CBSE Class 12 Physics Electromagnetic Induction And Alternating Current Worksheet Set C

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. In which of the following devices, the eddy current
effect is not used?
(a) electric heater
(b) induction furnace
(c) magnetic braking in train
(d) electromagnet 

Answer: A

Question. Eddy currents are produced when
(a) a metal is kept in varying magnetic field
(b) a metal is kept in steady magnetic field
(c) a circular coil is placed in a magnetic field
(d) current is passed through a circular coil

Answer: A

Question. The magnetic potential energy stored in a certain inductor is 25 mJ, when the current in the inductor is 60 mA. This inductor is of inductance
(a) 0.138 H
(b) 138.88 H
(c) 1.389 H
(d) 13.89 H 

Answer: D

Question. A current of 2.5 A flows through a coil of inductance 5 H. The magnetic flux linked with the coil is
(a) 0.5 Wb
(b) 12.5 Wb
(c) zero
(d) 2 Wb

Answer: B

Question. A long solenoid has 500 turns. When a current of 2 ampere is passed through it, the resulting magnetic flux linked with each turn of the solenoid is 4 × 10–3 Wb. The self-inductance of the solenoid is
(a) 1.0 henry
(b) 4.0 henry
(c) 2.5 henry
(d) 2.0 henry 

Answer: A

Question. A rectangular, a square, a circular and an elliptical loop, all in the (x – y) plane, are moving out of a uniform magnetic field with a constant velocity, V = v ^i . The magnetic field is directed along the negative z axis direction. The induced emf, during the passage of these loops, out of the field region, will not remain constant for
(a) the circular and the elliptical loops
(b) only the elliptical loop
(c) any of the four loops
(d) the rectangular, circular and elliptical loops

Answer: A

Question. A conducting circular loop is placed in a uniform magnetic field 0.04 T with its plane perpendicular to the magnetic field. The radius of the loop starts shrinking at 2 mm/s. The induced emf in the loop when the radius is 2 cm is
(a) 4.8p mV
(b) 0.8p mV
(c) 1.6p mV
(d) 3.4p mV

Answer: D

Question. For a coil having L = 2 mH, current flow through it is I = t2e–t then, the time at which emf becomes zero
(a) 2 sec
(b) 1 sec
(c) 4 sec
(d) 3 sec.

Answer: A 

Question. The total charge, induced in a conducting loop when    
it is moved in magnetic field depends on
(a) the rate of change of magnetic flux
(b) initial magnetic flux only
(c) the total change in magnetic flux
(d) final magnetic flux only.

Answer: C

Important Questions for NCERT Class 12 Physics Alternating Induction

Question. A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 amp, the efficiency of the transformer is approximately
(a) 50%
(b) 90%
(c) 10%
(d) 30%

Answer: B

Question. 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 minimised
(c) the weight of the transformer may be reduced
(d) rusting of the core may be prevented. 

Answer: B

Question. A step-up transformer operates on a 230 V line and supplies a load of 2 ampere. The ratio of the primary and secondary windings is 1 : 25. The current in the primary is
(a) 15 A
(b) 50 A
(c) 25 A
(d) 12.5 A 

Answer: B

Question. If we decrease the frequency of the applied A.C. with a purely capacitive load, do (1) the amplitude of Vc and (2) amplitude of Ic increase, decrease of remain the same. 
(a) (1) increase (2) same
(b) (1) same (2) increase
(c) (1) same (2) decrease
(d) (1) decrease (2) same

Answer: C

Question. In the circuit given in figure, 1 and 2 are ammeters. 
Just after key K is pressed to complete the circuit, the reading will be
(a) zero in 1, maximum in 2
(b) maximum in both 1 and 2
(c) zero in both 1 and 2
(d) maximum in 1, zero in 2.

Answer: D

Question. The time constant of C-R circuit is
(a) 1/CR
(b) C/R
(c) CR
(d) R/C

Answer : C

Question. In an ac circuit an alternating voltage 200 2 sin100 t volts is connected to a capacitor of capacity 1 mF. The r.m.s. value of the current in the circuit is 
(a) 10 mA
(b) 100 mA
(c) 200 mA
(d) 20 mA 

Answer: D

Question. A coil having an area A₀ is placed in a magnetic field which changes from B₀ to 4 B₀ in time interval t. The e.m.f. induced in the coil will be
(a) 3A₀ B₀ / t
(b) 4A₀ B₀ / t
(c) 3B₀ /A₀t
(d) 4A₀ / B₀t

Answer: A

Question. If coefficient of self induction of a coil is 1 H, an e.m.f. of 1V is induced, if
(a) current flowing is 1A
(b) current variation rate is 1 As–1
(c) current of 1A flows for one sec.
(d) None of these

Answer: B

Question. An electron moves along the line PQ which lies in the same plane as a circular loop of conducting wire as shown in figure.
What will be the direction of the induced current in the loop ?
(a) Anticlockwise
(b) Clockwise
(c) Alternating
(d) No current will be induced

Answer: A

Question. A long solenoid having 200 turns per cm carries a current of 1.5 amp. At the centre of it is placed a coil of 100 turns of cross-sectional area 3.14 × 10^–4 m² having its axis parallel to the field produced by the solenoid. When the direction of current in the solenoid is reversed within 0.05 sec, the induced e.m.f. in the coil is
(a) 0.48 V
(b) 0.048 V
(c) 0.0048 V
(d) 48 V

Answer: B

Question. A long solenoid of diameter 0.1 m has 2 × 10⁴ turns per meter. At the centre of the solenoid, a coil of 100 turns and radius 0.01 m is placed with its axis coinciding with the solenoid axis. The current in the solenoid reduces at a constant rate to 0A from 4 A in 0.05 s. If the resistance of the coil is 10π²Ω. the total charge flowing through the coil during this time is :- 
(a) 16 μC
(b) 32 μC
(c) 16 π μC
(d) 32 π μC

Answer: B

Question. In a circuit given in figure 1 and 2 are ammeters. Just after key K is pressed to complete the circuit, the reading is 

(a) zero in both 1 and 2
(b) maximum in both 1 and 2
(c) zero in 1 and maximum in 2
(d) maximum in 1 and zero in 2 

Answer: C

Question. According to Faraday’s law of electromagnetic induction
(a) electric field is produced by time varying magnetic flux.
(b) magnetic field is produced by time varying electric flux.
(c) magnetic field is associated with a moving charge.
(d) None of these

Answer: A

Question. The mutual inductance of a pair of coils, each of N turns, is M henry. If a current of I ampere in one of the coils is brought to zero in t second, the emf induced per turn in the other coil, in volt, will be
(a) MI/t
(b) NMI/t
(c) MN/It
(d) MI/Nt

Answer: A

Question. A copper disc of radius 0.1 m rotated about its centre with 10 revolutions per second in a uniform magnetic field of 0.1 tesla with its plane perpendicular to the field. The e.m.f. induced across the radius of disc is
(a) π/10 volt
(b) 2π/10 volt
(c) π x 10^-2 volt
(d) 2π x 10^-2 volt 

Answer: C

Question. Two solenoids of same cross-sectional area have their lengths and number of turns in ratio of 1 : 2. The ratio of self-inductance of two solenoids is
(a) 1 : 1
(b) 1 : 2
(c) 2 : 1
(d) 1 : 4

Answer: B

Question. Fig shown below represents an area A = 0.5 m² situated in a uniform magnetic field B = 2.0 weber/m² and making an angle of 60º with respect to magnetic field.

The value of the magnetic flux through the area would be equal to
(a) 2.0 weber
(b) √3 weber
(c) √3 / 2 weber
(d) 0.5 weber

Answer: D

Question. A metal rod moves at a constant velocity in a direction perpendicular to its length. A constant uniform magnetic field exists in space in a direction perpendicular to the rod as well its velocity. Select correct statements (s) from the following.
(a) The entire rod is at the same potential
(b) There is an electric field in the rod
(c) The electric potential is highest at the centre
(d) The electric potential is lowest at its centre and increases towards its ends

Answer: B

Question. A small square loop of wire of side l is placed inside a large square loop of side L (L >> l ). The loop are coplanar and their centres coincide. The mutual inductance of the system is proportional is
(a) l/L
(b) l²/L
(c) L/l
(d) L²/l

Answer: B

Question. As a result of change in the magnetic flux linked to the closed loop shown in the figure, an e.m.f. V volt is induced in the loop.
along the loop is
(a) QV
(b) 2QV
(c) QV/2
(d) zero

Answer: A

Question. A wire loop is rotated in a uniform magnetic field about an axis perpendicular to the field. The direction of the current induced in the loop reverses once each
(a) quarter revolution
(b) half revolution
(c) full revolution
(d) two revolutions

Answer: B

Question. If the number of turns per unit length of a coil of solenoid is doubled, the self-inductance of the solenoid will
(a) remain unchanged
(b) be halved
(c) be doubled
(d) become four times

Answer: D

Question. 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) N.A.B.R.w
(b) N.A.B.
(c) N.A.B.R.
(d) N.A.B.w

Answer: D

Question. Lenz’s law is consequence of the law of conservation of
(a) energy
(b) momentum
(c) charge
(d) mass

Answer: A

Question. Eddy currents are produced when
(a) a metal is kept in varying magnetic field
(b) a metal is kept in steady magnetic field
(c) a circular coil is placed in a magnetic field
(d) through a circular coil, current is passed

Answer: A

Question. An inductor may store energy in
(a) its electric field
(b) its coils
(c) its magnetic field
(d) both in electric and magnetic fields

Answer: C

Question. A 100 millihenry coil carries a current of 1 ampere. Energy stored in its magnetic field is
(a) 0.5 J
(b) 1 J
(c) 0.05 J
(d) 0.1 J

Answer: C

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. Two coils have a mutual inductance 0.005 H. The current changes in the first coil according to equation I = I0 sinwt, where I0 = 10 A and w = 100p rad/sec. The maximum value of e.m.f. in the second coil is
(a) p
(b) 5p
(c) 2p
(d) 4p 

Answer :  B

Question. If N is the number of turns in a coil, the value of self inductance varies as
(a) N 0
(b) N
(c) N 2
(d) N–2 

Answer :  C

Question. What is the self-inductance of a coil which produces 5 V when the current changes from 3 ampere to 2 ampere in one millisecond?
(a) 5000 henry
(b) 5 milli-henry
(c) 50 henry
(d) 5 henry 

Answer :  B

Question. A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced e.m.f. is
(a) four times per revolution
(b) six times per revolution
(c) once per revolution
(d) twice per revolution 

Answer :  D

Question. In a region of magnetic induction B = 10–2 tesla, a circular coil of radius 30 cm and resistance p2 ohm is rotated about an axis which is perpendicular to the direction of B and which forms a diameter of the coil. If the coil rotates at 200 rpm the amplitude of the alternating current induced in the coil is
(a) 4p2 mA
(b) 30 mA
(c) 6 mA
(d) 200 mA

Answer :  C

Question. If the number of turns per unit length of a coil of solenoid is doubled, the self-inductance of the solenoid will
(a) remain unchanged
(b) be halved
(c) be doubled
(d) become four times

Answer :  D

Question. A 100 millihenry coil carries a current of 1A. Energy stored in its magnetic field is
(a) 0.5 J
(b) 1 J
(c) 0.05 J
(d) 0.1 J 

Answer :  C

Question. In a coil of resistance 10 W, the induced current developed by changing magnetic flux through it, is shown in figure as a function of time. The magnitude of change in flux through the coil in weber is     
(a) 8
(b) 2
(c) 6
(d) 4

Answer :  B

Important Questions for NCERT Class 12 Physics Alternating Induction

Question. The primary winding of a transformer has 500 turns whereas its secondary has 5000 turns. The primary is connected to an A.C. supply of 20 V, 50 Hz. The secondary will have an output of
(a) 2 V, 50 Hz
(b) 2 V, 5 Hz
(c) 200 V, 50 Hz
(d) 200 V, 500 Hz.

Answer: C

Question. A 50 Hz a.c. source of 20 volt is connected across R and C as shown in figure. The voltage across R is 12 volt. The voltage across C is : 
(a) 8 V
(b) 16 V
(c) 10 V
(d) not possible to determine unless values of R and C are given

Answer: B

Question. Figure shows a circuit that contains three identical resistors with resistance R = 9.0 W each, two identical inductors with inductance L = 2.0 mH each, and an ideal battery with emf e = 18 V. The current i through the battery just after the switch closed is
(a) 0.2 A
(b) 2 A
(c) 0 ampere
(d) 2 mA

Answer: A

Question. In an A.C. circuit, the current flowing in inductance is I = 5 sin (100 t – p/2) amperes and the potential difference is V = 200 sin (100 t) volts. The power consumption is equal to 
(a) 1000 watt
(b) 40 watt
(c) 20 watt
(d) zero

Answer: D

Question. A coil of 40 henry inductance is connected in series with a resistance of 8 ohm and the combination is joined to the terminals of a 2 volt battery. The time constant of the circuit is
(a) 5 seconds
(b) 1/5 seconds
(c) 40 seconds
(d) 20 seconds

Answer : A

Question. An A.C. circuit containing only capacitance, the current : 
(a) lags the voltage by 90°
(b) leads the voltage by 90°
(c) remains in phase with voltage
(d) leads the voltage in 180°

Answer:  B

Question. The two rails of a railway track, insulated from each other and the ground, are connected to millivoltmeter. What is the reading of the millivoltmeter when a train passes at a speed of 180 km/hr along the track, given that the vertical component of earth’s magnetic field is 0.2 × 10^–4 wb/m² and rails are separated by 1 metre
(a) 10^–2 volt
(b) 10mV
(c) 1 volt
(d) 1 mV

Answer: D

Question. A current i = 2 sin (πt/3) amp is flowing in an inductor of 2 henry. The amount of work done in increasing the current from 1.0 amp to 2.0 amp is
(a) 1 J
(b) 2 J
(c) 3 J
(d) 4 J

Answer: C

Question. The self inductance of the motor of an electric fan is 10 H. In order to impart maximum power at 50 Hz, it should be connected to a capacitance of
(a) 8μF
(b) 4μF
(c) 2μF
(d) 1μF

Answer: D

Question. When the current in a coil changes from 2 amp. to 4 amp. in 0.05 sec., an e.m.f. of 8 volt is induced in the coil. The coefficient of self inductance of the coil is
(a) 0.1 henry
(b) 0.2 henry
(c) 0.4 henry
(d) 0.8 henry

Answer: B

Question. The total charge induced in a conducting loop when it is moved in a magnetic field depend on
(a) the rate of change of magnetic flux
(b) initial magnetic flux only
(c) the total change in magnetic flux
(d) final magnetic flux only

Answer: C

Question. Which of the following units denotes the dimension ML2/Q where Q denotes the electric charge?
(a) Wb/m²
(b) henry (H)
(c) H/m²
(d) weber (Wb)

Answer: B

Question. A rectangular coil of single turn, having area A, rotates in a uniform magnetic field B with an angular velocity w about an axis perpendicular to the field. If initially the plane of the coil is perpendicular to the field, then the average induced emf when it has rotated through 90° is
(a) ωBA/π
(b) ωBA/2π
(c) ωBA/4π
(d) 2ωBA/π

Answer: D

Question. Induced emf in the coil depends upon
(a) conductivity of coil
(b) amount of flux
(c) rate of change of linked flux
(d) resistance of coil

Answer: C

Question. Magnetic flux Φ in weber in a closed circuit of resistance 10Ω varies with time Φ (sec) as f = 6t² – 5t + 1. The magnitude of induced current at t = 0.25s is
(a) 0.2 A
(b) 0.6 A
(c) 1.2 A
(d) 0.8 A 

Answer: A

Question. A solenoid has 2000 turns wound over a length of 0.3 m. Its cross-sectional area is 1.2 × 10^–3 m². Around its central section a coil of 300 turns is wound. If an initial current of 2 A flowing in the solenoid is reversed in 0.25 s, the emf induced in the coil will be
(a) 2.4 × 10^–4 V
(b) 2.4 × 10^–2 V
(c) 4.8 × 10^–4 V
(d) 4.8 × 10^–2 V

Answer: B

Question. A coil having 500 square loops each of side 10 cm is placed normal to a magnetic field which increases at the rate of 1 Wb/m². The induced e.m.f. is
(a) 0.1 V
(b) 5.0 V
(c) 0.5 V
(d) 1.0 V

Answer: B

Question. In a coil of area 10 cm² and 10 turns with magnetic field directed perpendicular to the plane and is changing at the rate of 108 Gauss/second. The resistance of the coil is 20Ω. The current in the coil will be
(a) 0.5 A
(b) 5 A
(c) 50 A
(d) 5 × 10⁸ A

Answer: B

Question. If rotational velocity of a dynamo armature is doubled, then induced e.m.f. will become
(a) half
(b) two times
(c) four times
(d) unchanged

Answer: B

Question. A conducting square loop of side L and resistance R moves in its plane with a uniform velocity v perpendicular to one of its side. A magnetic induction B constant in time and space, pointing perpendicular and into the plane of the loop exists everywhere.

The current induced in the loop is
(a) B l v/R clockwise
(b) B l v/R anticlockwise
(c) 2B l v/R anticlockwise
(d) zero 

Answer: D

Question. There are two coils A and B as shown in figure a current starts flowing in B as shown, when A is moved towards B and stops when A stops moving. The current in A is counter clockwise. B is kept stationary when A moves. We can infer that
(a) there is a constant current in the clockwise direction in A
(b) there is a varying current in A
(c) there is no current in A
(d) there is a constant current in the counter clockwise direction in A 

Answer: D

Question. In the figure the flux through the loop perpendicular to the plane of the coil and directed into the paper is varying according to the relation Φ = 6t² + 7t + 1 where Φ is in milliweber and t is in second. The magnitude of the emf induced in the loop at t = 2 s and the direction of induce current through R are 

(a) 39 mV; right to left
(b) 39 mV; left to right 
(c) 31 mV; right to left
(d) 31 mV; left to right

Answer: D

Question. A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced e.m.f. is 
(a) twice per revolution
(b) four times per revolution
(c) six times per revolution
(d) once per revolution

Answer: A

Question. A conducting circular loop is placed in a uniform magnetic field, B = 0.025 T with its plane perpendicular to the loop.
The radius of the loop is made to shrink at a constant rate of 1 mm s^–1. The induced e.m.f. when the radius is 2 cm, is
(a) 2πμV
(b) πμV
(c) π/2 V
(d) 2μV

Answer: B

Question. If a current increases from zero to one ampere in 0.1 second in a coil of 5 mH, then the magnitude of the induced e.m.f. will be
(a) 0.005 volt
(b) 0.5 volt
(c) 0.05 volt
(d) 5 volt

Answer: C

Question. The armature of a dc motor has 20Ω resistance. It draws a current of 1.5 A when run by a 220 V dc supply. The value of the back emf induced in it is
(a) 150 V
(b) 170 V
(c) 180 V
(d) 190 V

Answer: D