#### 12th Standard Physics English Medium Current Electricity Reduced Syllabus Important Questions with Answer key 2021

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 105
Multiple Choice Questions
15 x 1 = 15
1. A wire connected to a power supply of 230 V has power dissipation P1. Suppose the wire is cut into two equal pieces and connected parallel to the same power supply. In this case power dissipation is P2. The ratio $\frac{P_2}{P_1}$ is

(a)

1

(b)

2

(c)

3

(d)

4

2. In India electricity is supplied for domestic use at 220 V. It is supplied at 110 V in USA. If the resistance of a 60W bulb for use in India is R, the resistance of a 60W bulb for use in USA will be

(a)

R

(b)

2R

(c)

$\frac{R}{4}$

(d)

$\frac{R}{2}$

3. When 'n' resistors of equal resistance (R) are connected in series and in parallel respectively,then the ratio of their effective resistance is

(a)

1: n2

(b)

n2: 1

(c)

n: 1

(d)

1: n

4. A potential difference is applied an the ends of a metallic wire. If the potential difference is doubled, the drift velocity

(a)

will be doubled

(b)

will be halved

(c)

(d)

will remain unchanged

5. A bird sitting on an insulated wire carrying a current feels quite safe because

(a)

the bird is a non-conductor of electricity

(b)

resistance of the bird is very large

(c)

there is a large potential difference between bird and wire

(d)

there is no potential difference between bird and wire

6. In an electric circuit fuse wire is connected in

(a)

parallel

(b)

star connection

(c)

delta connection

(d)

series

7. Two identical resistors are connected in parallel then connected in series. The effective resistance are in the ratio

(a)

1:2

(b)

2:1

(c)

1:4

(d)

4:1

8. The drift velocity is equal to mobility when__________

(a)

the electric field is parallel to the motion of electrons'

(b)

the electric field is unity

(c)

the absence of electric field

(d)

either (a) or (b)

9. Calculate the mobility of a free electron in an electric field of 10+2N/C.

(a)

10-4m2V-1s-1

(b)

10-5m2V-1s-1

(c)

10-3m2V-1s-1

(d)

105m2V-1s-1

10. A metal wire of current density is 3.2 x 107 Am-2 has 1028 electron/m3. The average drift velocity is________

(a)

0.02 m.s -1

(b)

200 m s-1

(c)

1.6 X 10-2 m s-1

(d)

3 x 10-2 m s -1

11. A potential of 1 kV is applied between the ends of conductor of length 20 cm. The drift velocity of electron in this field is 3.52 x 106 m/s. The relaxation time for the freeelectrons is

(a)

2 nano sec

(b)

4 nano sec.

(c)

6 nano sec

(d)

8 nano sec.

12. The unit of conductivity is________

(a)

mho

(b)

ohm

(c)

ohm-rn

(d)

mho-m-I

13. When three resistors are connected in parallel then the value of the effective resistance is________

(a)

less than or equal to individual resistance

(b)

greater than or equal to individual resistance

(c)

less than the individual resistance

(d)

greater than the individual resistance

14. Kirchhoff's I law is named as_____________

(a)

ohm's law

(b)

voltage law

(c)

resistance law

(d)

current law

15. Kirchhoff's II law is named as_______

(a)

voltage law

(b)

resistance in series

(c)

resistance in parallel

(d)

current law

16. 2 Marks
10 x 2 = 20
17. State microscopic form of Ohm’s law.

18. Define current density.

19. Define instantaneous current

20. Distinguish between ohmic & non-ohmic device.

21. Define electrical resistivity?

22. What is thermoelectric effect?

23. What is electric fuse?

24. How heating effect is used in electrical lamp.Name other lamps which use the heating effected?

25. What happens to the drift velocity of electron and to the resistance if length of conductor and to the resistance if length of conductor unchanged?

26. Nichrome and copper wire of same length and same radius are connected in series circuit I is passed through them. Which wire gets heated up more? Givereason.

27. 3 Marks
10 x 3 = 30
28. Compute the current in the wire if a charge of 120 C is flowing through a copper wire in 1 minute.

29. If an electric field of magnitude 570 N C-1, is applied in the copper wire, find the acceleration experienced by the electron.

30. A copper wire of cross-sectional area 0.5 mm2 carries a current of 0.2 A. If the free electron density of copper is 8.4 × 1028 m-3 then compute the drift velocity of free electrons.

31. When two resistances connected in series and parallel their equivalent resistances are 15 Ω and $\frac{56}{15}$Ω respectively. Find the individual resistances.

32. Obtain the macroscopic form of Ohm’s law from its microscopic form and discuss its limitation.

33. Explain the equivalent resistance of a series and parallel resistor network

34. Obtain the condition for bridge balance in Wheatstone’s bridge.

35. Derive an expression of drift velocity and write the relation between drift velocity and mobility.

36. (a) Distinguish between electric cells and batteries.
(b) Explain its function

37. Find the expression for the equivalent emf & internal resistance of the series combination of cells

38. 5 Marks
8 x 5 = 40
39. Two electric bulbs marked 20 W – 220 V and 100 W – 220 V are connected in series to 440 V supply. Which bulb will be fused?

40. A battery has an emf of 12 V and connected to a resistor of 3 Ω. The current in the circuit is 3.93 A. Calculate (a) terminal voltage and the internal resistance of the battery (b) power delivered by the battery and power delivered to the resistor

41. Calculate the effect internal resistance in series and parallel.

42. Obtain an expression for electrical conductor

43. A conductor of length I is connected to d.c. source of potential V. If the length of the conductor is doubled by streating it, keeping V constant, explain how do the following factors vary in the condcutor (I) Drift velocity. (ii) Resistor (ill) Resistivity.

44. Write mathematical relation between mobility & drift velocity of charge carriers in a conductor (ii) mobility & relaxation time (or) mean free time.

45. A potential difference of 3V is applied across a conductor through which SA of current is flowing. Determine the resistane of the conductor.

46. A cell of emf E and internal resistance 'r' gives a current of 0.5 A with an external resistance of 12 $\Omega$ and a current of 0.25 A with an external resistance of 25 with an external resistance of 25 $\Omega$ Calculate (i) internal resistance of the cell (ii) emf of the cell.