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

Conceptual and application type questions

1 What is the power dissipated in an AC circuit in which voltage and current are given by V = 230sin(ωt+π/2) and I = 10 sinωt? Give reason

2 Two identical loops ,one of copper and other of constantan ,are removed from a magnetic field within the same time interval. In which loop will the induced current be greater? Justify.

3 A conducting coil is moved relative to a magnetic field .Will there be induced emf and induced current always? Justify

4.What is the significance of power factor in i) electrical appliances ii) transmission of electric power?

5 Two identical bar magnets are dropped from the same height simultaneously , one falls through a copper tube and the other falls through air, will they reach ground at the same time? Justify.

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. A circular disc of radius 0.2 meter is placed in a uniform magnetic field of induction  1/∏ (Wb /m²) in such a way that its axis makes an angle of 60° with ^-B. The magnetic flux linked with the disc is
(a) 0.08 Wb
(b) 0.01 Wb
(c) 0.02 Wb
(d) 0.06 Wb

Answer :  C

Question. A 800 turn coil of effective area 0.05 m2 is kept perpendicular to a magnetic field 5 × 10^–5 T. When the plane of the coil is rotated by 90° around any of its coplanar axis in 0.1 s, the emf induced in the coil will be
(a) 0.02 V
(b) 2 V
(c) 0.2 V
(d) 2 × 10^–3 V

Answer :  A

Question. A coil of resistance 400 W is placed in a magnetic field. If the magnetic flux f (Wb) linked with the coil varies with time t (sec) as f = 50t2 + 4. The current in the coil at t = 2 sec is
(a) 0.5 A
(b) 0.1 A
(c) 2 A
(d) 1 A 

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 emf when the radius is 2 cm, is
(a) 2Π μV
(b) Π μV
(c) Π/2 μV
(d) 2 μV 

Answer :  B

Question. A rectangular coil of 20 turns and area of cross-section 25 sq. cm has a resistance of 100 W. If a magnetic field which is perpendicular to the plane of coil changes at a rate of 1000 tesla per second, the current in the coil is
(a) 1 A
(b) 50 A
(c) 0.5 A
(d) 5 A 

Answer :  C

Question. A magnetic field of 2 × 10–2 T acts at right angles to a coil of area 100 cm2, with 50 turns. The average e.m.f. induced in the coil is 0.1 V, when it is removed from the field in t sec. The value of t is
(a) 10 s
(b) 0.1 s
(c) 0.01 s
(d) 1 s

Answer :  B

Question. A metal ring is held horizontally and bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet is
(a) more than g
(b) equal to g
(c) less than g
(d) either(a) or (c) 

Answer :  C

Electromagnetic Induction and Alternating Currents 

1. The current passing through the wire A, B is increasing. In which direction does the induced current flow in the closed loop.

2. State Faraday’s Law of electromagnetic induction. Express it mathematically.

3. State Lenz Law. On which law of conservation is it based?

4. How are eddy currents produced?

5. Power factor of an a.c. circuit is 0.5. What will be the phase difference between voltage and current in the circuit?

6. What is the significance of a Q-factor in a series LCR resonant circuit?

7. What is the principle of a.c. generator?

8. What is the power consumed in (i) purely inductive and (ii) purely capacitive a.c. circuits?

9. Draw the graph to show the variation of Xc with the frequency of the a.c. source used.

10. Which is the best method of reducing current in an a.c. circuit and why?

11. If the no of turns of a solenoid is doubled, keeping the other factors constant how does the selfinductance of the solenoid change?

12. Sketch the variation of inductive reactance and capacitive reactance with the frequency of a.c. source.

13. What is the frequency of direct current?

14. What is the relation between peak value and root mean square value of alternating e.m.f.?

15. What is the mean value of an alternating current?

16. When are the voltage and current in LCR-circuit in same phase?

17. a capacitor blocks d.c. Why?

18. The frequency of a.c. source is doubled. How do R, XL and Xc get affected?

19. Which is more dangerous than d.c. for the same value?

20. A lamp is connected in series with a capacitor. Predict your observation for d.c. and a.c. connections. What happens in each case if the capacitance is reduced?

21. What is the unit of L/R?

22. As soon as the current is switched on in a high voltage wire, the bird sitting on it flies away, Why?

23. Fig shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph showing variation of induced e.m.f. E with time (t). What does the area enclosed by the E-t curve depict? M

24. A rectangular loop of wire is being withdrawn out of the magnetic field with velocity v. The magnetic field is perpendicular to the plane of paper. What will be the direction of induced current, if any, in the loop?

5 mark questions:-

25. Derive expression for self-inductance of a long air-cored solenoid of length l, radius r and having number of turns N.

26. What do you mean by mutual inductance of two nearby coils? Find an expression for mutual inductance of a solenoid-coil system.

27. Define the term capacitive reactance; show graphically the variation of capacitive reactance with frequency of applied voltage by angle π/2.
Describe briefly the principle, construction and working of a transformer. Why is its core laminated?
 

Important Questions for NCERT Class 12 Physics Electromagnetic Induction

Question. Faraday’s laws are consequence of conservation of
(a) energy
(b) energy and magnetic field
(c) charge
(d) magnetic field

Answer :  A.

Question. Two coils of self inductance 2 mH and 8 mH are placed so close together that the effective flux in one coil is completely linked with the other.
The mutual inductance between these coils is
(a) 16 mH
(b) 10 mH
(c) 6 mH
(d) 4 mH 

Answer :  D

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

Question. A straight line conductor of length 0.4 m is moved with a speed of 7 m/s perpendicular to a magnetic field of intensity 0.9 Wb/m2. The induced e.m.f. across the conductor is
(a) 5.04 V
(b) 25.2 V
(c) 1.26 V
(d) 2.52 V

Answer :  D

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 0 A from 4 A in 0.05 s. If the resistance of the coil is 10 p2 W, the total charge flowing through the coil during this time is
(a) 16 mC
(b) 32 mC
(c) 16p mC
(d) 32p mC

Answer :  B

Question. The magnetic flux through a circuit of resistance R changes by an amount Df in a time Dt. Then the total quantity of electric charge Q that passes any point in the circuit during the time Dt is represented by

(a) Q = 1/R . ΔΦ /Δt
(b) Q = ΔΦ/R
(c) Q = ΔΦ /Δt
(d) Q= R.ΔΦ /Δt

Answer :  B

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

Question. The magnetic flux through a circuit of resistance R changes by an amount Df in a time Dt. Then the total quantity of electric charge Q that passes any point in the circuit during the time Dt is represented by      
(a) Q = 1/R ^. ΔΦ//Δt 
(b) Q = ΔΦ//R
(c) Q = ΔΦ//Δt 
(d) Q = R .ΔΦ//Δt

Answer :  B

Question. A small loop lies outside a circuit. The key of the circuit is closed and opened alternately. The closed loop will show:

a. Clockwise pulse followed by another clockwise pulse
b. Anticlockwise pulse followed by another anticlockwise pulse
c. Anticlockwise pulse followed by a clockwise pulse
d. Clockwise pulse followed by an anticlockwise pulse
Answer : D

Question. A bar magnet is falling freely inside a long copper tube and a solenoid as shown in figure (i) and (ii) respectively then acceleration of magnet inside the copper tube and solenoid are respectively: (acceleration due to gravity = g)

a. g, g
b. Greater than g, lesser than g
c. Greater than g, g
d. Zero, lesser than g

Answer : D

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. A coil of area A = 0.5 m² is situated in a uniform magnetic field B = 4.0 wb/m² and area vector makes an angle of 60° with respect to the magnetic field as shown in figure. The value of the magnetic flux through the area A would be equal to:

a. 2 weber
b. 1 weber
c. 3 weber
d. 3/2 weber
Answer : B

Question. Consider the following figure, a uniform magnetic field of 0.2 T is directed along the positive x-axis. What is the magnetic flux through top surface of the figure?

a. Zero
b. 0.8 m-wb
c. 1.0 m-wb
d. – 1.8 m-wb
Answer : C

Question. An ideal transformer does not vary the output:
a. energy
b. power
c. frequency
d. current
Answer : A, B, C

Question. Consider the arrangement shown in figure in which the north pole of a magnet is moved away from a thick conducting loop containing capacitor. Then excess positive charge will arrive on:

a. Plate a
b. Plate b
c. On both plates a and b
d. On neither a nor b plates
Answer : B

Question. Alternating current can be used for:
a. heating
b. lighting
c. electrolysis
d. generate mechanical energy
Answer : A, B, D

Question. A square loop of side 1m is placed in a perpendicular magnetic field. Half of the area of the loop inside the magnetic field. A battery of emf 10 V and negligible internal resistance is connected in the loop. The magnetic field changes with time according to relation B = 0.01 – 2t Tesla. The resultant emf in the loop will be:

a. 1 V
b. 11 V
c. 10 V
d. 9 V
Answer : D

Question. Which of the following phenomenon cause loss of energy in a transformer?
a. heating
b. eddy currents
c. mechanical motion
d. hysteresis
Answer : A, B, D

Question. The core of the transformer is laminated to:
a. reduce the eddy currents
b. reduce self induction
c. increase the efficiency
d. decrease the weight of the transformer
Answer : A, C

Question. Which of the statements is/are true? Heat produced in a current-carrying conductor depends upon:
a. the time for which the current flows in the conductor
b. the resistance of the conductor
c. the strength of the current
d. the nature of current (A.C. or D.C.)
Answer : A, B, C

Question. Which of the followings does not decrease the efficiency of the transformer?
a. laminating the core
b. use of iron core
c. core made of material having narrow hysteresis loop
d. none of the above
Answer : A, B, C

Question. An alternating e.m.f. of frequency v ( = 1/2π√LC) is applied to a series LCR circuit. For this frequency of the applied e.m.f.
a. The circuit is at resonance and its impedance is made up only of a reactive part
b. The current in the circuit is in phase with the applied e.m.f. and the voltage across R equals this applied emf
c. The sum of the p.d.’s across the inductance and capacitance equals the applied e.m.f. which is 180° ahead of phase of the current in the circuit
d. The quality factor of the circuit is ωL / R or 1 / ωCRR and this is a measure of the voltage magnification (produced by the circuit at resonance) as well as the sharpness of resonance of the circuit
Answer : B, D

Assertion and Reason

Note: Read the Assertion (A) and Reason (R) carefully to mark the correct option out of the options given below:
a. If both assertion and reason are true and the reason is the correct explanation of the assertion.
b. If both assertion and reason are true but reason is not the correct explanation of the assertion.
c. If assertion is true but reason is false.
d. If the assertion and reason both are false.
e. If assertion is false but reason is true.

Question. Assertion: A rectangular loop and a circular loop are moved with a constant velocity from a region of magnetic field out into a field-free region. The field is normal to the loops. Then a constant emf will be induced in the circular loop and a time-varying emf will be induced in the rectangular loop.
Reason: The induced emf is constant if the magnetic flux changes at a constant rate.
Answer : D

Question. Assertion: The alternating current lags behind the e.m.f. by a phase angle of , π/2 when ac flows through an inductor.
Reason: The inductive reactance increases as the frequency of ac source decreases.
Answer : E

Question. Assertion: A coil is connected in series with a bulb and this combination is connected to an a.c. source. If an iron core is inserted in the coil, the brightness of the bulb will be reduced.
Reason: When an iron core is inserted in the coil, its inductance decreases.
Answer : C

Question. Assertion: Magnetic flux can produce induced e.m.f.
Reason: Faraday established induced e.m.f. experimentally.
Answer : E

Question. Assertion: A capacitor of suitable capacitance can be used in an ac circuit in place of the choke coil.
Reason: A capacitor blocks dc and allows ac only.
Answer : B

Question. Assertion: A magnetised iron bar is dropped vertically through a hollow region of a thick cylindrical shell made of copper. The bar will fall with an acceleration less than g, the acceleration due to gravity.
Reason: The emf induced in the bar causes a retarding force to act on the falling bar.
Answer : A

Question. Assertion: If the frequency of alternating current in an ac circuit consisting of an inductance coil is increased then current gets decreased.
Reason: The current is inversely proportional to frequency of alternating current.
Answer : A

Question. Assertion: When capacitive reactance is smaller than the inductive reactance in LCR current, e.m.f. leads the current.
Reason: The phase angle is the angle between the alternating e.m.f. and alternating current of the circuit.
Answer : B

Question. Assertion: An ac generator is based on the phenomenon of self-induction
Reason: In single coil, we consider self-induction only.
Answer : E

Question. Assertion: In series LCR circuit resonance can take place.
Reason: Resonance takes place if inductance and capacitive reactance’s are equal and opposite.
Answer : A

Question. Assertion: Chock coil is preferred over a resistor to adjust current in an ac circuit.
Reason: Power factor for inductance is zero.
Answer : A

Question. Assertion: A bulb connected in series with a solenoid is connected to ac source. If a soft iron core is introduced in the solenoid, the bulb will glow brighter.
Reason: On introducing soft iron core in the solenoid, the inductance increases.
Answer : E

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 or the cause the produced it.
Answer : A

Question. Assertion: An aircraft flies along the meridian, the potential at the ends of its wings will be the same.
Reason: Whenever there is change in the magnetic flux e.m.f. induces.
Answer : E

Question. Assertion: An alternating current does not show any magnetic effect.
Reason: Alternating current varies with time.
Answer : B