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SRHSS/2010/Magnetism Page 1 of 3 Sandhya.K
MAGNETIC DIPOLE
 Oersted discovered the magnetic effects of electric current.
Artificial Bar Magnet and its properties
 Natural magnets are not strong enough.
 Steel does not acquire magnetism easily but once acquired, retains it unless heated to a very high temperature.
 Soft iron acquires magnetism easily and loses it easily.
Magnetic Pole: The regions of attraction near the two ends of the magnet, where the magnetic force due to the bar magnet is
maximum.
Properties:
1. Attractive property: A magnet can attract small pieces of magnetic substances like iron, cobalt and nickel. The attraction
is
maximum at the poles.
2. Directive property: A magnet when suspended freely, aligns itself, approximately, along the geographical north-south
line such that the north pole of the magnet is towards the north of the earth and the south pole is
towards the south of the earth.
3. Unlike poles attract and like poles repel
4. Magnetic poles exists in pairs: If a magnet is cut into two equal parts,
transverse to its length, two magnets
of the same pole strength are obtained.
On the other hand, if the magnet is cut into two equal halves along the length
of the magnet, each part is found to be a magnet having pole strength exactly
half of the pole strength of the original magnet.
5. Inductive property: When a piece of a substance, such as soft iron, steel, cobalt, nickel
etc are placed near a bar magnet, it acquires magnetism. The magnetism
so acquired is called induced magnetism.
6. Repulsion is the surest test for distinguishing between a magnet and a piece of iron: The pole of a magnet attracts a piece
of iron as well as an unlike pole of another magnet. To identify the given specimen as a magnet, it
should repel the like pole of another magnet.
MAGNETIC DIPOLE AND MAGNETIC DIPOLE MOMENT
Since the bar magnet has two poles, it is sometimes called magnetic dipole.
Magnetic dipole: An arrangement of two magnetic poles of equal and opposite strengths separated by a finite distance is called a
magnetic dipole.
Magnetic length: The distance between the two poles of a bar magnet is called the
magnetic length of the magnet.
Magnetic length is a vector from S-pole of the magnet to the N-pole of the magnet and
is denoted by l

2 . Since the magnetic poles are situated within the magnet, the
magnetic length is always less than its geometric length.
Magnetic dipole moment: The product of the strength of either pole and the magnetic
length of the magnet is called magnetic dipole moment.
It is a vector quantity and is represented by M

. It is directed from S-pole to the N-pole.
Thus, )2( lmM


The S.I unit of magnetic dipole moment is ampere meter2 (Am2).
Magnetic Field
The space around a magnet (or current carrying conductor), in which it’s magnetic effect can be experienced, is called the
magnetic field.
N
S
N
S
S
N
S
S
On Breaking
transverse

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SRHSS/2010/Magnetism Page 2 of 3 Sandhya.K
The magnetic field in a region is said to be uniform, if the magnitude of its strength and direction is same at all points.
A uniform magnetic field is represented by equidistant parallel arrows.
The S.I unit of strength of magnetic field (also known as magnetic induction or magnetic flux density) is tesla (T).
1 tesla (T) = 1 newton ampere-1metre-1 (NA-1m-1) = 1 weber meter-2.
The strength of the magnetic field is also measured in gauss (G).
1 gauss (G) = 10-4 tesla
Magnetic field lines
The magnetic field line is the path along which an isolated north pole will tend to move, if it is free to do so.
Properties of magnetic field lines
1. The magnetic field lines are closed continuous loops extending through the body of the magnet.
2. Outside the body of the magnet, the magnetic field lines are directed from north-pole to the south-pole of the magnet.
3. The tangent to the magnetic field line at any point gives the direction of the magnetic field at that point.
4. No two magnetic field lines can intersect each other.
5. The magnetic field lines dilate laterally and contract longitudinally.
TORQUE ON A BAR MAGNET PLACED IN A MAGNETIC FIELD
Let a bar magnet having pole strength m and length 2l be placed in a uniform magnetic field of strength B making an angle θ
with the direction of the magnetic field.
Force on the N-pole = mB (along the direction of the magnetic field)
Force on the S-pole = mB (opposite to the direction of the magnetic field)
So the bar magnet is acted upon by two equal, parallel and opposite forces. This constitutes a
torque and tends to rotate the magnet in the clockwise direction. The magnitude of the torque
is given by-
torque, τ= either force x perpendicular distance between the two forces.
= mB x KN
From the right triangle KNS, we have-
l
KN
NS
KN
2
sin 
Or, sin2lKN 
Therefore,  sin)2.( lmB
Since, Mlm

)2( , the magnetic moment of the bar magnet, we get-
 sinMB --------------------- (1)
In vector notation, BM

 ---------------- (2)
Unit of magnetic dipole moment: from eqn. (1),


sinB
M 
Hence, unit of magnetic dipole moment =
tesla
newtonmetre
= 11 
 JTNmT
T
Nm
Current loop as a magnetic dipole
Consider a circular coil carrying current I in the anticlockwise direction. The magnetic field
lines due to each elementary portion of the circular coil will be of the shape of circular loops
and almost straight near the centre of the circular coil.
Applying, right hand thumb rule, it follows that the magnetic field lines seem to enter at the
lower face of the coil and leave at its upper face. Thus, lower face of the coil acts as the south
pole and upper face as the north pole. Hence, current carrying coil acts as a magnetic dipole.
It is found that magnetic dipole moment of the current loop,
(i) is directly proportional to the strength of the current loop i.e.
IM  -------------- (1)
(ii) is directly proportional to area of current loop i.e.
AM  -------------- (2)
Combining both the equations, IAM 
Or, kIAM  , where ‘k’ is the constant of proportionality.
When current is measured in amperes and area in metre2 and magnetic moment in ampere metre2 the constant of proportionality
‘k’ is found to be 1. Thus,
IAM  -------------- (3)
In case, the current carrying coil has ‘n’ turns, then-
nIAM  -------------- (4)
The factor ‘nI’ is called the ampere turns of the circular current loop.

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SRHSS/2010/Magnetism Page 3 of 3 Sandhya.K
Geo-magnetism or Earth’s Magnetism
Earth has its own magnetic field of the order of 10- 5 tesla on its surface.
The following points are for information only.
(i) The imaginary magnetic axis of earth makes an angle of 20° with the geographic rotational axis.
(ii) The magnetic south pole of earth is in north Canada at latitude 70.5° north and longitude 96° west and the magnetic
north pole is at latitude 70.5° south and longitude 84° east. The imaginary magnetic axis of earth makes an angle of
20° with the geographic rotational axis.
(iii) Magnetic poles are approximately 2000 kilometers away from the geographic poles.
(iv) Magnetic and geographic equators intersect each other at longitude 6° west and 174°east.
(v) Thumba near Trivandrum (India) is on the magnetic equator and hence it has been selected for rocket propulsion
experiments.
(1) Geographic Axis is a straight line passing through the geographical poles of the earth. It is the axis of rotation of the
earth. It is also known as polar axis.
(2) Geographic Meridian at any place is a vertical plane passing through the geographic north and south poles of the earth.
(3) Magnetic Axis is a straight line passing through the magnetic poles of the earth.
(4) Magnetic Meridian at any place is a vertical plane passing through the magnetic north and south poles of the earth.
Geo-magnetic Elements/ magnetic elements
The physical quantities, which determine the intensity of earth’s total magnetic field completely (both magnitude and direction),
are called magnetic elements.
(I) Declination(θ)
The angle between the magnetic meridian and the geographic meridian at a place is
declination at that place. It varies from place to place.
(ii) Dip or Inclination (δ):
The angle between the horizontal component of earth’s magnetic field and the earth’s resultant
magnetic field at a place is Dip or inclination at that place. It is zero at the equator and 90°at
the poles.
(iii) Horizontal Component BH:
The component of Earth’s total magnetic field in horizontal direction at any point on
earth's surface is called Horizontal component. This component is important as many
devices such as :Tangent Galvanometer, Vibration magnetometer, Magnetic compass
Mathematically, cosBBH  and sinBBV  (from the figure)
Squaring and adding the two equations,   2222222222
sincossincos BBBBBB VH  
Or,
22222
VHVH BBBBBB 