CBSE Class 12 Physics Electromagnetic Induction Assignment

 

 Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. A coil of copper having 1000 turns is placed in a magnetic field (B = 4 × 10^–5) perpendicular to its axis. The cross sectional area of the coil is 0.05 m². If it turns through 180° in 0.01 second, then the e.m.f. induced in the coil will be ; 
(a) 4 V
(b) 0.04 V
(c) 0.4 V
(d) 0.2 V

Answer :  C

Question. The current in self inductance L = 40 mH is to be increased uniformly from 1 amp to 11 amp in 4 milliseconds. The e.m.f. induced in inductor during process is
(a) 100 volt
(b) 0.4 volt
(c) 4.0 volt
(d) 440 volt 

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. In a coil of self inductance of 5 henry, the rate of change of current is 2 ampere per second, the e.m.f. induced in the coil is : 
(a) 5 V
(b) – 5V
(c) – 10 V
(d) 10 V

Answer :  C

Question. According to Lenz’s law there is a conversion of:
(a) momentum
(b) collision 
(c) voltage
(d) energy

Answer :  D

Question. A 50 turn circular coil has a radius of 3 cm, it is kept in a magnetic field acting normal to the area of the coil. The magnetic field B is increased from 0.10 T to 0.35 T in 2 milli second, the average induced emf will be: 
(a) 177 V
(b) 1.77 V
(c) 0.177 V
(d) 17.7 V

Answer :  B

Question. A solenoid is 1.5 m long and its inner diameter is 4.0 cm. It has 3 layers of windings of 1000 turns each and carries a current of 2.0 amperes. The magnetic flux for a cross-section of the solenoid is nearly 
(a) 4.1´10-5 weber
(b) 5.2´10-5 weber
(c) 6.31´10-3 weber
(d) 2.5´10-7 weber 

Answer :  C

Question. The current in self inductance L = 40 mH is to be increased uniformly from 1 amp to 11 amp in 4 milliseconds. The e.m.f. induced in inductor during process is   
(a) 100 volt
(b) 0.4 volt
(c) 4.0 volt
(d) 440 volt 

Answer :  A

 

Important Questions for NCERT Class 12 Physics Alternating Induction

 

Question. In an A.C. circuit, the current flowing is I = 5 sin (100t – p/2) ampere and the potential difference is V = 200 sin (100t) volts. The power consumption is equal to
(a) 20 W
(b) 0 W
(c) 1000 W
(d) 40 W

Answer   B

Question. A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produces oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be
(a) f/2
(b) f/4
(c) 8f
(d) f /2 2

Answer   D

Question. A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6 A, the voltage across the secondary coil and the current in the primary coil respectively are
(a) 300 V, 15 A
(b) 450 V, 15 A
(c) 450 V, 13.5 A
(d) 600 V, 15 A 

Answer   B

Question. In an ideal parallel LC circuit, the capacitor is charged by connecting it to a D.C. source which is then disconnected. The current in the circuit 
(a) becomes zero instantaneously 
(b) grows monotonically
(c) decays monotonically
(d) oscillates instantaneously

Answer   D

Question. With the decrease of current in the primary coil from 2 amperes to zero value in 0.01s the emf generated in the secondary coil is 1000 volts.
The mutual inductance of the two coils is C
(a) 1.25 H
(b) 2.50 H
(c) 5.00 H
(d) 10.00 H

Answer   C

Question. An AC source of angular frequency w is fed across a resistor R and a capacitor C in series.
The current registered is I. If now the frequency of source is changed to w/3 (but maintaining the same voltage), the current in the circuit is found to be halved. Calculate the ratio of reactance to resistance at the original frequency w A
(a) √3/5
(b) √2/5
(c) √1/5
(d) √4/5

Answer   A

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. The flux linked with a coil at any instant 't' is given by f = 10t2 – 50t + 250. The induced emf at t = 3s is
(a) –190 V
(b) –10 V 
(c) 10 V
(d) 190 V
Answer :  B

Question. In an AC generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency w 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. A conducting ring of radius 1 metre is placed in an uniform magnetic field B of 0.01 tesla oscillating with frequency 100 Hz with its plane at right angle to B. What will be the induced electric field ? 
(a) pvolt /m
(b) 2volt /m
(c) 10 volt/m
(d) 62 volt/m
Answer :  B

Question. A wheel with ten metallic spokes each 0.50m long is rotated with a speed of 120 rev/min in a plane normal to the earth’ s magnetic field at the place.
If the magnitude of the field is 0.40 G, the induced emf between the axle and the rim of the wheel is equal to 
(a) 1.256 × 10 ^{– 3} V
(b) 6.28 × 10 ^{– 4} V
(c) 1.256 × 10 ^{– 4} V
(d) 6.28 × 10 ^{– 5} V
Answer :  D

Question. The magnetic flux through a circuit carrying a current of 2.0 A is 0.8 weber. If the current reduces to 1.5 A in 0.1 s, the induced emf be :
(a) 2.0 V
(b) 4.0 V
(c) 8.0 V
(d) none of the above
Answer :  A

Question. A charged particle moves through a magnetic field in a direction perpendicular to it. Then the
(a) velocity remains unchanged 
(b) speed of the particle remains unchanged
(c) direction of the particle remains unchanged
(d) acceleration remains unchanged
Answer :  B

Question. In an inductor of self-inductance L = 2 mH, current changes with time according to relation i = t2e–t. At what time emf is zero? 
(a) 4s
(b) 3s
(c) 2s
(d) 1s
Answer :  C

Question. A cycle wheel of radius 0.5 m is rotated with constant angular velocity of 10 rad/s in a region of magnetic field of 0.1 T which is perpendicular to the plane of the wheel. The EMF generated between its centre and the rim is    
(a) 0.25 V
(b) 0.125 V
(c) 0.5 V
(d) zero
Answer :  B

 

Important Questions for NCERT Class 12 Physics Alternating Induction

Question. For the circuit shown in the fig., the current through the inductor is 0.9 A while the current through the condenser is 0.4 A. Then 
(a) current drawn from generator I = 1.13 A
(b) w = 1/(1.5 L C)
(c) I = 0.5 A
(d) I = 0.6 A
Answer   C

Question. The primary of a transformer when connected to a dc battery of 10 volt draws a current of 1 mA. The number of turns of the primary and secondary windings are 50 and 100 respectively. The voltage in the secondary and the current drawn by the circuit in the secondary are respectively
(a) 20 V and 2.0 mA
(b) 10 V and 0.5 mA
(c) Zero volt and therefore no current
(d) 20 V and 0.5 mA 
Answer   C

Question. A 220 volt input is supplied to a transformer. The output circuit draws a current of 2.0 ampere at 440 volts. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is
(a) 3.6 ampere
(b) 2.8 ampere
(c) 2.5 ampere
(d) 5.0 ampere 
Answer   D

Question. In an A.C. circuit, Irms and I0 are related as D
(a) Irms = pI0
(b) Irms = 2 I0
(c) Irms = I0/p
(d) Irms = I0 / 2 
Answer   D

Question. The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux f linked with the primary coil is given by f = f0 + 4t, where f is in webers, t is time in seconds and f0 is a constant, the output voltage across the secondary coil is
(a) 120 volts
(b) 220 volts
(c) 30 volts
(d) 90 volts 
Answer   A

Question. An ideal coil of 10H is connected in series with a resistance of 5W and a battery of 5V. After 2 sec the connection is made, the current flowing in ampere in the circuit is A
(a) (1 – e^–1)
(b) (1 – e)
(c) e
(d) e^–1
Answer   A

ASSERTION REASON TYPE QUESTIONS

Directions: These questions consist of two statements, each printed as Assertion and Reason. While answering these questions, you are required to choose any one of the following four responses.
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the
Assertion. (c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

Question. Assertion: Induced emf will always occur whenever there is change in magnetic flux.
Reason : Current always induces whenever there is change in magnetic flux.
Answer. C

Question. Assertion: Faraday’s laws are consequence of conservation of energy.
Reason : Self inductance of a coil depends on current flowing through the coil.
Answer. C

Question. Assertion : Only a change in magnetic flux will maintain an induced current in the coil.
Reason : The presence of large magnetic flux through a coil maintain a current in the coil if the circuit is continuous.
Answer. C

Question. Assertion: Lenz’s law violates the principle of conservation of energy.
Reason: Induced emf always opposes the change in magnetic flux responsible for its production.
Answer. A

Question. Assertion : In the phenomenon of mutual induction, self-induction of each of the coils persists.
Reason : Self -induction arises when strength of current in same coil changes. In mutual induction, current is changing in both the individual coils.
Answer. B

Question. Assertion : Acceleration of a magnet falling through a long solenoid decreases.
Reason : The induced current produced in a circuit always flow in such direction that it opposes the change to the cause that produced it.
Answer. A

Question. Assertion : An emf can be induced by moving a conductor in a magnetic field.
Reason : An emf can be induced by changing the magnetic field.
Answer. B

Question. Assertion : Eddy currents are produced in any metallic conductor when magnetic flux is changed around it.
Reason : Electric potential determines the flow of charge
Answer. B

 

CASE STUDY BASED QUESTION

1. Electromagnetic induction is defined as the production of an electromotive force across an electric conductor in the changing magnetic field. The discovery of induction was done by Michael Faraday in the year 1831. Electromagnetic induction finds many applications such as in electrical components which includes transformers, inductors, and other devices such as electric motors and generators. Alternating current is defined as an electric current which reverse in direction periodically. In most of the electric power circuits, the waveform of alternating current is the sine wave.

Question. How to increase the energy stored in an inductor by four times?
(a) By doubling the current
(b) This is not possible
(c) By doubling the inductance
(d) By making current 2 times
Answer. A

Question. Consider an inductor whose linear dimensions are tripled and the total number of turns per unit length is kept constant, what happens to the self-inductance?
(a) 9 times
(b) 3 times
(c) 27 times
(d) 13 times
Answer. B

Question. Lenz law is based on which of the following conservation
(a) Charge
(b) Mass
(c) Momentum
(d) Energy
Answer. D

Question. What will be the acceleration of the falling bar magnet which passes through the ring such that the ring is held horizontally and the bar magnet is dropped along the axis of the ring?
(a) It depends on the diameter of the ring and the length of the magnet
(b) It is equal due to gravity
(c) It is less than due to gravity
(d) It is more than due to gravity
Answer. C

 

2. Self Induction. When a current I flows through a coil, flux linked with it is φ = LI, where L is a constant known as self inductance of the coil. Any charge in current sets up an induced emf in the coil. Thus, self inductance of a coil is the induced emf set up in it when the current passing through it changes at the unit rate. It is a measure of the opposition to the growth or the decay of current flowing through the coil. Also, value of self inductance depends on the number of turns in the solenoid, its area of cross-section and the permeability of its core material.

Question. The inductance in a coil plays the same role as
(a) inertia in mechanics
(b) energy in mechanics
(c) momentum in mechanics
(d) force in mechanics
Answer. A

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. The inductance L of a solenoid depends upon its radius R as
(a) L ∝ R
(b) L ∝ 1/R
(c) L ∝ R2
(d) L ∝ R3
Answer. C

Question. The unit of self-inductance is
(a) Weber ampere
(b) Weber-1 ampere
(c) Ohm second
(d) Farad
Answer. C

Question. The induced emf in a coil of 10 henry inductance in which current varies from 9 A to 4 A in 0.2 second is
(a) 200 V
(b) 250 V
(c) 300 V
(d) 350 V
Answer. B

NUMERICALS

Question. Current in a circuit falls from 5.0 A to 0.0 A in 0.1 s. If an average emf of 200 V is induced, calculate the self-induction of the circuit.
Answer. 4H

Question. A long solenoid with 15 turns per cm has a small loop of area 2.0 cm² placed inside normal to the axis of the solenoid. The current carried by the solenoid changes steadily from 2 A to 4 A in 0.1 s, what is the induced emf in the loop while the current is changing? 
Answer. M= 120X3.14x10^-9 H and e₂ = 7.5x 10^-6V

Question. An air cored solenoid with length 30 cm, area of cross-section 25 cm² and number of turns 500 carries a current of 2.5 A. The current is suddenly switched off in a brief time of 10^-3 s. How much is the average back emf induced across the ends of the open switch in the circuit ? Ignore the variation in magnetic field near the ends of the solenoid.
Answer. 6.5V

Question. A rectangular loop of sides 8 cm × 2 cm with a small cut is stationary in a uniform magnetic field produced by an electromagnet. If the current feeding the electromagnet is gradually reduced so that the magnetic field decreases from its initial value of 0·3 T at the rate of 0·02 Ts^-1. If the cut is joined and the loop has a resistance of 1·6 Ω, how much power is dissipated by the loop as heat ? What is the source of this power?
Answer. 6.4 x 10^-10 W

Question. A 0.5 m long solenoid of 10 turns/cm has area of cross-section 1 cm² . Calculate the voltage induced across its ends if the current in the solenoid is changed from 1A to 2A in 0.1s.
Answer. 0.628mV

 

1. When current in a coil changes with time, how is the back e.m.f. induced in the coil related to it?

2. Two identical loops one of copper and the other of aluminium are rotated with the same angular speed in some magnetic field. Compare

(a) induced e.m.f and

(b) the current produced in the two loops.

3. A jet plane is traveling west at 450 ms ^-1. If the horizontal component of earth’s magnetic field at that place 4x10^-4 Tesla and the angle of dip is 300, find the e.m.f. induced between the ends of wings having a span of 30m.

4. A bar magnet is falling under gravity through an air cored coil C. Plot a graph between induced e.m.f E and time t. What does the area enclosed by curve depict?

5. An a.c. generator consists of a coil of 50 turns and area 2.5 m² rotating at an angular speed of 60 rad/s in a uniform magnetic field B = 0.30 T between two fixed pole pieces. The resistance of the circuit including that of the coil is 500 Ω.

(a) What is the maximum current drawn from the generator?

(b) What is the flux through the coil when the current is zero? What is the flux when the current is maximum?

(c) Would the generator work if the coil were stationary and instead the pole pieces rotated together with the same speed as above?

6. A circular coil of radius 8.0 cm and 20 turns is rotated about its vertical diameter with an angular speed of 50 rad/s in a uniform horizontal magnetic field of magnitude 3.0x10^-2 T. Obtain the maximum and average e.m.f induced in the coil. If the coil forms a closed loop of resistance 10Ω, calculate the maximum value of current in the coil. Calculate the average power loss due to joule heating. Where does the power come from?

7. An air cored solenoid is of length 0.3m, area of cross section 1.2 x 10^-3 m² and has 2500 turns. Around its central section, a coil of 350 turns is wound. The solenoid and the coil are electrically insulated from each other. Calculate the e.m.f induced in the coil if the initial current of 3A in the solenoid is reversed in 0.25s.