Magnetism and Matter
Some important facts about magnet:
1. The Black Rock were found in magnesia up place in Asia minor.
2. These rocks were made of ore of iron cold magnetite ore black iron oxide (Fe 3O4).
3. These rocks were attract small pieces of iron.
4. Letter Chinese called it loadstone.
5. If a piece of loadstone or magnetite is suspended freely by thread it would set itself
in the direction of earth's geometrical North and South.
6. A magnet attract not only things made up of iron but it also attracts things made of
steel nickel cobalt etc.
7. The object made of steel, nickel, cobalt, iron are called magnetic substances.
Substance which attracts magnetic substances is called magnet and this property is
called magnetism.
 The attractive property of magnet was discovered by Greeks and its directional
property was discovered by Chinese.
 In ancient time the directional property of magnet were used by sailors.
Types of magnets:
1. Natural magnet: The magnet found in nature are called natural magnets. Magnetite
which is the ore of iron is a natural magnet.
 These magnet are of uneven odd shapes and they cannot be moulded in desired
shape
 Their magnetism is wear hence generally natural magnet is not used.
 Earth behaves like a permanent natural magnet.
2. Artificial magnet: Man made magnets are called artificial magnets. These are strong
and different shapes.
Magnetic poles: The points inside the magnet where the attraction is maximum are called
magnetic poles. Every magnets has two poles (i) North Pole (N - Pole) and (ii) South Pole
(S - Pole)
Properties of magnets:
1. A magnet attracts the magnetic substances.
2. A magnet when suspended freely, align itself along the geographical North and
South direction of the earth.
3. Like poles of magnet repel each other.
4. Unlike poles of magnet attract each other.
5. Repulsion is the shortest test of magnetism.
6. If a bar of soft iron brought near to the pole of magnet then the iron bar becomes a
magnet this property is called magnetic induction.
7. If a bar magnet is beaten overheated, then it loses its magnetic property.
Reason: Repulsion is the shortest test of magnetism because magnet attracts every
magnetic substances and it is impossible to investigate that that substance is magnet or
the actual magnet is magnet but if this magnet gets closer to the another magnets like
pole then it will repel and it is the surest test that this is the magnet.
Magnetism and Matter Page 1
It is not possible to separate the poles of magnet because each molecule of magnet is itself a small magnet.
Some important definitions related to magnet:
1. Magnetic axis: The line joining the 2 poles of magnet is called its magnetic axis.
2. Effective length: The distance between the poles of magnet is called effective length of magnet. It is denoted by 2l.
The magnetic pole are not exactly on the ends of magnet, hence the effective length is always less than geometric length
of magnet.
Effective length = Geometric length.
3. Pole strength: the pole strength of pole of magnet is the strength with which it attract the magnetic substance
towards it.
It is denoted by m.
It's SI unit is ampere meter (Am).
It's dimensional formula is [M0LT0A]
4. Magnetic Meridian: it is a vertical plane that passes through the magnetic North and South poles of the earth.
5. Magnetic dipole: arrangement of two equal and opposite magnetic poles
separated by small distance (2l) is called magnetic dipole. Actually every
magnet or simply bar magnet is the magnetic dipole.
Magnetic dipole moment (M) of a magnetic dipole is defined as the product of its pole strength (m) and
magnetic length (2l)
Similar to electrostatics, the North Pole has magnetic pole strength +m and the South Pole has magnetic pole strength -m.
A current carrying loop behave like a bar magnet whose one face is North Pole and another is South Pole.
Comparison between torque acting on a magnetic dipole and electric dipole.
Coulomb's inverse square law of magnetism.
Magnetism and Matter Page 2
The force of attraction or repulsion between 2 magnetic poles held near me is directly proportional to the product of
pole strength add inversely proportional to the square of distance between them.
Actually monopoles of magnet is not possible. To solve this problem Coulomb took long bar magnets so that far
poles off that magnets has a negligible pole strength hence the nearby polls can behaves like 2 separate poles.
Unit pole strength:
In SI system: when 2 similar poles are kept one metre apart in air or vacuum and
repel each other by force of 10-7 Newton, then the polls are called unit pole
strength.
In CGS System: When 2 similar poles are kept one centimetre apart in air or
vacuum and repel each other by the force of one dyne then the poles are called
unit pole strength.
Magnetic Field: The region around a magnet in which its effect can be
experienced is non as magnetic field.
Magnetic field intensity or magnetic field strength: The force experienced by
a free imaginary unit North Pole at a point in the magnetic field is known as
magnetic field intensity for magnetic field strength at that point.
Magnetic field lines or magnetic lines of force: magnetic field lines or magnetic
lines of force are the curve in the magnetic field on which if a unit North Pole
is placed then it will follow the imaginary curve drawn.
Or
These are the imaginary lines drawn around a magnet to represent the magnetic field around it.
Properties of magnetic field lines:
1. The lines drawn are imaginary but the field represented by it is real.
2. They are closed and continuous curves.
3. Outside the magnet the direction is from north to South and inside the magnet the direction is from
South to north.
4. The tangent drawn at any point on the curve gives the direction of resultant force at that point.
5. 2 magnetic field lines can never intersect each other if they were intersect each other then add the
point of intersection there will be 2 tangents and 2 directions of magnetic field which is not possible.
Magnetism and Matter Page 3
Differences between electric and magnetic field lines:
Electric lines of force.
Magnetic lines of force.
1. These are open smooth curves.
These are closed smooth curves.
2. These are normal to the conductor.
These are not normal to the magnet always.
3. They are not found inside the conductor.
They are found inside the magnetic substances also.
Uniform Magnetic Field: If the magnetic field vector B at different point in
the field is same in magnitude and direction, then the field in that region is
said to be uniform. The uniform magnetic field is represented by parallel
lines.
If the magnetic field vector at different points in the magnetic field is different in magnitude and
direction then the field in that region is said to be non-uniform magnetic field. The field lines of nonuniform magnetic field is not parallel to each other.
Magnetic field due to a monopole:
Magnetic field due to monopole is defined as the force experienced by it per unit test pole.
Units:
1. Si unit:
2. Cgs unit: gauss or oersted.
Dimensional formula:
Pure magnetic field is represented by H and the net magnetic field is represented by B. Since the cure
non-magnetic environment is not possible hence the magnetic field is usually represented by B
1. Axial position, End on position or Gauss' A position:
When the point where the intensity of magnetic field is to be found, is on the magnetic axis, then it is called end on
position or Gauss' A position.
Magnetism and Matter Page 4
The direction of net magnetic field due to
a bar magnet at its axis is along the
direction of magnetic moment.
Magnetic field due to monopole is directly proportional to r2 but due to
dipole is directly proportional to r3.
2. Equatorial position or broad side on position or Gauss B position: when the point where the intensity of
magnetic field has to be found, lies on the perpendicular bisector of magnetic axis, that is, it is on the neutral axis,
then it is called broadside on position for Gauss' B position.
It's direction is opposite to
the direction of magnetic
dipole moment.
Comparison between magnetic field due to bar magnet at Gauss A and Gauss B position.
Difference between axial and Equatorial position:
Axial Position:
Equatorial position:
1. The observation point lies on the extended magnetic
axis.
The observation point lies on the perpendicular bisector of
magnetic axis.
2. In this position, the magnetic field let us certain position
is twice that of Equatorial position, when the unit pole is
kept at equal distance.
In this position, the magnetic field at a certain point is half as
compared to the axial position, when the unit North Pole is
kept at equal distance.
3. The direction of net magnetic field acts along the
direction of magnetic axis from South to North Pole.
The direction of net magnetic field acts parallel to the direction
of magnetic axis from north to South Pole.
Magnetism and Matter Page 5
Torque on a bar magnet suspended in uniform magnetic field.
If the bar magnet is parallel to the magnetic field then no torque will act on it
If the magnet is held perpendicular to the magnetic field then the torque
acting on the magnet will be maximum.
Magnetic moment (M) is numerically equal to the maximum torque acting on the bar magnet, when it is held
perpendicularly in a uniform magnetic field of unit intensity.
Potential energy of a bar magnet in a uniform magnetic field is defined as work done in deflecting it from standard
position (that is magnet makes 90o angle with the direction of field) to any other position.
In this position the magnet remains under position of stable
equilibrium.
In this position the potential energy of magnet is zero. It is the reason due to which it is
called standard position.
In this condition magnet is said to be under
the position of unstable equilibrium.
The main difference between the magnetic field of bar magnet and a solenoid is that magnetic field of bar magnet is
stronger at poles, while the magnetic field inside the solenoid is almost uniform and strong little less and the ends.
Magnetism and Matter Page 6
Comparison of solenoid (Electromagnet) and a bar magnet.
Bar magnet:
Current carrying solenoid (Electromagnet)
Similarities:
1. A bar magnet has 2 poles North and South poles.
Current carrying solenoid also has 2 poles North Pole and South
Pole.
2. It align itself in north South direction when
suspended freely.
It also align itself in north South direction when suspended
freely.
3. In a bar magnet like poles repel each other while
unlike poles attract each other.
In current carrying solenoid also like poles repel each other
while unlike poles attract each other.
4. It shows induction.
It also shows induction.
5. It attracts magnetic substances towards it.
It also attracts magnetic substances towards it.
Dissimilarities:
1. In a bar magnet the intensity of magnetic field is
maximum at its poles and is minimum at its centre.
Solenoid the intensity is uniform and maximum inside it. At its
ends the intensity is slightly less.
2. Its magnetism is a stable.
Its magnetism depends on the value of current flowing through
it.
3. The polarity of its ends is fixed.
Its polarity can be changed by changing the direction of current.
The capability of magnetising field to magnetise the substance is
represented in the term of magnetising force or magnetic intensity H.
Magnitude of magnetising force is defined as the number of empire turns flying around unit length of
solenoid to produce the magnetic field in the solenoid
It is defined as the magnetic moment developed per unit volume when the magnetic
specimen is subjected to a magnetising field. It is represented by I
Intensity of magnetization of a magnetic material is also defined as the pole strength developed per unit area of cross
section of the specimen.
Magnetism and Matter Page 7
The magnetic susceptibility of a material is defined as the ratio of
the intensity of magnetization I to the applied magnetic field H.
The magnetic induction is defined as the number of magnetic lines of force inside
the material crossing per unit area normally through the magnetic substance.
Magnetic permeability of a magnetic substance is defined as the ratio of
magnetic induction (B) to that of magnetic intensity (H). It is denoted by
Magnetic properties of substances:
• In the year 1864 Michael Farady discovered that almost all the substances are affected by external magnetic field.
• On the basis of his discovery substances are divided into 3 categories:
1. Diamagnetic substances:
2. Paramagnetic substances:
3. Ferromagnetic substances:
1. Diamagnetic substances: The diamagnetic substances are those substances which when placed
in a magnetic field are feebly (weakly) magnetised in the direction opposite to the direction of
that magnetizing field.
• The magnetic field inside the diamagnetic substance is found to be slightly less than the external
magnetic field but in opposite direction.
• Property of diamagnetism is shown by those in which the individual atoms or molecules do not
possess any net magnetic moment of their own that is non polar atoms. Examples: bismuth,
phosphorus antimony copper etc.
Properties of diamagnetic substances:
a. The diamagnetic substances are feebly repelled by powerful magnet.
b. When a rod of diamagnetic substances is suspended freely inside a magnetic field, it slowly align
itself at right angle to the direction of magnetic field.
c. When are diamagnetic liquid contained in a watch glass is placed on 2 closely spaced pole pieces
of magnet, it shows a slight depression at the middle. However the liquid rises in the middle if
the pole pieces are moved apart
Magnetism and Matter Page 8
• When a diamagnetic liquid is filled in U tube and it's one arm is kept in strong
magnetic field then the liquid level in the arm is slightly depressed.
• The relative permeability ( ) of diamagnetic substances is a slightly less than one.
• The magnetic susceptibility of diamagnetic substances has small negative value.
• The diamagnetic substances do not obey Curie's law.
• The susceptibility of these substances does not depend upon the magnetising field
H. Hence the graph between I&H is straight line.
Diamagnetic substances
Paramagnetic substances
Ferromagnetic substances
1. These substances when placed in
magnetic field acquire feeble
magnetism opposite to the direction
of magnetic field.
These substances when placed in
magnetic field, acquire feeble
magnetism in the direction of
magnetic field.
These substances when placed in magnetic
field are strongly magnetised in the
direction of magnetic field.
2. These substances are feebly repelled These substances are feebly
by the magnet.
attracted by the magnet.
3. Examples of diamagnetic substances
are: bismuth, phosphorus, antimony,
copper, zinc, gold, mercury, lead
common salt water air hydrogen
common nitrogen etc.
These are strongly attracted by the magnet.
Examples of paramagnetic
Examples of ferromagnetic substances are:
substances are: aluminium,
iron, steel, nickel, cobalt etc
platinum, liquid oxygen, manganese,
chromium, copper sulphate, crown
glass etc
4. .
5. A rod of diamagnetic material is
6. Paramagnetic road become
suspended freely between 2
parallel to the magnetic field.
magnetic poles, its axis becomes
perpendicular to the magnetic field.
Ferromagnetic material rod also becomes
parallel to the magnetic field quickly.
6. In the non-uniform magnetic field,
the diamagnetic substances are
attracted towards the weaker
magnetic field, that is there moves
from stronger to weaker magnetic
field.
In non-uniform magnetic field they
move from weaker to stronger part
of the magnetic field slowly.
In a non-uniform magnetic field they moves
from weaker to stronger magnetic field
rapidly.
7. Their magnetic permeability is less
than one slightly.
Their magnetic permeability is
greater than one slightly.
Their magnetic permeability is very very
greater than one.
8. Their susceptibility is a small and
negative.
Their susceptibility is small and
positive.
Their susceptibility is very large positive.
In these substances the magnetic
lines of force are closer than air.
In these substances magnetic lines of force
are much closer than air
9. .
10. In these substances the magnetic
lines of force are farther then air.
Magnetism and Matter Page 9
Curie’s Law:
Magnetic susceptibility of a material varies inversely with the absolute temperature.
cm α 1 / T
cm = C / T
(where C is Curie constant)
Curie temperature for iron is 1000 K, for cobalt 1400 K and for nickel 600 K.
Intensity of Magnetisation (I) increases with increase
in Magnetising Force (H) initially through OA and
reaches saturation at A.
When H is decreased, I decreases but it does not
come to zero at H = 0.
The residual magnetism (I) set up in the material
represented by OB is called Retentivity.
To bring I to zero (to demagnetise completely),
opposite (negative) magnetising force is applied.
This magetising force represented by OC is called
coercivity.
After reaching the saturation level D, when the
magnetising force is reversed, the curve closes to the
point A completing a cycle.
The loop ABCDEFA is called Hysteresis Loop.
The area of the loop gives the loss of energy due to
the cycle of magnetisation and demagnetisation and
is dissipated in the form of heat.
The material (like iron) having thin loop is used for
making temporary magnets and that with thick loop
(like steel) is used for permanent magnets.
Magnetism and Matter Page 10