#### 12th Standard Physics English Medium Magnetism and Magnetic Effects of Electric 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. Th e force experienced by a particle having mass m and charge q accelerated through a potential diff erence V when it is kept under perpendicular magnetic field $\vec { B }$ is

(a)

$\sqrt { \frac { 2{ q }^{ 3 }BV }{ m } }$

(b)

$\sqrt { \frac { { q }^{ 3 }{ B }^{ 2 }V }{ 2m } }$

(c)

$\sqrt { \frac { 2{ q }^{ 3 }{ B }^{ 2 }V }{ m } }$

(d)

$\sqrt { \frac { { 2q }^{ 3 }BV }{ { m }^{ 3 } } }$

2. Th ree wires of equal lengths are bent in the form of loops. One of the loops is circle, another is a semi-circle and the third one is a square. Th ey are placed in a uniform magnetic fi eld and same electric current is passed through them. Which of the following loop confi guration will experience greater torque ?

(a)

circle

(b)

semi-circle

(c)

square

(d)

all of them

3. A wire of length l carries a current I along the Y direction and magnetic field is given by $\vec { B } =\frac { \beta }{ \sqrt { 3 } } =(\hat { i } +\hat { j } +\hat { k } )T.$ The magnitude of Lorentz force acting on the wire is

(a)

$\sqrt { \frac { 2 }{ \sqrt { 3 } } } \beta Il$

(b)

$\sqrt { \frac { 1 }{ \sqrt { 3 } } } \beta Il$

(c)

$\sqrt { 2 } \beta Il$

(d)

$\sqrt { \frac { 1 }{ 2 } } \beta Il$

4. Direction of magnetic force on a positive charge moving in a magnetic field is given by

(a)

thumb rule

(b)

left hand rule

(c)

right hand rule

(d)

cork screw rule

5. Consider the motion of a charged particle in a uniform magnetic field directed into the paper. If velocity v of the particle is in the plane of the paper, the charged particle will describe a

(a)

straight line

(b)

circle

(c)

ellipse

(d)

hyperbola

6. Lorentz force generally refers to force experienced by a charge due to combined action of

(a)

magnetic fields

(b)

electric fields

(c)

electric, magnetic & gravitational fields

(d)

electric and magnetic fields

7. The unit of magnetic field is

(a)

ampere-turn

(b)

ampere

(c)

newton coulomb

(d)

tesla

8. The magnetic induction at the center of a circular coil having 5 turn and radius 2π cm carrying a current of 50 mA is

(a)

2π x 10-7 T

(b)

50π x 10-7 T

(c)

25π x 10-7 T

(d)

2.5π x 10-7 T

9. Magnetic flux density at the center of a circular coil of diameter 20 cm carrying a current 5 A kept in air is

(a)

4π x 10-7 tesla

(b)

3.14 x 10-5 tesla

(c)

10-7 tesla

(d)

2π x 10-7 tesla

10. The direction of the magnetic field due to a solenoid is given by

(a)

Amperes circuital law

(b)

Biot-Savart law

(c)

Right hand palm rule

(d)

Flemings right hand law

11. The unit of pole strength (magnetic charge) is _______

(a)

A/m

(b)

Am2

(c)

A/m2

(d)

Am

12. The angular frequency of a charged particle moving in a uniform magnetic field ______

(a)

depends upon the mass and velocity of the particle

(b)

depends upon mass and radius of circular path

(c)

depends upon the charge and velocity of the particle

(d)

neither depends upon the velocity nor the radius of circular path

13. To convert a galvanometer into an ammeter we connect which one of the following to the galvanometer?

(a)

a low resistance in series

(b)

a high resistance in series

(c)

a low resistance in parallel

(d)

a high resistance in parallel

14. The direction of force on a current carrying conductor placed in a magnetic field is given by _____

(a)

Fleming's Left Hand Rule

(b)

Fleming's Right Hand Rule

(c)

Ampere's velocity

(d)

Biot-Savart Law

15. The magnitude of the magnetic Lorentz force does not depend on ______

(a)

mass of the charged particle

(b)

velocity of the charged particle

(c)

magnetic induction

(d)

direction of motion of the charged particle

16. 2 Marks
10 x 2 = 20
17. What is magnetic susceptibility?

18. What are the causes for earth's magnetic field according to Gover?

19. What is magnetic axis, magnetic meridian & magnetic equator.

20. Define magnetic flux density.

21. State Right hand thumb rule.

22. What is magnetic Lorentz force?

23. When is a galvanometer said to be sensitive?

24. Why the Phosphor - bronze wire is used as the suspension wire in moving coil galvanometer?

25. Why is the path of a charged particle not a circle when its velocity is not perpendicular to the magnetic field?

26. How is a galvanometer converted into (i) an ammeter and (ii) a voltmeter?

27. 3 Marks
10 x 3 = 30
28. Let the magnetic moment of a bar magnet be $\overset { \rightarrow }{ { p }_{ m } }$ whose magnetic length is d = 2l and pole strength is qm. Compute the magnetic moment of the bar magnet when it is cut into two pieces
(a) along its length
(b) perpendicular to its length.

29. Calculate the magnetic flux coming out from the surface containing magnetic
dipole (say, a bar magnet) as shown in figure.

30. Consider a magnetic dipole which on switching ON external magnetic field orient only in two possible ways i.e., one along the direction of the magnetic field (parallel to the field) and another anti-parallel to magnetic field. Compute the energy for the possible orientation. Sketch the graph.

31. A coil of a tangent galvanometer of diametre 0.24 m has 100 turns. If the horizontal
component of Earth’s magnetic field is 25 × 10-6 T then, calculate the current
which gives a deflection of 60o.

32. Explain the principle and working of a moving coil galvanometer.

33. Calculate the magnetic field inside and outside of the long solenoid using Ampere’s circuital law.

34. Write the value of
(i) Horizontal component &
(ii) vertical component of Earth's magnetic field.

35. Write the expression in a vector from for the (i) Lorentz magnetic force $\vec { F }$ due to a charge moving with velocity $\vec { v }$ in a magnetic field $\vec { B }$ What is the direction of force. (ii) If the magnetic force $\vec { F }$ acting an it is non-zero would the particle gain any energy?

36. Two identical charged protrudes moving with the same speed enter a region of uniform magnetic field. If one of these enters normal to the field direction and the other enters along a direction at 30° with the field. What would be the ratio of their angular frequencies.

37. How will the magnetic field intensity at the contre of a circular coil carrying current change if the current through the coil is doubled and the radius of the coil is halved?

38. 5 Marks
8 x 5 = 40
39. A particle of charge q moves with velocity $\vec { v }$ along positive y -direction in a magnetic field $\vec { B }$ .Compute the Lorentz force experienced by the particle
(a) when magnetic field is along positive y-direction
(b) when magnetic field points in positive z - direction
(c) when magnetic field is in zy - plane and making an angle θ with velocity of the particle. Mark the direction of magnetic force in each case.

40. An electron moving perpendicular to a uniform magnetic field 0.500 T undergoes circular motion of radius 2.80 mm. What is the speed of electron?

41. Let E be the electric field of magnitude 6.0 × 106 N C-1 and B be the magnetic field
magnitude 0.83 T. Suppose an electron is accelerated with a potential of 200 V, will
it show zero deflection?. If not, at what potential will it show zero deflection.

42. The coil of a moving coil galvanometer has 5 turns and each turn has an effective area of 2 × 10-2 m2. It is suspended in a magnetic field whose strength is 4 × 10-2 Wb m-2. If the torsional constant K of the suspension fibre is 4 × 10-9 N m deg-1.
(a) Find its current sensitivity in degree per micro - ampere.
(b) Calculate the voltage sensitivity of the galvanometer for it to have full scale deflection of 50 divisions for 25 mV.
(c) Compute the resistance of the galvanometer

43. A circular coil with cross-sectional area 0.1 cm2 is kept in a uniform magnetic field of strength 0.2 T. If the current passing in the coil is 3 A and plane of the loop is perpendicular to the direction of magnetic field. Calculate
(a) total torque on the coil
(b) total force on the coil
(c) average force on each electron in the coil due to the magnetic field of the free electron density for the material of the wire is 1028 m-3.

44. A non - conducting sphere has a mass of 100 g and radius 20 cm. A flat compact coil of wire with turns 5 is wrapped tightly around it with each turns concentric with the sphere. This sphere is placed on an inclined plane such that plane of coil is parallel to the inclined plane. A uniform magnetic field of 0.5 T exists in the region in vertically upward direction. Compute the current I required to rest the sphere in equilibrium.

45. Explain the Motion of a charged particle in a uniform magnetic field.

46. A rectangular coil of area 2 x 10-4 m2 and 40 turns is pivoted about one of its vertical sides. The coil is in a radial horizontal field of 60G. What is the torsional constant of the hair springs connected to the coil if a current of 4.0 mA produces an angular deflection of 16°?