#### 12th Standard Physics English Medium Electromagnetic Induction and Alternating Current Reduced Syllabus Important Questions With Answer Key 2021

12th Standard

Reg.No. :
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Physics

Time : 01:00:00 Hrs
Total Marks : 105

Multiple Choice Questions

15 x 1 = 15
1. When the current changes from +2A to −2A in 0.05 s, an emf of 8 V is induced in a coil. Th e co-efficient of self-induction of the coil is

(a)

0.2H

(b)

0.4H

(c)

0.8H

(d)

0.1H

2. A circular coil with a cross-sectional area of 4 cm2 has 10 turns. It is placed at the centre of a long solenoid that has 15 turns/cm and a cross-sectional area of 10 cm2. The axis of the coil coincides with the axis of the solenoid. What is their mutual inductance?

(a)

7.54 μH

(b)

8.54 μH

(c)

9.54 μH

(d)

10.54 μH

3. In a transformer, the number of turns in the primary and the secondary are 410 and 1230 respectively. If the current in primary is 6A, then that in the secondary coil is

(a)

2 A

(b)

18 A

(c)

12 A

(d)

1 A

4. In a series resonant RLC circuit, the voltage across 100 Ω resistor is 40 V. The resonant frequency ω is 250 rad/s. If the value of C is 4 μF, then the voltage across L is

(a)

600 V

(b)

4000 V

(c)

400 V

(d)

1 V

5. In an oscillating LC circuit, the maximum charge on the capacitor is Q. The charge on the capacitor when the energy is stored equally between the electric and magnetic fields is

(a)

$\frac{Q}{2}$

(b)

$\frac{Q}{\sqrt3}$

(c)

$\frac{Q}{\sqrt2}$

(d)

Q

6. An emf of 12V is induced when the current in the coil changes at the rate of 40 A S-1. The coefficient of self induction of the coil is

(a)

0.3 H

(b)

0.003 H

(c)

30 H

(d)

4.8 H

7. In Fleming's right hand rule, the forefinger represents the direction of

(a)

motion of the conductor

(b)

magnetic field

(c)

induced current

(d)

induced emf

8. The self inductance of a coil is

(a)

directly proportional to N

(b)

inversely proportional to N

(c)

directly proportional to N2

(d)

inversely proportional to N2

9. A phenomenon in which a varying current in one coil induces an emf in the neighbouring coil is

(a)

mutual induction

(b)

self induction

(c)

electrostatic induction

(d)

electromagnetic induction

10. If two coils are wound on a soft iron core the mutual induction is

(a)

small

(b)

very large

(c)

infinity

(d)

zero

11. Eddy currents was first observed by

(a)

Foucault

(b)

Newton

(c)

(d)

Fleming

12. The property of a coil which enables to produce an opposite induced emf in it when the current in the coil changes is called _______________

(a)

electromagnetism

(b)

self - induction

(c)

mutual induction

(d)

parallel- induction

13. The necessary magnetic field for a low power a.c generator is produced by _________

(a)

electric coil

(b)

permanent magnets

(c)

electromagnets

(d)

batteries

14. A transformer

(a)

converts energy

(b)

changes voltage

(c)

changes current

(d)

changes both current and voltage

15. In an a.c. circuit with an inductor

(a)

voltage lags current by π/2

(b)

voltage and current are in phase

(c)

(d)

current lags voltage by π/2

16. 2 Marks

10 x 2 = 20
17. State Fleming’s right hand rule.

18. Mention the ways of producing induced emf.

19. What do you mean by self-induction?

20. List out the advantages of stationary armature-rotating field system of AC generator.

21. How will you define RMS value of an alternating current?

22. Explain the use of transformers in long distance transmission of electric power

23. What is the signifienance of Quality factor or Q-factor.

24. A bar magnet is moved in the direction indicted by the arrow between two coils PQ and CD predict the directions of induced current in each coil.

25. A rectangular loop of wire is moved within the region of a uniform magnetic field acting perpendicular to its plane. What is the direction and magnitude of current induced in it?

26. Three ac circuits in which equal amount of circuits are flowing. If the frequency of emf is increased how will the current the affected in these circuits? Give reason.

27. 3 Marks

10 x 3 = 30
28. A circular antenna of area 3 m2 is installed at a place in Madurai. The plane of the area of antenna is inclined at 47o with the direction of Earth’s magnetic field. If the magnitude of Earth’s field at that place is 40773.9 nT find the magnetic flux linked with the antenna.

29. A closed coil of 40 turns and of area 200 cm2, is rotated in a magnetic field of flux density 2 Wb m-2. It rotates from a position where its plane makes an angle of 30o with the field to a position perpendicular to the field in a time 0.2 sec. Find the magnitude of the emf induced in the coil due to its rotation.

30. A copper rod of length l rotates about one of its ends with an angular velocity ω in a magnetic field B as shown in the figure. The plane of rotation is perpendicular to the field. Find the emf induced between the two ends of the rod.

31. The self-inductance of an air-core solenoid is 4.8 mH. If its core is replaced by iron core, then its self-inductance becomes 1.8 H. Find out the relative permeability of iron.

32. A circular metal of area 0.03 m2 rotates in a uniform magnetic field of 0.4 T. The axis of rotation passes through the centre and perpendicular to its plane and is also parallel to the field. If the disc completes 20 revolutions in one second and the resistance of the disc is 4 Ω, calculate the induced emf between the axis and the rim and induced current flowing in the disc.

33. Establish the fact that the relative motion between the coil and the magnet induces an emf in the coil of a closed circuit.

34. Give the uses of Foucault current.

35. What do you understand by self-inductance of a coil? Give its physical significance.

36. Show mathematically that the rotation of a coil in a magnetic field over one rotation induces an alternating emf of one cycle.

37. Obtain an expression for average power of AC over a cycle. Discuss its special cases

38. 5 Marks

8 x 5 = 40
39. A rectangular coil of area 70 cm2 having 600 turns rotates about an axis perpendicular to a magnetic field of 0.4 Wb m-2. If the coil completes 500 revolutions in a minute, calculate the instantaneous emf when the plane of the coil is
(i) perpendicular to the field
(ii) parallel to the field and
(iii) inclined at 60o with the field.

40. A straight metal wire crosses a magnetic field of flux 4 m Wb in a time 0.4 s. Find the magnitude of the emf induced in the wire

41. A rectangular coil of area 6 cm2 having 3500 turns is kept in a uniform magnetic field of 0.4 T. Initially, the plane of the coil is perpendicular to the field and is then rotated through an angle of 180o. If the resistance of the coil is 35 Ω, find the amount of charge flowing through the coil.

42. A fan of metal blades of length 0.4 m rotates normal to a magnetic field of 4 x 10-3T. If the induced emf between the centre and edge of the blade is 0.02 V, determine the rate of rotation of the blade.

43. A magnetic field of flux density 10T alts normally to the coil of 50 turns having 100cm2 area. Find emf induced, if the coil is removed fro the magnetic field in 0.15S.

44. The magnetic flux through a coil perpendicular to the plane is given by Φ = 5t3 +4t2 +2t calculate the induced emf through the coil at t = 2S

45. What is the self - inductance of a solenoid of length 40cm, area of cross - section 20cm2 and total number of turns is 800?

46. An AC generator consists of a coil of 1000 turns and cross. sectional area of 100 cm2, rotating at an angular speed of 100 rpm in a uniform magnetic field of 1.6 x 10-2 T calculate the maximum emf produced in the coil