2. Can You Recall?
1. What are magnetic lines of force?
Magnetic Lines of Force is a an imaginary line
representing the direction of magnetic field such
that the tangent at any point is the direction of the
field vector at that point.
2. Why magnetic monopoles do not exist?
A magnetic monopole does not exist . Just as the two faces of a current loop
cannot be physically separated, magnetic North pole and the South pole can
never be separated even on breaking a magnet to its atomic size.
3. Which materials are used in making magnetic compass needle?
The needle of magnetic compass is made of steel. This material can be
magnetised for an extended period. On the other hand, plastic, wood and paper
are non-magnetic substances and hence, can not be magnetised in order to
indicate directions.
3. β’ Toque on a bar magnet in a magnetic field
If a bar magnet placed in uniform magnetic field
π = ππ΅ sin π
Bar magnet will perform rotational motion
Due to any displacement
Work will be done
(Work β Stored in the form of P.E.)
We say magnetic Potential Energy.
ππ =
0
π
π ππ
ππ = 0
π
ππ΅π ππ π ππ = βππ΅ sin π
4. β’ Toque on a bar magnet in a magnetic field
Case I: At π = 00
ππ = βπ π΅
Case II: At π = 1800
ππ = π π΅
Case III: At π = 900
ππ = 0
Where, m β Magnetic Moment
B β Magnetic Field
π β π΄ππππ
β’ Two magnet suspended Freely β
Mutual Perpendicular to each other
5. β’ Two magnet suspended Freely β Mutual Perpendicular to each other
At equilibrium: π = πΌ πΌ
π = πΌ
π2π
ππ‘2 __________ (i) πΌ =
ππ
ππ‘
=
π
ππ‘
ππ
ππ‘
πΌ β ππππππ‘ ππ πππππ‘ππ
πΌ β π΄πππ’πππ πππππππππ‘πππ
π β π΄πππ’πππ π·ππ πππππππππ‘
π β π΄πππ’πππ πππππππ‘π¦
If we rotate magnet in opposite direction:
Torque will be opposite
π = βππ΅ sin π ______________(ii)
(-ve sign: Bar magnet is rotating in opposite direction)
From equation (i) and (ii)
β΄ πΌ
π2π
ππ‘2 = βππ΅ sin π
7. β΄ π =
ππ΅
πΌ
Time period for Bar Magnet, β΄ π =
2π
π
β΄ π =
2π
ππ΅
πΌ
= 2π Γ
πΌ
ππ΅
β΄ π = 2π
πΌ
ππ΅
Examples
A bar magnet of moment of inertia of 500 π ππ2 makes 10 oscillations per
minute in a horizontal plane. What is its magnetic moment, if the horizontal
component of earth's magnetic field is 0.36 gauss?
Ans: π = 2π
πΌ
ππ΅
8. Examples
A bar magnet of moment of inertia of 500 π ππ2
makes 10 oscillations
per minute in a horizontal plane. What is its magnetic moment, if the
horizontal component of earth's magnetic field is 0.36 gauss?
Given: Moment of Inertia I = 500 π ππ2
Frequency n = 10 oscillation per minute = 10/60 oscillations per
second
Time period T = 6 sec
π΅π» = 0.36 gauss
π = 2π
πΌ
ππ΅
π =
4π2πΌ
π2π΅
=
4 Γ (3.14)2 Γ500 Γ10β7
36 Γ 0.36 Γ 10β4
m = 1.524 π΄ π2
9. ππππππ‘ππ π β π΄π‘ππππ π π‘ππ’ππ‘π’ππ β π·πππππ
Origin of Magnetism in Materials:
Electron Revolving around the nucleus
β
Magnetic Dipole Moment
(m = IA)
Where, I β Moment of Inertia
π β ππππππ‘ππ ππππππ ππππππ‘
A β π΄πππ π ππ‘ππ‘ππ ππ¦ πβ
β’ Magnetism depends upon atom.
10. Magnetic Moment of an Electron Revolving Around the
Nucleus of an Atom:
Intrinsic Angular
Momentum
Magnetic dipole moment due to current loop is,
m = I A
Where, m β magnetic Dipole
(Orbital Magnetic Moment)
I β Current
A β Area enclosed by loop
Consider, An electron moving in an orbit with constant
velocity (v),
Having radius r about nucleus
Electron covers distance of circumference i.e. 2ππ in time
πππππ =
π·ππ π‘ππππ
ππππ
=
2ππ
π
11. Magnetic Moment of an Electron Revolving Around the
Nucleus of an Atom:
Intrinsic Angular
Momentum
Current associated with charge πβ is,
πΌ =
π
π
_______(i)
β΄ πΉππ ππππ’πππ π ππππ, π =
2π
π
We get from (i), πΌ =
π
2π
π
=
ππ
2π
πΌ =
ππ
2π
As π£ = π π i.e. π =
π£
π
β΄ πΌ =
ππ£
2ππ
Now, For orbital magnetic moment: ππππ = πΌπ΄
ππππ =
ππ£
2ππ
ππ2 =
1
2
evr ________ (ii)
12. Magnetic Moment of an Electron Revolving Around the
Nucleus of an Atom:
Intrinsic Angular
Momentum
For some electron, orbital angular momentum:
πΏ = πππ£π
Where, ππ β πππ π ππ πβ
From equation (ii), ππππ =
1
2
ππ£π
ππππ =
1
2
ππ£π Γ
ππ
ππ
=
π
2ππ
Γ (πππ£π)
ππππ =
π
2ππ
Γ πΏ
Where, ππππ β πππππ‘ππ ππππππ‘ππ ππππππ‘
Conclusion: πππππ‘ππ ππππππ‘ππ ππππππ‘ β π΄πππ’πππ ππππππ‘π’π
π. π. ππππ β πΏ
Where,
π
2ππ
β πΆπππ π‘πππ‘ πΆπππππ ππ πΊπ¦ππππππππ‘ππ πΆπππ π‘πππ‘
14. Similarly, From Quantum Mechanics: From 2ππ
postulates of Bohrβs theory:
Angular momentum of electron is n multiple of
β
2π
i.e. πΏ = πππ£π = π
β
2π
We know that, ππππ =
π
2ππ
πΏ
β΄ ππππ =
π
2ππ
.
πβ
2π
=
ππβ
4πππ
For 1π π‘ orbit, n = 1
β΄ ππππ =
πβ
4πππ
The Value
πβ
4πππ
β π΅βππ ππππππ‘ππ β 9.274 Γ 10β24 π΄/π2
Example
Calculate the gyromagnetic ratio of electron
(given π = 1.6 Γ 10β19 πΆ, ππ = 9.1 Γ 10β31 ππ)
15. Example
Calculate the gyromagnetic ratio of electron
(given π = 1.6 Γ 10β19
πΆ, ππ = 9.1 Γ 10β31
ππ)
Ans: Gyromagnetic Ratio =
π
2 ππ
Gyromagnetic Ratio =
1.6 Γ 10β19
2 Γ 9.1 Γ 10β31
πΊπ¦ππππππππ‘ππ πππ‘ππ = 8.8 Γ 1010
πΆ/ππ
16. Magnetization
β’ Unpaired electron having magnetic dipole moment
β’ ππππππ‘ππ§ππ‘πππ =
ππππππ‘ππ ππππππ‘
ππππ’ππ ππ πππ‘πππππ
β’ π =
ππππ‘
ππππ’ππ
β’ M is vector quantity
β’ Unit -
π΄
π
β’ Dimension - [πΏβ1π΄]
17. Magnetization
Consider, a rod placed in solenoid
β΄ Magnetic field inside solenoid π΅π = π0ππΌ
Where, n = No. of turns on solenoid
I = Current Passing
β΄ Magnetic field due to material kept inside solenoid (π΅π)
Here, Net Magnetic Field π΅ = π΅π + π΅π
We know that, π΅π β ππππππ‘ππ§ππ‘πππ
β΄ π΅π = πππ
Where, ππ β πππππππππππ‘π¦ ππ ππππ π ππππ
β΄ π΅ = ππππΌ + πππ
Here, new quantity called as magnetic field intensity βHβ
β΄ π» = π πΌ
Does not depend upon on material rod.
18. Magnetization
Hence, π΅ = πππ» + πππ
π΅ = ππ(π» + π) β¦β¦β¦..(1)
β΄ π» + π =
π΅
π0
β΄ π» =
π΅
π0
β π
Unit of H and M is same β A/m
If H is not too strong, So
Magnetization induced in material β Magnetic Intensity
β΄ π β π»
π = ππ»
Where, π β ππππππ‘ππ ππ’π ππππ‘ππππππ‘π¦
21. β’ Magnetic permeability and susceptibility are quantitative measures of
magnetic properties of materials.
β’ The key difference between magnetic permeability and susceptibility
is that magnetic permeability describes the ability of a material to
support the formation of a magnetic field inside itself whereas
susceptibility describes whether a material is attracted to a magnetic
field or is repelled from it.
β’ Magnetic susceptibility is a dimensionless measure.
Summary β Magnetic Permeability vs Susceptibility
β’ Magnetic permeability is given by the units Henries per meter, and magnetic
susceptibility is a dimensionless property of materials.
β’ The key difference between magnetic permeability and susceptibility is that
magnetic permeability describes the ability of a material to support the
formation of a magnetic field inside itself whereas susceptibility describes
whether a material is attracted to a magnetic field or is repelled from it.
22. Magnetic Permeability vs Susceptibility
Magnetic permeability of a material is the ability of
a material to support the formation of a magnetic
field inside itself.
Magnetic susceptibility is the measure of magnetic
properties of a material which indicates whether
the material is attracted or repelled from an
external magnetic field.
Units of Measurement
Magnetic permeability is measured by the SI unit
Henries per meter (H/m or HΒ·mβ1) which is
equivalent to Newtons per ampere squared (NΒ·Aβ2).
The magnetic susceptibility is a dimensionless
property.
Value for Diamagnetic Materials
The value of magnetic permeability for diamagnetic
materials is less than 1.
The value of magnetic susceptibility for diamagnetic
materials is less than zero.
Value for Paramagnetic Materials
The value of magnetic permeability for
paramagnetic materials is greater than 1.
The value of magnetic susceptibility for
paramagnetic materials is greater than zero.
23. Example: The region inside a current carrying toroid winding is filled with Aluminium
having susceptibility π = 2.3 Γ 10β5. What is the percentage increase in the
magnetic field in the presence of Aluminium over that without it?
Solution: The magnetic field inside the solenoid without Aluminium
π΅π = πππ»
The magnetic field inside the solenoid with Aluminium B = Β΅ H
π΅βπ΅0
π΅0
=
πβππ
ππ
π = ππ(1 + π)
π
ππ
β 1 = π
πβππ
ππ
= π
therefore
π΅βπ΅0
π΅0
=
πβππ
ππ
= π
Percentage increase in the magnetic field after inserting Aluminium is
π΅βπ΅0
π΅0
Γ 100 = 2.3 Γ 10β5 Γ 100 = 0.0023 %
25. Magnetic properties of material
β’ Diamagnetic: Susceptibility is Negative
β’ Paramagnetic: Susceptibility is Positive but small
β’ Ferromagnetic: Susceptibility is Positive but large
26. Diamagnetism
β’ Repels magnetic line of forces
β’ These material having completely filled electron
β’ Having no net magnetic dipole moment
β’ When these material placed in magnetic field, then they move from
stronger to weaker
β’ These material are repelled by a magnet
β’ Eg. β Copper, gold, metal etc
27. Diamagnetic Substances
β’ Diamagnetic substances are those which are repelled by magnets and
when placed in a magnetic field move from the stronger to the
weaker part of the field.
β’ Diamagnetic materials examples
β’ Bismuth
β’ Phosphorus
β’ Antimony
β’ Copper
β’ Water
β’ Alcohol
β’ hydrogen
28. Properties of Diamagnetic materials
β’ When a diamagnetic substance is placed in a magnetic field it sets
itself at right angles to the direction of the lines of force.
β’ When diamagnetic material is placed within a magnetic field the lines
of force tend to go away from the material.
29. Paramagnetism
β’ Those substances which are weekly magnetized when placed in an external magnetic
field in the same direction as the applied field are called paramagnetic substances.
β’ They tend to move from weaker to the stronger part of the field.
β’ According to Piere Curie (Applied for paramagnetic material)
β’ ππππππ‘ππ§ππ‘πππ π β ππππππ‘ππ πΉππππ β¦β¦β¦β¦β¦β¦.(1)
β’ ππππππ‘ππ§ππ‘πππ π β
1
π΄ππ πππ’π‘π ππππππππ‘π’ππ
β¦ β¦ β¦ β¦ . . 2
β’ i.e. M β
π΅
π
β¦β¦From eq. (1) and (2)
β’ β΄ π = πΆ
π΅
π
β’ Where, πΆ β πΆπππ π‘πππ‘ β¦..(3)
We know that, π΅ = πππ»
From equation (3), π =
πΆπππ»
π
β΄
π
π»
= π = πΆ
π0
π
β΄ π = ππ β 1 = πΆ
π0
π
(a) No (b) Weak (c) Strong
External Magnetic Field
30. Paramagnetism
Paramagnetic substances are those which are attracted by magnets and when
placed in a magnetic field move from weaker to stronger parts of the field.
Paramagnetic materials examples
Aluminum
Manganese
platinum,
crown glass
the solution of salts of iron and oxygen
Properties of paramagnetic materials
If a bar of paramagnetic material is suspended in between the pole pieces of
an electromagnet, it sets itself parallel to the lines of force.
When a bar of paramagnetic material is placed in a magnetic field the lines of
force tend to accumulate in it.
31. Ferromagnetism
Ferromagnetic materials are those materials which exhibit a spontaneous net
magnetization at the atomic level, even in the absence of an external
magnetic field.
When placed in an external magnetic field, ferromagnetic materials are
strongly magnetized in he direction of the field.
Having strong magnetic property, they become permanent magnet.
Domain Theory: In ferromagnetic material
There is strong exchange of magnetic dipole moment
Due to this exchange, small area formed in which all atoms are present.
Small area (domain) Line which separate it called as domain axis.
(a) Unmagnetised and (b) Magnetised
Ferromagnetic material with domain
32. Ferromagnetic Substances
Ferromagnetic substances are those which are attracted by the
magnets and can also be magnetized.
Ferromagnetic materials examples
β’ Iron
β’ Nickel
β’ cobalt and their alloys
Properties of Ferromagnetic Substances
The ferromagnetic substance shows the properties of the paramagnetic
substance to a much greater degree.
The susceptibility has a positive value and the permeability is also very large.
The intensity of magnetization I is proportional to the magnetizing field H for
a small value.
33. Example: A domain in ferromagnetic iron is in the form of cube of side 1 Β΅m.
Estimate the number of iron atoms in the domain, maximum possible dipole
moment and magnetisation of the domain. The molecular mass of iron is 55
g/mole and density is 7.9 π/ππ3. Assume that each iron atom has a dipole
moment of 9.27 Γ 10β24
π΄π2
.
Solution: The volume of the cubic domain (10β6)3 = 10β18 π3 = 10β12ππ3
mass = volume Γ density = 7.9 Γ 10β12 π
An Avogadro number (6.023 x 1023) of iron atoms has mass 55 g.
The number of atoms in the domain N =
7.9 Γ10β12 Γ 6.023 x 1023
55
= 8.56 Γ 1010
The maximum possible dipole moment ππππ₯ is achieved for the case when all
the atomic moments are perfectly aligned (though this will not be possible in
reality).
ππππ₯ = 8.65 Γ 1010 Γ 9.27 Γ 10β24 = 8 Γ 10β13 π΄π2
Magnetisation M =
ππππ₯
domain volume
= 8 Γ 105 π΄/π
34. Effect of Temperature
β’ Ferromagnetic material depends upon temperature.
β’ Temperature β Increased β Domain Distorted β Dipole moment weaker β
It becomes paramagnetic material
β’ The temperature at which a ferromagnetic material transforms into a
paramagnetic substance is called Curie temperature (ππΆ) of that material.
β’ Curie Temperature β Temperature at which ferromagnetic material β
paramagnetic material
β’ Magnetic Susceptibility, π =
πΆ
π β ππΆ
for π > ππΆ
β’ Where, C is a constant
Fig.: Curie Temperature of
some Ferromagnetic material.
Curie Temperature of
some materials
35. Hysteresis
β’ The behaviour of ferromagnetic material when placed in external magnetic
field is quite interesting and informative.
β’ It is nonlinear and provides information of magnetic history of the sample.
β’ Consider an unmagnetized ferromagnetic material in the form of a rod
placed inside a solenoid.
β’ On passing the current through solenoid,
magnetic field is generated which
magnetises the rod.
β’ Knowing the value of Ο of the material of the
rod, M (magnetization) = Ο H. From Eq. (1)
β’ π» =
π΅
π0
β π
Fig.: Hysteresis cycle (loop)
36. Hysteresis
β’ Knowing the value of H (= ππ) and M, one can calculate the corresponding
magnetic field B.
β’ Figure shows the behaviour of the material as we take it through one cycle
of magnetisation.
β’ At point O in the graph the material is in nonmagnetised state.
Fig.: Hysteresis cycle (loop)
β’ As the strength of external magnetic intensity
H is increased, B also increases.
β’ But the increase is non linear.
β’ Near point a, the magnetic field is at its
maximum value which is the saturation
magnetization condition of the rod.
β’ This represents the complete alignment and
merger of domains.
37. Hysteresis
β’ If H is increased, (by increasing the current flowing through the solenoid)
there is no increase in B. This process is not reversible.
β’ At this stage if the current in the solenoid is reduced, the earlier path of the
graph is not retraced. (Earlier domain structure is not recovered).
β’ When H = 0 (current through the solenoid is made zero, point b in the
figure) we do not get B = 0.
Fig.: Hysteresis cycle (loop)
β’ The value of B when H = 0 is called retentivity or
remanence.
β’ This means some domain alignment is still retained
even when H = 0.
β’ Next, when the current in the solenoid is increased
in the reverse direction, point c in the graph is
reached, where B = 0 at a certain value of H.
β’ This value of H is called coercivity.
38. Hysteresis
β’ At this point the domain axes are randomly oriented with respect to each other.
β’ If the current is further increased, in the reverse direction, B increases and again
reaches a saturation state (point d).
β’ Here if H is increase further, B does not increase.
β’ From this point d onwards, when H is reduced, B also reduces along the path de.
Fig.: Hysteresis cycle (loop)
β’ At this point e, again H = 0 but B is not zero.
β’ It means domain structure is present but
the direction of magnetisation is reversed.
β’ Further increase in the current, gives the
curve efa.
β’ On reaching point a, one loop is complete.
β’ This loop is called hysteresis loop and the
process of taking magnetic material through
the loop once is called hysteresis cycle.
39.
40.
41. Permanent Magnet and Electromagnet:
β’ Soft iron having large permeability (>1000) and small amount of retaining
magnetization, is used to make electromagnets.
β’ For this purpose, a soft iron rod (or that of a soft ferromagnetic material) is
placed in a solenoid core.
β’ On passing current through the solenoid, the magnetic field associated with
solenoid increases thousand folds.
β’ When the current through the
solenoid is switched off, the
associated magnetic field
effectively becomes zero.
β’ These electromagnets are used in
electric bells, loud speakers,
circuit breakers, and also in
research laboratories.
42. Permanent Magnet and Electromagnet:
β’ Giant electromagnets are used in cranes to lift heavy loads made of iron.
β’ Superconducting magnets are used to prepare very high magnetic fields of
the order of a few tesla.
β’ Such magnets are used in NMR (Nuclear Magnetic Resonance) spectroscopy.
β’ Permanent magnets are prepared by using a hard ferromagnetic rod instead
of soft used in earlier case.
β’ When the current is switched on, magnetic
field of solenoid magnetises the rod.
β’ As the hard ferromagnetic material has a
property to retain the magnetization to
larger extent, the material remains
magnetised even after switching off the
current through the solenoid.
43. Do you know?
β’ Soft ferromagntic materials can be easily magnetized and
demagnetized.
β’ Hysteresis loop for hard and soft ferramagnetic materials
44. Magnetic Shielding:
Fig. Magnetic Shielding.
β’ When a soft ferromagnetic material is kept in a uniform magnetic field, large
number of magnetic lines crowd up inside the material leaving a few outside.
β’ If we have a closed structure of this material, say a spherical shell of iron kept in
magnetic field, very few lines of force pass through the enclosed space.
β’ Most of the lines will be crowded into the iron shell. This effect is known as
magnetic shielding.
β’ The instrument which need to be protected from magnetic field is completely
surrounded by a soft ferromagnetic substance.
β’ This technique is being used in space ships.
β’ Some scientific experiments require the experiment
to be protected from magnetic field in the
laboratory.
β’ There, high magnetic fields of magnets need to be
shielded by providing a case made up of soft
ferromagnetic material.
45. Do you know?
β’ There are different types of shielding available like electrical
and accoustic shielding apart from magnetic shielding.
β’ Electrical insulator functions as an electrical barrier or shield
and comes in a wide array of materials.
β’ Normally the electrical wires used in our households are also
shielded.
β’ In case of audio recording it is necessary to reduce other stray
sound which may interfere with the sound to be recorded.
β’ So the recording studios are sound insulated using acoustic
material.