Class 11 Important Questions for Physics: Magnetism

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Magnetism â?– Illustrative Examples 1) A circular coil has l000 turns each of area 2m2. If a current of 3mA flows through the coil, find the magnetic moment of a coil, find the magnetic moment of a coil.

Data : n = 1000, A = 2m2, I = 3

A

(Ans. : M = 6Am2)

2) A bar magnet of geometric length 18 cm has pole strength 100Am. Find the magnetic dipole moment of the bar magnet.

Data : Geometric length = 18 cm, m = 100 Am, M = ?

(Ans.: M = 15Am2)

3) A magnet of magnetic dipole moment 2 Am2 is deflected through 300 from the direction of a magnetic field of induction 2 Wb/m2. Find the magnitude of the torque or couple.

Data : M = 2Am2 ,

, B = 2 Wb/m2

( Ans.:

)

4) A magnetic needle free to rotate in r vertical plane parallel to the magnetic meridian has its north tip pointing down at 21048'with the horizontal. If the horizontal component of the earth's magnetic induction at that place is 3.5 Wb/m2, determine the earth's magnetic induction at that place.

Data : Angle of dip ( ) = 210480 BH = 3.5

wb/m2 (Ans.: B = 1.4

Wb/m2)

5) A bar magnet has pole strength 10 Am and a magnetic length of 5 cm. Find the magnetic induction at equidistance point of 10 cm from the either of its two poles.

Data : m = 10Am, 2l = 5

m , r = 10

m=0.1m

(Ans.

=

T)

6) Find the magnetic induction due a bar magnet of magnetic moment 0.5 Am2 at a point on its axis at a distance of 15 cm from the nearest pole. The magnetic length of the magnet is 10 cm. (Given: Data :

SI unit).

= ?, r = 15 + 5 = 20cm, 2l = 0.1 cm M = 0.5 Am2, r = 0.2m,2l = 0.l m (Ans.

= 1.422

Wb/m2)

â?– Theory Questions 1) Explain the concept of origin of magnetism on the basis of circulating charges.

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2) Explain the equivalence between a magnetic dipole and (a) current carrying circular coil (b) solenoid.

3) Define (i) Magnetic dipole (ii) Magnetic length (iii) Magnetic moment. State S.I. units of pole strength and magnetic moment.

4) Derive an expression for the torque acting on a magnet placed in a uniform magnetic induction.

5) Derive an expression for the magnetic induction at a point along (i) axis (ii) equator of a bar magnet. Also deduce expressions for magnetic induction due to a short magnetic dipole.

6) What are magnetic elements? Explain in detail.

7) What are electromagnets? State the factors affecting their strength.

8) What is magnetism?

9) Why does a freely suspended magnet always points in north - south direction?

10) Explain the following terms. i. Poles of magnet ii. Magnetic dipole iii. Magnetic length iv. Geometric length v. Magnetic axis vi. Magnetic equator 11) What are magnetic lines of force? State the properties of magnetic lines of force.

12) In a magnet, poles are not situated at the ends of the magnet. Why? 13) Explain the following terms. a. Magnetic flux b. Magnetic induction c. Pole strength 14) Explain the term magnetic moment of a bar magnet in terms of torque experienced by a bar magnet.

15) State right handed screw rule.

16) A thin iron nail is attracted at the ends of the magnet. But it is not attracted to the middle portion of the magnet. Why? 17) Write a short note on terrestrial magnetism (magnetic field of the earth).

18) Draw neat and well labelled diagram to show earth’s magnetic field.

19) Define the following terms: i. Geographic axis ii. Geographic meridian iii. Geographic equator

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iv. Magnetic axis v. Magnetic meridian vi. Magnetic equator 20) Answer the following questions regarding earth’s magnetism: i. Which direction would a compass needle point to, if located right on the geomagnetic north or south pole? ii. Geologists claim that besides the main magnetic N-S poles, there are several local poles on the earth’s surface oriented in different directions. How is such a thing possible at all?

21) What is a dip needle?

22) State the uses of electromagnets.

23) Magnetic equator and geographical equator of the earth are same. Is this true or false?

❖ Textbook problems 1) A rectangular coil of length 8 cm and breadth 5 cm has 200 turns of insulated wire. Find the magnetic moment of the coil, when a current of 2A, flows through it.

(Ans.: 1.6 Am2) 2) A bar magnet of moment 7.5 Am2 experiences a torque of 1.5

Nm, when

placed inclined at 300 in a uniform magnetic field. Find the magnetic induction of the field.

(Ans.: 4

T)

3) Show that the magnitude of magnetic induction at a point on the axis of a short bar magnet is twice the magnitude of magnetic induction at a point on the equator at the same distance.

4) The vertical and horizontal components of earth's magnetic field at a place are 2

T and 3.464

T respectively. Calculate the angle of dip and resultant

earth's magnetic field at that place.

(Ans.:

T)

5) A short magnetic dipole has magnetic moment 0.5 Am2. Calculate the magnetic induction by it at a distance of 20 cm from the centre of the magnetic dipole on (i) the axis and (ii) the equatorial line of the short magnetic dipole. (Given S.I. units)

(Ans.:

)

❖ Practice problems F.Y.J.C. Science

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➢ Type-I : Problems based on magnetic moment and pole strength 1) A bar magnet of magnetic moment 5.0 Am2 has the poles 20 cm apart. Calculate the pole strength.

(Ans : 25 Am)

2) A bar magnet has magnetic moment 3.6 Am2 and pole strength 10.8 Am. Determine its magnetic length and geometric length.

(Ans : i. The magnetic length of the bar magnet is 0.33 m. ii. The geometric length of the bar magnet is 0.4 m.)

3) A magnetic dipole of length 10 cm has pole strength of 20 Am. Find i. the magnetic moment of the dipole,

ii. geometric length.

(Ans : i. The magnetic moment of the dipole is 2.0 Am2. ii. The geometric length of the dipole is 12 cm.)

4) A coil has radius of 3 cm and 200 number of turns. If it carries a current of 2 A, what is its magnetic moment?

(Ans : 1.13 A-m2)

5) Find the magnetic induction at a point distant 10 cm on the axis of a short bar magnet of magnetic moment 0.2 A-m2 ?

(Ans :

)

6) Determine the magnetic length of a bar magnet having a pole strength of 200 Am and magnetic moment of 10 Am2.

(Ans : 0.05 m)

7) A bar magnet has a magnetic moment of 2.5 Am2. Find its pole strength if its geometric length is 5 cm.

(Ans : 60 Am)

8) A flat circular coil has 500 turns, each of radius 25 cm. Find the value of the magnetic moment of the coil, when a current of 50 mA is passed through it.

(Ans : 4.91 A-m2) 9) A bar magnet has geometric length 18 cm and magnetic moment 1.5 A-m2. Find its pole strength.

(Ans : 10 A-m)

➢ Type-II : Problems based on torque 10) A magnet of length 10 cm and pole strength 20 Am is deflected through 30° from magnetic meridian. Calculate the deflecting couple. [BH = 0.4 × 10-4 tesla] (Ans : 11) A couple of moment

)

Nm acts on a magnet suspended in a uniform

magnetic field of induction 0.5 Wb/m2 when making an angle of 30° with the field.

Find the magnetic moment of the magnet.

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(Ans : 1 Am2.)

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12) A short bar magnet placed with its axis at 30° with a uniform external magnetic field of 0.25 T experiences a torque of magnitude equal to

J. What is the

magnitude of magnetic moment of the magnet?

(Ans :

)

13) A flat circular coil has 50 turns each of radius of 5 cm. The coil is placed in a uniform magnetic field of induction 10-4 Wb/m2, with its plane inclined at an angle of 60° with the direction of the field. Find the value of the torque acting on the coil when a current of 0.2 A is passed through it.

(Ans :

)

14) A bar magnet of magnetic length 0.14 m and pole strength 20 A-m is placed in uniform magnetic field of induction

T. If the angle between the magnetic

induction and magnetic moment is 30°, find the magnitude of torque acting on the magnet.

(Ans :

)

15) A magnet is kept in a uniform magnetic field of induction 0.006 Wb/m2 with the axis of the magnet inclined to the direction of the field at an angle of 45°. Find the torque acting on the magnet if its moment is 10 Am2.

(Ans :

)

16) A bar magnet of moment 2.5 A-m2 is in a uniform induction of the maximum torque that can act on it.

Find (Ans :

)

17) What torque is needed to hold a single-turn coil of area 5 cm2 in equilibrium when it carries a current of 10 A and is placed with its axis at right angles to a field of flux density 0.15 T? What torque would be required if the coil had 20 turns?

(Ans :

)

18) A coil having 200 turns and an average area 250 cm2 carries a current of 0.5 A. It is placed in a uniform magnetic field of induction

with its plane

parallel to the direction of the field. Find the increase in the torque acting on the coil when the current passing through it is increased by 10%.

(Ans :

)

➢ Type-III : Problems based on earth’s magnetic field 19) A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22° with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.35 G.

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Determine the magnitude of the earth’s field at the place.

(Ans : 0.38 G.)

20) The vertical component of earth’s magnetic field at a place is

times the

horizontal component of earth’s magnetic induction. What is the value of angle of dip at this place?

(Ans : 60°.)

21) The horizontal component and vertical component of earth’s magnetic field at a place are equal. What is the angle of dip at this place?

(Ans :

)

22) Horizontal and vertical components of earth’s magnetic field at a place are 0.22 T and 0.38 T respectively. Find the resultant intensity of earth’s magnetic field and angle of dip at that place.

(Ans :

)

➢ Type-IV : Problems based on magnetic induction at a point along the axis and along the equator 23) The values of magnetic induction at two points on the axis of a bar magnet at distances of 10 cm and 20 cm respectively, from its centre are in the ratio 25 : 2.

Find the magnetic length of the magnet.

(Ans : 10 cm)

24) Find the magnetic induction due to a bar magnet of magnet moment 0.5 A-m2 at a point on its axis at a distance of 17 cm from the nearest pole. The magnetic length of the magnet is 6 cm. (Given:

S.I. unit)

(Ans :

)

25) A bar magnet has pole strength of 10 A-m and a magnetic length of 8 cm. Find the value of magnetic moment and magnetic induction at equidistant point of 16 cm from either of the two poles.

(Ans : 0.8 Am2)

26) A bar magnet has a dipole moment of 5 A-m2. Calculate the magnetic induction produced by it at a point in air at a distance of 50 cm from either pole.

(Ans :

)

27) A short bar magnet has a dipole moment of 2 Am2. Find the magnetic induction at a point on its axis at a distance of 20 cm from its centre in air.

(Ans :

)

28) A bar magnet has magnetic length of 10 cm. Compare the values of magnetic induction at two points on its axis at a distance of 30 cm and 40 cm from its centre.

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(Ans : 2.37 : 1)

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29) Find the magnetic induction at a point at a distance of 10 cm on the axis of a short bar magnet of magnetic moment 5 A-m2.

(Ans :

)

30) A bar magnet of length 10 cm has a pole strength of 10 Am. Calculate the magnetic induction at a distance of 0.2 m from its centre at a point on its i. axial line and

ii. equatorial line (Ans : (i)

(ii)

)

31) A magnet of length 20 cm has magnetic moment 10 Am2. Calculate magnetic induction in air at a point at a distance of 40 cm from its centre on the axis of the magnet.

(Ans :

)

32) A short bar magnet has magnetic moment 40 Am2. The axis of magnet is in magnetic meridian with the south pole pointing north. Find the point on the axis of magnet at which resultant magnetic field will be zero.

(Ans : 0.6436 m) 33) Find the magnetic moment of a very short magnet which produces a magnetic induction of

at an equatorial point at a distance of 20 cm from its

centre.

(Ans : 24 A-m2)

➢ Type-V : Miscellaneous 34) Find the pole strength of a bar magnet of geometric length 18 cm and magnetic moment 6 A-m2?

(Ans : 40 A m)

35) A short bar magnet has a magnetic moment of 0.48 JT-1. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10 cm from the centre of the magnet on (i) the axis (ii) the equatorial lines (normal bisector) of the magnet.

(Ans :

)

36) A magnetic dipole has pole strength 20 A-m and magnetic length 20 cm. Find the magnetic induction at a point at a distance of 40 cm from the centre of the dipole and i. on the axis and

ii. on the equator of the dipole (Ans : (i)

(ii)

)

37) The greatest torque acting on a current carrying coil held in a uniform magnetic field of induction

is 0.005 N-m. Find the dipole moment of the

equivalent dipole formed by the coil. Hence, find the current passing through the

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coil, if it has 2000 turns and area of 0.25 m2.

(Ans :

)

❖ Multiple Choice Question 1) The ratio of magnetic induction along the axis to magnetic induction along the equator of a magnet is (A) 1:1

(B) 1:2

(C) 2 : 1

(D) 4 : 1

2) Magnetic field lines (A) do not intersect each other.

(C) intersect each other at 90°.

(B) intersect each other at 45°.

(D) intersect each other at 60°.

3) Magnetic fields do not interact with (A) stationary electric charge.

(C) stationary permanent magnets.

(B) moving electrical charges.

(D) moving permanent magnets.

4) A steel wire of length / has a magnetic moment M. It is then bent into a semicircular arc. The new magnetic moment is (A) M

(B)

(C)

(D)

5) A current of 3 A is flowing in a plane circular coil of radius 4 cm and number of turns 20. The coil is placed in a uniform magnetic field of magnetic induction 0.5 T. Then the dipole moment of the coil is (A) 3000 A m2

(B)0.3 A m2

(C) 300 A m2

(D) 75 A m2

6) A current carrying loop is placed in a uniform magnetic field. The torque acting on it does not depend upon (A) area of loop

(C) value of current

(B) shape of loop

(D) magnetic field

7) The points A and B are situated perpendicular to the axis of 2 cm long bar magnet at large distances x and 3 x from the centre on opposite sides. The ratio of magnetic fields at A and B will be approximately equal to (A) 27: 1

(B) 1 : 27

(C) 9 : 1

(D) 1 : 9

8) A compass needle is placed at the magnetic pole. It (A) points N - S.

(C) becomes vertical.

(B) points E - W.

(D) may stay in any direction.

9) Magnetic lines of force originate from ________ and end at ________.

(A) north pole, north pole

(C) south pole, north pole

(B) north pole, south pole

(D) south pole, south pole

10) Two isolated point poles of strength 30 A-m and 60 A-m are placed at a distance of 0.3 m. The force of repulsion between them is (A)

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11) The magnetic dipole moment has the dimensions of (A) current × length

(C) current × area

(B) charge × time × length.

(D)

12) Magnetic lines of force are (A) continuous.

(B) discontinuous.

(C) sometimes continuous and sometimes discontinuous.

(D) such that nothing can be said about their nature.

13) Units of magnetic moment are (A) JT

(B) A m

(C) A m2

(D) A m-2

14) Unit of pole strength of a magnet are (A) A m-1

(B)A m2

(C) A m-2

(D) A m

15) Magnetic field intensity due to a dipole varies as dn, where n = (A) 2

(B) -2

(C) 3

(D) -3

16) A short bar magnet placed with its axis at 30° with a uniform external magnetic field of 0.25 T experiences a torque of

Magnetic moment of the

magnet is (A) 0.36 JT-1

(B) 0.72 JT-1

(C) 0.18 JT-1

(D) zero

17) When a magnet is heated, its strength (A) always decreases.

(C) may increase or decrease.

(B) always increases.

(D) remains unaffected.

18) A magnetic needle is kept in a non-uniform magnetic field. It experiences (A) a force and a torque.

(C) a torque but not a torque

(B) a force but not a torque

(D) neither a force nor a torque

19) The magnetic induction for a point at a distance d along the equator of a bar magnet is

(A)

(C)

(B)

(D)

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20) Two points A and B are situated at a distance x and 2x respectively from the nearer pole of a magnet 2 cm long. The ratio of magnetic fields at A and B is (A) 4 : 1

(B) 2 : 1

(C) 8 : 1

(D) 6 : 1

21) A large magnet is broken into two pieces so that their lengths are in the ratio 2 : 1. The pole strengths of the two pieces will have the ratio (A) 2 : 1

(B) 1 : 2

(C) 4 : 1

(D) 1 : 1

22) In the unmagnetised state, magnetic domains of a magnetic substance are oriented at (A) at. 60°

(B) at 90°

(C) randomly

(D) at 150°

23) Magnetic moment of a bar magnet is equal to moment of a couple required to keep it (A) parallel to uniform magnetic field.

(B) perpendicular to uniform magnetic field.

(C) perpendicular to uniform magnetic field of unit intensity.

(D) parallel to uniform magnetic field of unit intensity.

24) A magnetic field exerts no force on (A) magnet.

(C) a moving charge.

(B) an unmagnetised iron bar.

(D) a charge at rest.

25) The magnetic induction B and the force F on a pole of strength m are related by (A) B = m F

(C) F = m B

(B) F = nIABm

(D)

26) Suppose we wish to have a current carrying coil equivalent to a magnet with moment 8 Am2. When the coil has 50 turns and carries a current of 100 mA, the area of the coil is (A) 1600 cm2 27) A torque of

(B)160 cm2

(C) 1.6 m2

(D) 0.16 m2

N-m is required to hold a bar magnet at right angles to a

magnetic field of induction 0.5 Wb/m2. The value of magnetic moment of the magnet is (A)

(B)

(C)

(D)

28) A short bar magnet has magnetic moment 2.5 A-m2. The magnetic induction at a distance 20 cm along the axis of the bar magnet is (A)

(C)

(B)

(D)

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29) The torque required to keep the magnet perpendicular to a magnetic field of induction 2 x

is

The magnetic moment of the magnet

is (A) 0.2 A-m2

(B) 2 A-m2

(C) 20 A-m2

(D) 0.02 A-m2

30) The magnetic induction at an axial point 20 cm from a short magnetic dipole of magnetic moment 20 A-m2 is (A)

(C)

(B)

(D)

31) A magnetic dipole has magnetic length 10 cm and pole strength 100 A-m. Its magnetic dipole moment is (A) 1000 A-m2

(B)500 A-m2

(C) 10 A-m2

(D) 5 A-m2

32) The pole strength of a bar magnet having geometric length of 6 cm and magnetic moment of 2 A-m2 is (A) 3.7 A-m

(B) 37 A-m

(C) 4 A-m

(D) 40 A-m

33) A circular coil of 50 turns and 10 cm radius carries a current of 2 A. The magnetic moment of the coil is (A) 6.284 A-m2

(B) 1 A-m2

(C) 3.142 A-m2

(D) 0.1 A-m2

34) The geometric length of a bar magnet having half magnetic length 5 cm is (A) 12 cm

(B) 10 cm

(C) 6 cm

(D) 4.2 cm

35) The axis of earth’s magnetic dipole is inclined to axis of rotation of the earth nearly by (A) 90°

(B) 45°

(C) 11.5°

(D) 0°

36) Out of the following physical terms, ________ is NOT a magnetic element.

(A) declination

(C) horizontal component of earth’s magnetic induction

(B) Dip

(D) magnetic equator

37) The angle between magnetic meridian and the geographic meridian is known as _________.

(A) angle of dip

(C) magnetic (or field) declination

(B) magnetic field inclination

(D) angle of magnetism

38) The angle of dip at the equator is (A) 90°

(B) 45°

(C) 30°

(D) 0°

39) The strong magnet is made with a material having hysteresis loss of material _________.

(A) small

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(B) large

(D) depends on use

40) The angle of dip at the magnetic poles of the earth is (A) 90°

(B) 45°

(C) 30°

(D) 0°

❖ Answer to Multiple Choice Question 1. (C)

2. (A)

3. (A)

4. (B)

5. (B)

6. (B)

7. (A)

8. (D)

9. (B)

10. (A)

11. (C)

12. (A)

13. (C)

14. (D)

15. (D)

16. (A)

17. (A)

18. (A)

19. (D)

20. (C)

21. (A )

22. (C)

23. (C)

24. (D)

25. (C)

26. (C)

27. (A)

28. (B)

29. (C)

30. (D)

31. (C)

32. (D)

33. (C)

34. (A)

35. (C)

36. (D)

37. (C)

38. (D)

39. (A)

40. (A)

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