produces oscillations of frequency f. t
(2009) doubled and C is changed to 4C, the frequency wu
(a) fi2 (b) fi4
27. In an a.c. circuit the e.m.f. (E) and the current (i) at any
(c) 8f (d) f/2w2 (200
instant are given respectively by
e E,sinot, i= 1hsin(t - ))
The average power in the circuit over one cycle of
7.9 Transformers
34. A transformer
a.c. is
having eticiency of 90% is worsi
curent
on 200 V and 3 kW the
power Pply. It
(a)
Fo'o cos (b) Eglo in the
secondary coil is 6 A, the voltage
oss
the

2 secondary coil and the current in the prima

Folo d) sin (2008) respectively are
(c)2 inductive reactance 31 2 has a resistance
(a) 300 V, 15 A (b) 450 V, 15 A
coil of (c) 450 V, 13.5 A (d)600V, 15 A
28. A aced
placed in sseries
in er with a condenser ol
is ted to
2. t 35. The primary ot a transtormer when
of 8
capacitative react. ce 25 2. The
combination
is cont The

de battery of 10 volt draws a current

to a n d.c. sOurce of
soure
T10 V. The
ne power o d
number of turns ot the
primary anu e
c o n n e c t c d

circuitis

factor ofthe (b) 0.56 windings are 50 and 100 respectively. The
the secondary and the current drawn by the
(a) 0.33 (d) 0.80
(c) 0.64
(2006) the secondary are respectively
ernating Curent
65

(a) 20 V
and 2.0 mA (b) 10 V and 0.5 mA
7.A RCIRL Circuits with DC Source
Zero volt and therefore no current
(c)
0.5 mA (Karnataka NEET 2013) 42. Figure shows a circuit that contains three identical
(d) 20 V and
resistors with resistance R 9.0 Q each. two identi-
=

A 220 volt input is supplied to a transformer. The

cal inductors with inductance L = 2.0 mH each, and
output circuit draws a current of 2.0 ampere at
an ideal battery with emf e = 18 V. The current i
40 volts. If the efficiency of the transformer is 80%,
the current drawn by the primary windings of the through the battery just after the switch closed is
transtormer is

(a) 3.6 ampere (b) 2.8 ampere

(c) 2.5 ampere (d) 5.0 ampere (2010)
7. The primary and secondary coils of a transformer have
50 and 1500 turns respectively. If the magnetic flux o (a) 0.2 A (b) 2A
linked with the primary coil is given by Ù = Oo + 4,
where is in webers, t is time in seconds and o, is a
(c) 0 ampere (d) 2 mA (NEET2017)
43. A coil of 40 henry inductance is connected in series
constant, the output voltage across the secondary coil is oil is
(a) 120 volts with a resistance of 8 ohm and the combination is
(b) 220 volts
(c) 30 volts joined to the terminals of a 2 volt battery. The time
(d) 90 volts (2007)
constant of the circuit is
38. Atransformer is used to light a 100 Wand 110V lamp (a) 5 seconds (b) 1/5 seconds
from a 220 V mains. If the main current is 0.5 amp,
(c) 40 seconds (d) 20 seconds
the (2004)
efficiency of the transformer is approximately
(a) 50% 14. In the circuit given in I and 2 are ammeters.
(b) 90% hgure.
(c) 10% (d) 30% (2007) Just after key K is pressed to R

.
complete the circuit, the
Ihe core of a transformer is laminated because
reading will be
(a) ratio of voltage in primary and secondary may (a) zero in 1, maximum in 2
00000 wO
be increased
(b) maximum in both I and 2
(b) energy losses due to eddy currents may be (c) zero in both I and 2
minimised (d) maximum in l, zero in 2.
(1999)
Cthe weight of the transformer may be reduced 45. When the key K is
pressed at time t 0,
d) rusting of the core
may be prevented. (2006) then which
=

of the following statement about the

40. current I in the
AStep-up transformer operates on a 230 V line and resistor AB of the given circuit is true?
Supplies a load of 2 ampere. The ratio of the primary
andsecondary windings is 1: 25. The current in the
1000 9 C
primary is
(a) 15 A (b) 50 A
(c) 25 A (d) 12.5 A (1998) (a) I oscillates between I mA and 2 mA
. ne
primary winding of a transformer has 500 turns (b) Att=0, I =2 mA and with time it
goes to I mA
whereas its secondary has 5000 turns. The primary (c) I= 1 mA at all t
Connected to an A.C. supply of 20 V, 50 Hz. The (d) I= 2 mA at all t.
(1995)
Secondary will have an output ot 46. The time constant ot C-R circuit is
(a) 2V, 50 Hz (b) 2 V, 5 Hz (a) 1/CR (b) CIR
c) 200 V, 50 Hz (d) 200 V, 500 Hz. (1997) (c) CR (d) RIC (1992)

ANSWER KEY
(c) 2. (d) 3. (a) 4. (b) 5. (a) 6. (c) 7. (d) 8. (d) 9. (c) 10. (c)
(a)
(c) 12. 13. (d) 14. (b) 15. (b) 16. (b) 17. (d) 18. (d) 19. (a.b) 20. (b)
2 2 . (c) 23. (c) 24. (c) 25. (d) 26. (d) 27. (a) 28. (d) 29. (c) 30. (b)
b)
A. c) 32. (d) 33. (d) 34. (b) 35. (c) 36. (d) 37. (a) 38. (b) 39. (b) 40. (b)
42. (") 43. (a) 44. (d) 45. (b) 46. (c)