MAGNETIC PROPERTIES OF MATERIALS

MAGNETIC PROPERTIES OF MATERIALS

1. Magnetic Properties:-
The important magnetic properties of a magnet are:
1. Magnetic Permeability.
2. Magnetic Susceptibility.

2. Intensity of Magnetisation (I):-
The magnetic moment acquired by a substance per unit volume is defined as intensity of magnetisation.

3. Magnetic Susceptibility:-
Magnetic susceptibility of a substance is defined as the ratio of the intensity of magnetisation (I) and the intensity of applied magnetic field.

Note:-
Magnetic susceptibility is constant for a given substance and it has no units.

4. Retentivity:-
Retentivity of a magnetic material is a property by virtue of which it retains certain amount of intensity of magnetisation even after the removal of applied magnetic field.

5. Classification of Magnetic materials:-
Magnetic materials are classified into three types. They are
1. Dia-magnetic substances.
2. Para-magnetic substances.
3. Ferro-magnetic substances.

1. Dia-Magnetic substances:-
Dia magnetic substances are those in which the resultant magnetic moment of individual atoms is zero.
Example:-
Copper, Bismuth, Gold, Air, Water, Mercury, Hydrogen, etc.,.
Properties:-
1. Dia magnetic substances are repelled by magnets.

2. When dia magnetic substance is suspended freely in an external magnetic field it moves at right angles to the field direction.

3. The relative permeability (μr) of a diamagnetic substance is approximately equal to 1 or less than 1 (μr ≤1).

4. The magnetic susceptibility (χ) of diamagnetic substance is small and negative.

5. In a magnetic field, they move from high intensity region to low intensity region.

2. Para-Magnetic substances:-
Para magnetic substances are those in which the resultant magnetic moment of individual atoms is not zero.
Examples:-
Aluminum, Platinum, Chromium, Manganese, Oxygen.
Properties:-
1. Paramagnetic substances are feebly attracted by magnets.

2. When a Para magnetic substance in the form of rod is suspended in the magnetic field it comes to rest along the direction of the field.

3. the relative permeability (μr) of a paramagnetic substance is slightly greater than (one) (μr>1).

4. The magnetic susceptibility (χ) of paramagnetic substance is small and positive

5. In a magnetic field, they move from low intensity region to high intensity region.

3. Ferro magnetic substances:-
Ferro Magnetic substances are those in which the resultant magnetic moments of individual atoms align themselves in parallel because of a special effect present in them giving rise to spontaneous magnetisation.
Examples:-
Iron(Fe),Cobalt(Co), Nickel(Ni), Gadolinium (Gd), Dysprosium(Dy) etc.,.
Properties:-
1. Ferro magnetic substances are strongly attracted by magnets.

2. When a rod of ferromagnetic substance is freely suspended in a magnetic field, it comes to rest in the direction of the field.

3. The relative permeability (μr) of a ferromagnetic substance is very high (μr>>1)

4. The magnetic susceptibility (χ) of a ferromagnetic substance is very large and positive.

5. In a magnetic field, they move from a low intensity region to high intensity region.
6. Differences between Dia and Para magnetic substances?

Dia magnetic substances Para magnetic substances
1.  Dia magnetic substances are those in which the resultant magnetic moment of individual atoms is zero. 1.  Para magnetic substances are those in which the resultant magnetic moment of individual atoms is not zero.
2.  Dia magnetic substances are repelled by magnets. 2.  Para magnetic substances are feebly attracted by magnets
3.  When Dia magnetic substance is suspended freely in an external magnetic field it moves at right angles to the field direction. 3.  When a paramagnetic substance in the form of rod is suspended in a magnetic field it comes to rest along the direction of the magnetic field.
4.  The relative permeability (μr)  of a diamagnetic substance is approximately equal to 1 or less than 1 (μr≤1) 4.  The relative permeability  (μr) of a paramagnetic substance is slightly greater than 1  (μr>1).
5.  The magnetic susceptibility of a diamagnetic substance is small and negative. 5.  The magnetic susceptibility of a paramagnetic substance is small and positive.
6.  In a magnetic field they move from high intensity to low intensity region. 6.  In a magnetic field they move from low intensity to high intensity region.

7. Differences between Dia magnetic and ferromagnetic substances?

Dia magnetic substances Ferro  magnetic substances
1.  Dia magnetic substances are those in which the resultant magnetic moment of individual atoms is zero. 1.  Ferro magnetic substances are those in which the resultant magnetic moments of individual atoms align themselves in parallel because of a special effect present in them giving rise to spontaneous magnetisation.
2.  Dia magnetic substances are repelled by magnets. 2.  Ferro magnetic substances are strongly attracted by magnets
3.  When Dia magnetic substance is suspended freely in an external magnetic field it moves at right angles to the field direction. 3.  When a rod of ferromagnetic substance is freely suspended in a  magnetic field, it comes to rest in the direction of the field.

 

4.  The relative permeability (μr)  of a diamagnetic substance is approximately equal to 1 or less than 1 (μr≤1) 4.  The relative permeability  (μr)  of a ferromagnetic substance is very high  (μr>>1)
5.  The magnetic susceptibility of a diamagnetic substance is small and negative. 5.  The magnetic susceptibility (χ) of a ferromagnetic substance is very large and positive.
6.  In a magnetic field they move from high intensity to low intensity region. 6.  In a magnetic field, they move from a low intensity region to high intensity region.

8. Differences between paramagnetic and ferromagnetic substances

Para magnetic substances Ferro magnetic substances
Refer 6th answer 6  points of paramagnetic substances. Refer 7th answer  6 points of Ferro magnetic substances.

9. What is Spontaneous Magnetisation? In which material you observe it
1. In a ferromagnetic substance, there is always some amount of intensity of magnetisation even though the applied magnetic field is zero. This inherent magnetisation is called ‘ spontaneous magnetisation’.

2. This is due to the special effect called ‘exchange coupling’ between the adjacent atoms in the solid.

3. We can observe spontaneous magnetisation in Fe, Co, Ni, Gd, Dy etc.,.

10. What are the domains in the ferromagnetic substances

1. Domain in ferromagnetic specimen is a small local region in which all the magnetic dipoles align parallel to each other giving rise to certain magnetisation with in the domain.

2. In a ferromagnetic substance large number of domains occur.

3. The direction of magnetisation within a domain is different from that in the neighboring domains as shown in the figure.

4. In this figure, the boundary lines indicate the domains and the arrow with in a domain indicates total magnetisation.

11. “Only magnetic substances like Fe, Co, Ni. Can be made into magnets while non-magnetic substances like brass, paper cannot be made into magnets in any means”. Why
The property of magnetism exhibited by magnetic substances have origin in configuration of electrons in atoms or group of atoms called ‘molecules’ . Thus a molecule of magnetic substance is not the same as that of non-magnetic substance. So, magnetic substances like Fe, Co, Ni etc., can be made into magnets while non-magnetic substances like brass, paper cannot be made into magnets.

12. “After a magnetic substance is completely magnetised, any further magnetisation does not bring any change in its magnetism”? Why
Repetition of the process of magnetisation allows the maximum alignment among the molecular magnets. This means the magnetisation of iron-bar cannot be increased further. This limit of a substance is called ‘magnetic saturation’ . After this point any magnetisation of the magnetic substance will not further improve its magnetism.

13. Why relative permeability has no units?
The relative permeability is the ratio of the magnetic force in the medium to the magnetic force in the free space. Since it is a ratio of μ and μ0, it has no units.

14. “An iron rod is not a magnet by itself”. Why?
In a ferromagnetic substance, a large number of domains occur. The direction of magnetisation with in a domain is different from that in neighbouring domain. As a result, a net magnetisation of the specimen as a whole is almost zero. That is why, an iron rod is not a magnet by itself.

15. “Steel is used to make permanent magnets. Soft iron is used to make electro magnets”. Why?
In the steel the alignment of molecular magnets produced during the magnetisation remains permanent. Therefore, steel is used to make permanent magnets. But in some substances like soft iron the alignment is disturbed easily. There fore, soft iron is used to make electro magnets. (Temproary magnets).

16. Compare the values of relative permeability and susceptibility of dia, para and ferro magnetic substances.

Types of substances Relative permeability(μr) Magnetic susceptibility
1.  Dia magnet μr ≤1 Small and negative(-ve)
2.  Para magnet μr>1(slightly greater than 1) Small and positive (+ve)
3.  Ferro magnet μr>>1(very large) Large and positive (+ve)

17. Units of physical quantities?

Sno Physical quantities S.I. units Symbol of unit
1 Magnetic pole strength(m) Ampere metre A-m.
2 Magnetic moment(M) Ampere sq metre A-m2.
3 Magnetic field strength (H) or intensity of magnetisation  (or) Am-1.
4 Magnetic induction (or) Magnetic flux density  (or) Tesla (or) Wb/m2 (or)

wbm-2 (or) N/Am .

5 Magnetic flux Weber (or) Wb  (or)  .
6 Permeability (μ) (or) H/m (or) Hm-1 (or)N/A2
7 Magnetic susceptibility (χ) No units
8 Relative permeability (μr) No units

III. Problems:-

1. A unit pole is placed at a distance of 5 cm from a like pole of strength 500 CGS units. Find the magnetic force between them.
Given,
Pole strength of the first pole=m1=1 CGS unit.
Pole strength of the second pole=m2=500CGS units
Distance between the two poles=r=5cm.
In CGS units K=1.
Magnetic force between the two poles=F=?
We know that,

F=20 dynes.
The magnetic force between two poles=20 dynes.

2. Two poles of strengths 240 and 300 CGS units respectively are kept apart at a certain distance. The magnetic force between them is 2000 dynes. Find the distance between the poles?
Given,

Pole strength of the first pole=m1=240 CGS unit.

Pole strength of the second pole=m2=300 CGS units

Magnetic force between the two poles=F=2000 dynes.

Distance between the two poles=r=?

In CGS units K=1.

We know that,

3. Calculate the value of magnetic induction at a distance of 0.2 m on the axial line of a short bar-magnet of length 6.5 cm and pole strength 4×10-3Am.

Given,

Given

4. Calculate the magnetic induction at a distance of 0.4 m from the centre of a short bar-magnet on the equatorial line of a bar-magnet of length 4cm and pole strength 4×10-3 Am.

Magnetic induction on equatorial line of a bar magnet is given by

5. Calculate the magnetic induction at a distance of 40 cm on the equatorial line of a magnet having length 6 cm and pole strength 5×10-6 Am.
Magnetic induction of an equatorial line is given by

Magnetic induction on the equatorial line =0.468 x10-12 N/Am.

6. The distance between two magnetic poles of pole strengths 3×10-4 Am and
4×10-4 m is 10 cm in air. What is the force of repulsion between the two poles.
Given,

7. Two north poles of pole strength 1 Am are separated by a distance 1 metre in air. Calculate the force of repulsion between them.
Given,
Pole strength of two north poles m1=m2=1Am.
Distance between the two poles=1m.
μ0=4Пx10-7 Henry/metre.
Force of repulsion between two poles =F=?
We know that,

F=10-7 Newtons
Force of repulsion between the two north poles =10-7 Newtons.

8. A short magnet of magnetic moment 5×10-1 Am2 is placed in the magnetic meridian with its south pole pointing north (earth’s). Calculate the magnetic induction at a distance of 10 cms from the centre of the magnet.
Given,
Magnetic moment=M=5×10-1 Am2.
Distance=d=10cm=10×10-2 m=0.1m.
Since the south pole of a magnet is pointing towards the north pole of the earth, magnetic induction is given by

9. Each pole of a bar magnet experiences the force of 4×10-4 N when placed in a Uniform magnetic field of induction 2×10-5 N/Am. Calculate the pole strength of the magnet.
Given,

10. If each pole of a bar magnet having magnetic moment 7.8 Am2 experiences the force of 15.6×10-5 N when placed in a magnetic field of induction 0.4×10-5 N/Am. Calculate the length of the magnet.
Given,

11. Calculate the value of magnetic induction at a distance of 0.5 m on the axial line of a short bar-magnet of length 5 cm and pole strength 2×10-3 Am.
Magnetic induction of the axial line is given by
Given,

12. Calculate the magnetic induction at a distance of 0.5 m from the centre of a short bar-magnet on the equatorial line of a bar magnet of length 5cm and pole strength 2×10-3 Am.
Magnetic induction of an equatorial line is given by

13. The north pole of short magnet of length 5 cm is facing the geographical north. The pole strength of the magnet is 3×10-2 Am. Find the position of the neutral points.
Given,


III. Match the following:-

         Group-A                                              Group-B
1. Ferro magnetic substance              (b) a)unit of magnetic flux.

2. Para magnetic substance                (d) b)Godolinium.

3. Dia magnetic substance                  (f) c)CGS unit of intensity
of magnetic field.

4. Weber                                              (a) d)Platinum

5. Gauss                                               (c) e)ether.

f)Bismuth.

2.       Group-A                                            Group-B

1. 104 gauss                                        (b) a) .

2. Horizontal component of earth’s    (d) b)Tesla
magnetic field induction in A.P.

3. Permeability of free space.              (c) c)4Пx10-7 Henry/metre.

4. Magnetic field induction on axial    (f) d)0.39 x10-4 Tesla

5. Magnetic field induction on

equatorial                                            (a) e)10-10 Henry/metre.
Line of a bar magnet.
f)

3.       Group-A                                              Group-B

1. Magnetic saturation                          (f) a)Ampere metre.

2. Magnetic field induction                   (d) b)Newton.

3. Ewing                                                 (c) c)Molecular theory.

4. Magnetic pole strength                     (a) d)Weber/metre2.

5. Magnetic moment                             (g) e)Lumen.

f)maximum limit of magnetisation.

g) product of magnetic length and pole                                                                         strength.

4.         Group-A                                           Group-B

1. Paper                                                (c) a) repelled by magnet

2. Demagnetisation                              (e) b) Ewing.

3. Copper                                              (a) c) Non-magnetic.

4. Pole strength                                     (d) d) Ampere metre.

5. Molecular magnet                             (b) e) Heat.

5.        Group-A                                        Group-B

1. m                                                       (d) a)permeability of free space.

2. μ0                                                      (a) b)Magnetic flux.

3. B                                                        (e) c)Intensity of magnetic field.
4. H                                                        (c) d)pole strength.

5. Φ                                                        (b) e)magnetic flux density.

6.     Group-A                                          Group-B

1. M                                                        (b) a)mxB
2. B                                                         (e) b)mx2l.
3. μ0xμr                                                  (d) c)χxH
4. F                                                          (a) d)μ
5. I                                                           (c) e)μ0xH

7.      Group-A                                            Group-B

1. I                                                           (d) a)Tesla.

2. F                                                           (e) b)Ampere metre2.

3. B                                                           (a) c)Henry/metre.

4. μ0                                                         (c) d)Ampere/metre

5. M                                                          (b) e)Newton.

IV. Fill in the blanks:-

1. Identify the magnetic substance among the following                   (a).
a) Nickel                          b) Aluminum.
c)Chromium                    d) Copper.

2. The limit of magnetisation of a substance is called magnetic saturation.
3. The material used to make permanent magnets is steel.
4. Any magnet, however small consists of both north and south poles. So a magnet is also called dipole.

5. If a magnet is hammered for a long time its magnetism becomes zero.

6. The ability of a magnet to attract or repel another magnetic pole is called pole strength.

7. Unit of pole strength in SI system is ampere metre.
8. Unit of pole strength in MKS system is Weber.
9. Unit of pole strength in CGS system is Gauss.
10. The magnetic permeability of free space (air) is represented by the symbol μ0.
11. The ratio of the magnetic force in a medium to magnetic force in free space is relative permeability.

12. If “A” is area through which a flux Φ (Weber) passes normally, the magnetic flux density is given by .

13. The magnetic flux passing through a unit normal area is called magnetic field induction.

14. Weber/metre2= Tesla.

15. 1 Tesla= 10 Gauss.

16. CGS unit of intensity of magnetic field is Gauss.

17. The relation between magnetic flux density B and intensity of magnetic field H is given by B=μ0H.

18. SI unit of intensity of magnetic field is Ampere/metre.

19. The magnetic force acting on a unit north pole at a point independent of the medium is known as intensity of magnetic field.

20. If ‘m’ is the pole strength of a very short bar-magnet of length 2l then the magnetic moment is given by M=mx2l.

21. Magnetic field induction ‘B’ on the axial line at a distance of ‘d’ from the centre of a bar-magnet of magnetic moment ‘M’ is given by .

22. Magnetic field induction ‘B’ on the equatorial line at a distance of ‘d’ from the centre of a bar-magnet of magnetic moment ‘M’ is given by .

23. When north pole of the bar-magnet points towards the geographical north pole of the earth, the neutral points lie on equatorial line.

24. When south of the bar magnet points towards the geographical north pole of the earth, the neutral points lie on axial line.

25. The value of horizontal component of earth’s magnetic field in A.P. is
0.39×10-4 Tesla.

26. The motion of the electron that gives rise to magnetic moment is spin.

27. In case of a non-magnetic substance, the net magnetic moment due to the molecule is zero.

28. The magnetic moment acquired by a substance per unit volume is called intensity of magnetisation .

29. Substances in which the resultant magnetic moment of individual atom is zero are called diamagnetic substances.

30. The magnetic susceptibility of diamagnetic substance is small and negative.

31. An example of diamagnetic substance is Bismuth.

32. The substances in which the resultant magnetic moment of individual atom is not zero are called paramagnetic substances.

33. The relative permeability of paramagnetic substances is slightly greater than 1.

34. A small local region in which all the magnetic dipoles align parallel to each other giving rise to certain magnetisation within the substance is called domain.

35. An example of paramagnetic substance is chromium.

36. Force of attraction between the two opposite poles of strengths is 20 and 25 C.G.S units situated 5cm away is 20 dynes.

37. The force between two magnetic poles situated 8cm a part in air is 360 dynes. If the distance between the poles is decreased by half, the force will become 1440 dynes.

Note:-
1. If the distance between the poles is increased ‘n’ times then the force will become times the previous force.

2. If the distance between the poles decreases by ‘n’ times, then the force will become n2 times the previous force.

38. Force between two poles of strengths 10 and 50, the C.G.S units situated 2cm apart in air is 125 dynes.

39. The unit magnetic pole is defined as that pole which repels an identical pole at 1metre distance with a force of 10-7 Newtons.

40. The magnetic moment of a magnet is 100 C.G.S units. If the length of the magnet is 10 cm, the pole strength is 10 Ampere metre.

41. Two like poles of same pole strengths separated by a distance two centimeters from one another experiences the force of repulsion F. If the distance between them is doubled then the force of repulsion will be .
42. Permanent magnets are prepared from steel.

43. If a magnet is dropped from the top of a building, its magnetic power after reaching the ground decreases.

44. The north pole of a bar magnet is placed pointing the North and parallel to the earth’s magnetic field, then the null points are formed on equatorial line.

45. A magnet of moment 200 C.G.S units is divided into two magnets of equal length the moment of each magnet is 100 C.G.S units.

46. Magnetic moment of a magnet is 1000 C.G.S units. The distance between the magnetic poles is 20 cm, then the pole strength is 50 C.G.S units.

47. In order to get neutral points on the equatorial line of the bar magnet, the bar magnet should be kept along the magnetic meridian with north pole pointing north pole of the earth.

48. Isolated magnetic poles do not exist.

49. In steel, the alignment of molecular magnets produced during the magnetisation remains permanent.

50. Soft iron is used to make electro magnet.

51. The two poles of a magnet have equal strength.

52. Ewing’s molecular theory failed to explain distinction between magnetic and non-magnetic substances.

53. The force of attraction or repulsion between two magnetic poles is directly proportional to product of pole strengths.

54. The force of attraction or repulsion between two magnetic poles is inversely proportional to square of the distance between them.

55. In the formula ,μ0 is called magnetic permeability of free space.

56. One Weber= μ0 Ampere metre.

57. μ0=4Πx10-7 Henry/metre.

58. For air or vacuum the value of relative permeability μr=1.

59. Unit magnetic pole is defined as that pole which repels an identical pole at one metre distance with a force of 10-7N.

60. The magnetic field induction due to a magnet at a point in the medium is defined as the force acting on a unit North pole placed at that point.

61. The equatorial line is perpendicular to the axial line of the bar-magnet.

62. In a laboratory it is the horizontal component of the earth’s magnetic field that influences the magnetic field induction due to a bar magnet.

63. The intensity of magnetisation is ‘I’ and the intensity of the applied magnetic field is ‘H’. Then magnetic susceptibility

64. Ferro magnetic substances are those in which the resultant magnetic moment of the individual atoms align themselves in parallel because of a special effect present in them giving rise to spontaneous magnetisation.

65. The resultant magnetic field at a neutral point is zero.

66. Iron is not a magnet by itself.

67. Oxygen is a Para magnetic substance.

68. Neutral points are identified with magnetic compass.

69. A magnetic substance can be magnetised to a maximum limit called magnetic saturation.

70. The surest test for magnetism is repulsion.

71. The relation between B and H is B= μ0H.

72. Platinum is a Para magnetic substance.

73. Copper is repelled by the magnet in magnetic field.

74. The relative permeability of a diamagnetic substance is approximately equal to 1 or less than 1.

75. The magnetic susceptibility of paramagnetic substance is small and positive.

76. The relative permeability of ferromagnetic substance is very high (μr>>1).

77. The magnetic susceptibility of ferro magnetic substance is large and positive.

78. The residual magnetisation absorbed in ferromagnetic substance is retentivity.