#### Electromagnetic Induction and Alternating Current Important Questions

12th Standard EM

Reg.No. :
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Physics

Time : 01:00:00 Hrs
Total Marks : 50
5 x 1 = 5
1. Th e fl ux linked with a coil at any instant t is given by $\Phi$B =10t2−50t+ 250 . The induced emf at t = 3s is

(a)

−190 V

(b)

−10 V

(c)

10 V

(d)

190 V

2. 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

3. 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

4. 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

5. A step-down transformer reduces the supply voltage from 220 V to 11 V and increase the current from 6 A to 100 A. Then its efficiency is

(a)

1.2

(b)

0.83

(c)

0.12

(d)

0.9

6. 5 x 1 = 5

8. (1)

9. Ac circuit with resistor

10. (2)

Predominantly Inductive

11. Ac circuit with inductor

12. (3)

13. Ac circult with capacitor

14. (4)

Electromagnetic Induction

15. RLC circuit

16. (5)

Voltage and current are in phase

5 x 2 = 10
17. What do you mean by self-induction?

18. What is meant by mutual induction?

19. What is inductive reactance X1? How does it depend on frequencei Give its unit.

20. Distinguish between AC and DC.

21. What is resonant frequency?

22. 5 x 3 = 15
23. A cylindrical bar magnet is kept along the axis of a circular solenoid. If the magnet is rotated about its axis, find out whether an electric current is induced in the coil.

24. A straight conducting wire is dropped horizontally from a certain height with its length along east – west direction. Will an emf be induced in it? Justify your answer.

25. If the current i flowing in the straight conducting wire as shown in the figure decreases, find out the direction of induced current in the metallic square loop placed near it.

26. How does the capacitive reactance depend on frequency? & What is the reactance of a capacitor at hertz to the study at?

27. What is the phase relation between current and emf in an AC circuit containing a cpacitor only? Sketch a graph showing the variation the reactance of a capacitor with frequency.

28. 3 x 5 = 15
29. An inverter is common electrical device which we use in our homes. When there is no power in our house, inverter gives AC power to run a few electronic appliances like fan or light. An inverter has inbuilt step-up transformer which converts 12 V AC to 240 V AC. The primary coil has 100 turns and the inverter delivers 50 mA to the external circuit. Find the number of turns in the secondary and the primary current.

30. Obtain an expression for angular frequency of LC oscillations?

31. Explain the mutual induction between two long solenoids. Obtain an expression for the mutual inductance.