PHYSICS
UNIT - 2
Chapter 3: MAGNETISM
Terms and Definitions
Magnetizing Field (H)
The magnetic field in which a material is kept is called magnetizing filed. The strength (or
intensity) of magnetic field is denoted by H.
Its SI unit is ampere per meter (A/m).
Magnetic Moment (pm)
Magnetic dipole moment of a bar magnet is given by pm = m.2l
where m – pole strength, 2l – effective length of bar magnet.
The magnetic dipole moment of a current loop is given by pm = I.A
where I – current in the loop, A – area of the loop.
Its unit is A.m2
Magnetization (M)
The magnetic moment per unit volume developed inside a solid is called magnetization or
intensity of magnetization.
M = pm / V.
Its unit is A/m.
Magnetic Susceptibility (χ)
Since the magnetization is induced by the applied magnetizing field, so M is proportional to H.
MαH
M = χH where χ is proportionality constant, known as magnetic susceptibility.
χ=M/H
It is defined as magnetization produced in the material per unit applied magnetizing field.
It has no units.
(The magnetic susceptibility of a material is a measure of the ease with which the material can be
magnetized)
Magnetic flux (φ)
The total number of magnetic lines of force passing through a given area is called magnetic flux
through that area.
Its units are Weber (W)
Magnetic Induction or magnetic flux density (B)
The total number of magnetic lines of force per unit area is called magnetic induction or magnetic
flux density.
B = φ / A.
Its SI unit is Weber / m2 or Tesla (T)
Its CGS unit is Gauss (G) 10000 G = 1Wb/ m2 = 1T
Relationship between B and H
When a material is kept in a magnetic field, two types of induction arise: one due to applied
magnetizing field H and the other as a consequence of magnetization M of the material itself.
B = µo (H +M)
where µo is permeability of free space = 4 π x 10-7 (henry / meter) (H/ m)
Magnetism Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, B = µo (H + χH)
= µo (1+ χ) H
= µo µr H
where µr = 1+ χ = relative permeability
Thus, B = µ H
where µ = absolute permeability of the medium
For free space M = 0, therefore B = µoH
Origin of Magnetization
Magnetic properties of a solid arise because the atoms of solid act as tiny magnetic dipoles and
they possess magnetic dipole moment. The magnetic moment of the atom arises from 3 sources:
(i) orbital motion of electron (ii) electron spin (iii) nuclear spin (too small hence neglected)
The resulting mag.moment of an atom is the sum of the orbital and spin magnetic moments of its
electrons.
Different atomic dipoles may align themselves in different directions to give a net zero magnetic
moment. When substance is placed in magnetic field, the atomoc dipoles are aligned in the
direction of external magnetic field. Thus the material is magnetized.
Types of Magnetic Materials
On the basis of their behavior in external mag.fields, Faraday classified the various substances
into 3 categories:
(i) Diamagnetic Substances: These are substances which develop weak magnetization in a
direction opposite to the magnetizing field. Such substances are weakly repelled by magnets.
Ex: Bismuth, copper, lead, zinc, tin, gold, silicon, water, sodium chloride.
(ii) Paramagnetic Substances: Paramagnetic substances are those which develop weak
magnetization in the direction of magnetizing field. Such substances are weakly attracted by
magnets
Ex: Manganese, Aluminum, Chromium, Platinum, Sodium, Copper chloride, Oxygen
(iii) Ferromagnetic substances: Ferromagnetic substances are those which develop strong
magnetization in the direction of magnetizing field. They are strongly attracted by magnets.
Ex: Iron, Cobalt, Nickel, Gadolinium and alloys like alnico.
Diamagnetism
In the absence of external magnetic field, the atoms in diamagnetic materials do not possess
permanent magnetic moment (Fig a). In such material atoms consists of even number of electrons.
The electrons of such atoms are paired with opposite spin (Fig c) so that the magnetic moments
cancel each other.
When the material is placed in magnetic field, the orbital motion of electron undergoes changes
and a magnetic moment is induced in the atoms in a direction opposite to the applied magnetic
field (according to Lenz’s Law). (Fig b). The material therefore gets magnetized in a direction
opposite to the applied magnetic field. (M is opposite to H)
H=0 H
Fig c
Fig a Fig b
Magnetism Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
UNIT - 2
Chapter 3: MAGNETISM
Terms and Definitions
Magnetizing Field (H)
The magnetic field in which a material is kept is called magnetizing filed. The strength (or
intensity) of magnetic field is denoted by H.
Its SI unit is ampere per meter (A/m).
Magnetic Moment (pm)
Magnetic dipole moment of a bar magnet is given by pm = m.2l
where m – pole strength, 2l – effective length of bar magnet.
The magnetic dipole moment of a current loop is given by pm = I.A
where I – current in the loop, A – area of the loop.
Its unit is A.m2
Magnetization (M)
The magnetic moment per unit volume developed inside a solid is called magnetization or
intensity of magnetization.
M = pm / V.
Its unit is A/m.
Magnetic Susceptibility (χ)
Since the magnetization is induced by the applied magnetizing field, so M is proportional to H.
MαH
M = χH where χ is proportionality constant, known as magnetic susceptibility.
χ=M/H
It is defined as magnetization produced in the material per unit applied magnetizing field.
It has no units.
(The magnetic susceptibility of a material is a measure of the ease with which the material can be
magnetized)
Magnetic flux (φ)
The total number of magnetic lines of force passing through a given area is called magnetic flux
through that area.
Its units are Weber (W)
Magnetic Induction or magnetic flux density (B)
The total number of magnetic lines of force per unit area is called magnetic induction or magnetic
flux density.
B = φ / A.
Its SI unit is Weber / m2 or Tesla (T)
Its CGS unit is Gauss (G) 10000 G = 1Wb/ m2 = 1T
Relationship between B and H
When a material is kept in a magnetic field, two types of induction arise: one due to applied
magnetizing field H and the other as a consequence of magnetization M of the material itself.
B = µo (H +M)
where µo is permeability of free space = 4 π x 10-7 (henry / meter) (H/ m)
Magnetism Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 1
, B = µo (H + χH)
= µo (1+ χ) H
= µo µr H
where µr = 1+ χ = relative permeability
Thus, B = µ H
where µ = absolute permeability of the medium
For free space M = 0, therefore B = µoH
Origin of Magnetization
Magnetic properties of a solid arise because the atoms of solid act as tiny magnetic dipoles and
they possess magnetic dipole moment. The magnetic moment of the atom arises from 3 sources:
(i) orbital motion of electron (ii) electron spin (iii) nuclear spin (too small hence neglected)
The resulting mag.moment of an atom is the sum of the orbital and spin magnetic moments of its
electrons.
Different atomic dipoles may align themselves in different directions to give a net zero magnetic
moment. When substance is placed in magnetic field, the atomoc dipoles are aligned in the
direction of external magnetic field. Thus the material is magnetized.
Types of Magnetic Materials
On the basis of their behavior in external mag.fields, Faraday classified the various substances
into 3 categories:
(i) Diamagnetic Substances: These are substances which develop weak magnetization in a
direction opposite to the magnetizing field. Such substances are weakly repelled by magnets.
Ex: Bismuth, copper, lead, zinc, tin, gold, silicon, water, sodium chloride.
(ii) Paramagnetic Substances: Paramagnetic substances are those which develop weak
magnetization in the direction of magnetizing field. Such substances are weakly attracted by
magnets
Ex: Manganese, Aluminum, Chromium, Platinum, Sodium, Copper chloride, Oxygen
(iii) Ferromagnetic substances: Ferromagnetic substances are those which develop strong
magnetization in the direction of magnetizing field. They are strongly attracted by magnets.
Ex: Iron, Cobalt, Nickel, Gadolinium and alloys like alnico.
Diamagnetism
In the absence of external magnetic field, the atoms in diamagnetic materials do not possess
permanent magnetic moment (Fig a). In such material atoms consists of even number of electrons.
The electrons of such atoms are paired with opposite spin (Fig c) so that the magnetic moments
cancel each other.
When the material is placed in magnetic field, the orbital motion of electron undergoes changes
and a magnetic moment is induced in the atoms in a direction opposite to the applied magnetic
field (according to Lenz’s Law). (Fig b). The material therefore gets magnetized in a direction
opposite to the applied magnetic field. (M is opposite to H)
H=0 H
Fig c
Fig a Fig b
Magnetism Prof. Harison Cota, Don Bosco College of Engineering, Fatorda Page 2
