12 pius augustine magnetism

2
Dr. Pius Augustine, SH College, Kochi

Introduction
Shepherd boy – Magnus
Obtained from – Magnesia, a coastal
district of ancient Thessaly, Greece.
Named ‘magnetite‘
Chemical name iron oxide (Fe2O3)
Chinese used first as direction finders –
lodestone means leading stone
3

i. attracting small pieces of iron
ii. setting itself along a definite
direction when it is suspended
freely.
Word Magnet – derived from magnesia.4

5

Applications
Electromagnets – electric motor,
microphone, loudspeaker,
Magnetic tapes - audio, video
recording, and computer
memory.
Permanent magnets – labs,
dynamo, galvanometer etc
6

Natural magnets
Exhibit property of magnetism
by themselves.
Lodestone or magnetite is the
only natural magnet
Disadvantage is that, is brittle
and cannot be obtained in
different geometrical shapes.
7

Artificial magnets
Substances made by people to
exhibit the property of
magnetism are artificial
magnets.
i. Bar magnet – rectangular bar
ii. Horse – Shoe (shaped) magnet
iii. Robson’s magnet – dumb-bell shape
iv. Cylindrical magnet
v. Magnetic needle – freely pivoted. 8

9

Neodymium magnets
Magnetic compass
Magnetic needle
10

Horse shoe magnet has twice
the attractive power than the bar
magnet?
Both poles are facing each other
11

Artificial magnets are preferred
to natural magnets?
1. Stronger
2. Magnetic strength can be increased
3. Can be made in any shape or size
according to the requirements
Electromagnets are also artificial magnets 12

Activity -1
Hang a bar magnet freely.
What is the direction in which
it comes to rest.
Disturb and try again.
What did you understand?
End towards north is N pole or
North seeking pole 13

Activity -2
Bring two bar magnets likes poles,
unlike poles and understand that like
poles repel and unlike poles attract.
Try with a bar magnet and iron bar.
How will you identify whether given two
pieces are magnets or magnet and
iron?
Repulsion is sure way of testing
magnets
14

Activity -3
Put iron filings on a piece of paper .
Roll bar magnet in iron filing and lift
it up.
In which regions do the maximum
iron filings cling to the magnet?
Try with horse shoe magnet. What
do you observe?
15

Properties of magnet
i. Attractive property
ii. Directional property
iii. Strength is maximum at the ends
called poles.
iv. Monopoles donot exist (when a
bar magnet is broken, every piece is
a complete magnet)
v. Like poles repel and unlike poles
attract
16

Design experiments using these figures to
explain magnetic properties.
17

• Some magnets have even
more than one pole
• eg. Refrigerator magnets have
narrow strips of alternating
north and south poles.
18

Theoretical physicists have speculated
for more than 70years about the
possible existence of discrete magnetic
“charges”, called magnetic monopoles.
Various attempts have been made to
find monopoles, but none has proved
successful. 19

• What is a lodestone?
• What is natural magnet? Why are
artificial magnets required?
• State and explain four properties of
magnet?
20

Poles of a magnet
• The points of a magnet where the attraction
appears to be maximum.
• Poles are not at the ends of the magnet, but
slightly inside.
• Distance from centre of magnet to one of the
poles – length of the magnet (half the effective
length)
21

Effective length and magnetic axis.
Distance b/w the poles of a magnet is
called its effective length which
nearly 5/6th the geometrical length of
magnet.
Imaginary straight line connecting poles
of a magnet is called magnetic axis.22

Length of the magnet (l)
Effective length of the magnet (2l)
Geometrical length/physical
length/actual length of the magnet (L)23

Geometric pole: Geometric end of a bar
magnet
Magnetic pole: The point situated slightly
inside the bar magnet, where most of its
magnetic power is concentrated
24

• Is it possible to isolate the poles of a
magnet?
If a magnet is carefully broken into two
equal pieces lengthwise/breadthwise. How
does the strength of each piece compare
with that of the original magnet?
Ans: 1:2
25

memomagnetics
Collecting magnets
as a hobby
Collector of
magnets would
be called
memomagnetist.
26

Does every magnet necessarily
have a north and south pole?
Yes.
Some ‘trick’ magnets have
more than one pair of
poles, but ,always occur
in pairs.
27

Magnetic materials
Those that are attracted by a
magnet and can be converted into
magnets. (Ferromagnetic
materials)
Eg. Iron, cobalt, nickel, alloys
containing these with other
metals.
Iron shows strong magnetic
properties. 28

Non magnetic materials
Those that are not attracted
by a magnet
Classified into three main
groups
i. Diamagnetic
ii. Paramagnetic.
iii.Ferromagnetic 29

Diamagnetic substance
They are feebly repelled by either of the poles
of a strong magnet
Eg: Copper, Gold, Bismuth, Antimony, Water.
Magnetic field strength decrease in it compared
to outside when placed in external field.
Super conductor exhibits perfect diamagnetism-
food for thought. 30

Paramagnetic substance
They are very feebly attracted by either of the
poles of a strong magnet
Eg: Platinum, manganese, aluminium, zinc,
certain kinds of plastic and wood.
31

Ferromagnetic substance
Strongly attracted by magnet and can be easily
magnetized to form strong magnets
Eg: Iron, steel, nickel, cobalt, and their alloys .
32

Magnetic properties of the materials.
Decided by the following parameters and/or
magnetic response of the material when
placed in magnetic field.
i. Magnetizing field (H)
ii. Permeability (μ)
iii. Intensity of magnetization (M)
iv. Susceptibility (χ) 33

Activity - 4
Clamp a magnet
vertically down.
Make a chain of nails.
34

Magnetic induction.
Is a process by which a
piece of magnetic
material becomes a
magnet when it is placed
near or touching a
permanent magnet.
35

Induction always precedes attraction.
Explain
In magnetic induction, magnetic
substance will be magnetised in such
a way that, pole near to inducing
magnet will be opposite to that of
inducing magnet.
So there will be attraction b/w
inducing magnet and induced
magnet. 36

Magnetic induction is temporary.
Comment.
Induced magnetism prevails as
long as inducing magnet is in
the vicinity.
If inducing magnet is removed,
chain of iron nails collapse.
Induced magnetism is
temporary. 37

Explain the term “induced magnetism”.
How does induction play role in the attraction of a
piece of iron by a magnet?
Explain the mechanism through which the
unmagnetized iron nail attracted to a magnet
when brought near to it.
38

Comment on these figures
Free ends of the hanging pins repel
in the first figure and attract in the
second figure.
39

Magnetic field.
Region around a magnet in
which magnetic influence
can be felt is called
magnetic field.
Strongest near the magnet
and decreases with
distance. 40

Magnetic field is due to the
‘distortions’ in the electric field
caused by motion of charge
and was explained by Albert
Einstein in 1905 in his special
theory of relativity. Magnetism
is relativistic
• “On the Electrodynamics of Moving Bodies”
41

Activity -4
Place a magnet on a
cardboard.
Sprinkle iron filings evenly.
Tap the board, note pattern
Remove iron filings and plot
the lines around with
magnetic compass 42

Magnetic field lines (lines of force)
Line along which an isolated
north pole would move, if it
is free to move.
ie. direction of field line is
from north pole to south
pole (outside the magnet) 43

Magnetic field lines – Will never intersect?
Magnetic field is a vector quantity and the
direction of tangent at any point on the
field line is the direction of magnetic field.
So field lines will never intersect. If so, there
will be two tangents or two directions for
magnetic field at intersecting point, which is
not possible.
44

45

46

47

48

49

Neutral point.
B/w two like poles, magnetic field
lines experience lateral
repulsion.
Neutral point is a point where
effective magnetic field is
zero.
Compass is not affected. 50

51

Plotting of the combined magnetic
field line of earth and bar magnet
Case 1 North pole of the magnet is
pointing toward geographical north
Case 2 North pole of the magnet is
pointing towards geographical south.
52

Please add the steps for plotting combined magnetic
lines
Student activity
53

Explain the formation of neutral points in the
plot of combined magnetic field of earth and
bar magnet in the two cases.
North pole of bar magnet pointing north – neutral
point on the equitorial line
North pole of bar magnet pointing geographical
south – on the axial line.
54

North pole of bar magnet pointing north – neutral point on the equitorial line
North pole of bar magnet pointing geographical south – on the axial line.
Magnetic field due to earth is from Magnetic north
to magnetic south. (Magnetic north is near
geographic south and vice versa)
At neutral point field due to earth and that det to
bar magnet are equal in magnitude but in
opposite direction. So nullified. 55

Quadrupole magnets consist of groups of
four magnets laid out so that in the multipole
expansion of the field the dipole cancel
The simplest magnetic quadrupole is
two identical bar magnets parallel to
each other such that the north pole of
one is next to the south of the other and
vice versa. Such a configuration would
have no dipole moment, and its field will
decrease at large distances faster than
that of a dipole. 56

57

58

59

How will you get neutral
point with single bar
magnet. Draw a diagram
to support your answer.
60

In the figure, the magnetic field lines in the right and left extremes
are nearly parallell lines. Give reason.
Ans: Parallell field lines represent uniform magnetic field.
Earth’s magnetic field in a small region of space will be
uniform.
61

Parallell field line – uniform magnetic field
Crowded lines (inward) – Non uniform magnetic field
increasing
Diverging lines – Non uniform magnetic field
(decreasing)
X
indicates
neutral point
where there is
no field line
62

Properties of magnetic field lines
i.Continuous and closed curve.
ii. Never intersect
iii. From N to S outside and S to
N inside
iv. Crowded near poles.
v. Uniform field – parallel lines 63

Magnetic field line is the path along which
an isolated north pole would move.
A long magnetised needle passing
through a cork, floating vertically on
water in a beaker, such that, south
pole is inside water and north pole
near the edge of the beaker.
64

Magnetic field line is the path along which
an isolated north pole would move.
Place a bar magnet on the edge
such a way that north pole is close
to needle.
Needle moves along a curve
towards south pole of magnet. 65

Magnetic compass
compass is a navigational instrument
for determining direction relative to
the Earth's magnetic poles.
It consists of a magnetized pointer
(usually marked on the North end)
free to align itself with Earth's
magnetic field kept in a nonmagnetic
case with glass on the top. 66

Making of compass :
i. Ordinary iron nail is pointed
along the direction of earths
magnetic field.
ii. Repeatedly strike it with a
hammer or a rock. 67

Plot magnetic lines in the following
case.
i.Two identical bar magnets
north poles facing each other
ii. S – S
iii. Magnets are arranged
parallel NN SS
iv. Magnets are arranged
parallel NS SN 68

Earth’s magnetic field?
Earth behaves like a gigantic
bar magnet with its south
pole near geographical
north pole (slightly to west).
69

70

Magnetic merididan
Vertical plane passing
through a freely
suspended magnet at
rest and magnetic axis. 71

Geographic meridian
Vertical plane passing
through geographic
axis
72

Magnetic declination or Angle of declination
Angle b/w the magnetic
meridian and geographic
meridian at a place
It varies over the earth.
Declination of a given place
vary over time 73

Isogonals
Lines joining places
having the same
magnetic declination
are known as
isogonals.
74

Agonic line
Line which joins all the places
on earth, having zero angle of
declination.
75

Magnetic dip
Magnetic needle free to rotate about
horizontal axis takes rest in the
direction of resultant field, which
makes an angle with the horizontal
called dip.
Dip is measured with dip circle
0o at equator 90o at poles 76

Magnetic equator
Imaginary line passing
through all such places
on the earth where angle
of dip is 0o. 77

Isoclinics
Lines joining places
having the same
magnetic dip are
known as isoclinics.78

Aclinic lines
Line joining all the palces,
having zero angle of dip is
aclinic line
It is magnetic equator
79

Horizontal intensity Bh
B at a place can be resolved in to
two components.
Horizontal and vertical.
It is Bh affect magnetic
measurements and expts.
Bh =0.38 x 10-4T 80

Uniform magnetic field
If B is same in magnitude
and direction in a region.
Represented by equidistant
parallel lines.
Eg. Earths magnetic field in small region81

Combined field – Earth and bar magnet
i. N pointing N(geographic)
Neutral point – on equitorial line
ii. N pointing S (geographic)
Neutral point – on axial line82

Theories of magnetism
18th century – due to magnetic fluid.
Abandoned like caloric theory (heat)
19th cent. In 1850 Wilhelm Weber
proposed and in 1898 Ewing
developed molecular theory.
Modern theory – due to orbital and
spin motion of electrons
83

Molecular theory
Molecules are tiny magnets,
even when material is not
magnetised.
Molecular magnets are in closed
chain forms with opposite poles
are close to each other.
When magnetised – molecular
magnets allign or reoreient. 84

If the motion of electric charges
produces magnetism, where is this
motion in a common bar magnet?
Electrons of atom.
Electron spin and revolution.
Electron spin is the chief contributor.
A pair of electrons spinning in the same
direction makes up a stronger
magnet, opposite directions, work
against each other. 85

• Most materials electrons spin in
opposite and fields cancel.
• Iron : each iron atom has four
electrons whose spin magnetism
not cancelled. Lesser amount in
cobalt and nickel.
• Rare earth metal Gadolinium
orbital motion is more significant.
86

Evidence for molecular theory
i. Magnet is broken each piece
is a magnet. Even smallest
piece ie. molecule is a
magnet.
ii. Magnetising iron filings in a
test tube by single touch
iii.Magnetic saturation
87

Magnetic domains
Magnetic field of an individual iron
atom is so strong that interactions
among adjacent atoms cause
large clusters of them to line up
with one another.
These clusters of aligned atoms are
called magnetic domains.
Billions of domains. 88

89

• It is interesting to listen with an
amplified stethoscope to the
clickity- clack of domain
undergoing alignment in a
piece of iron when a strong
magnet approaches.
• Like charge in paper when
charged rod is brought near. 90

Magnetic domains
91

Activity -5
Fill TT with iron filings.
Magnetise by single touch.
Filings at top align horizontally.
Ends of TT exhibit opp. polarity
Shake – loose magnetism.
Each iron filing act like
molecular magnet. 92

Methods for demagnetising
i. Dropping or hammering
ii. Heating
iii.Keep magnet side by side with
like poles next to each other
iv.Placed inside a solenoid
carrying alternating current.
93

Taking care of magnets
i. Shouldn’t be dropped or
hammered
ii. Should not be heated.
iii.Should not store with like
poles each other. 94

Taking care of magnets
Should be kept in pairs with their
opposite poles lying side by side.
Piece of wood in between, and two
pieces of soft iron called keepers,
placed across the poles.
Single keeper for horse shoe 95

Self Demagnetisation
Magnets stored without
keepers have a tendency to
lose their magnetism on
their own called self
demagnetisation. 96

Magnetic keepers
Magnetic keepers are used to protect the
magnets from self demagnetization.
They are soft iron pieces, which attach
opposite poles of two bar magnets (horse
shoe) kept together with a wooden piece
in between.
97

Biomagnetism
Pigeons have multiple
domain magnets in
their skull connected
with large no. of nerves.
Pigeons have magnetic
sense – can discern
longitudinal directions
along earth’s magnetic
field and also detect
latitude by magnetic
dip.
98

99

You are given two identical
bars, one is a magnet and the
other piece of soft iron. How
will you distinguish between the
two without using any other
magnet? 100

Keep one piece perpendicular to
other and move from one end to
other, if force vary and minimum in
the middle, that piece is magnet.
If iron is placed horizontally and
magnet is moved from one end to
other force remains same. 101

All magnets are
electromagnet.
Comment.
102

All atoms have moving
electric charges. Why,
then aren’t all materials
magnetic?
103

Magnetising field (H)
Ability of an applied field to magnetize a
material is denoted by (H).
Note: For magnetizing a material magnetic field
inside a current carry coil in the form of a spring
(solenoid) is used. And strength of the field
depends on number of turn/unit length of the
spring and current through it. H = nI 104

Permeability (μ)
The ratio of the magnetic flux density (B)
(magnetic lines/unit area) in a material
to the magnetising field (H) is called
permeability.
μ = B/H
105

Intensity of magnetization (M)
Magnetic moment developed per
unit volume of the specimen, which
is subjected to a magnetic field.
M = m/V
Strength of a magnetic dipole is expressed
in terms of dipole moment (m) 106

Magnetic susceptibility (χ)
It is the ratio of the intensity of
magnetization produced (M) in a
material to the magnetizing field
(H).
χ = M/H
107

108
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Dr. Pius Augustine, Asst. Professor, Sacred Heart College, Thevara, Kochi.Dr. Pius Augustine, SH College, Kochi

12 pius augustine magnetism

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