6.1. MAGNET
A
brown coloured stone which is found naturally on earth's surface is called
magnetite (Fe3O4) or magnet. First of all such stone was
found in the city called Magnesia (in Asia), hence it was given the name
magnetite. Later on, the word magnetite was reduced to magnet. In India, it is
found in Tamilnadu and Orissa states. The properties of a magnet are called
magnetism.
6.2. PROPERTIES OF A MAGNET
A
magnet has the following properties:
1.
It attracts iron filings. The point in a magnet, where the power of attraction
is maximum, is called a pole. A magnet has two poles.
2.
A freely suspended magnet (in horizontal plane) always rests in north-south
direction. The north seeking pole of the magnet is termed as north-pole and the
south seeking pole is termed as south-pole. A magnet is also known as leading
stone or load-stone on account of its directional property.
3.
Like poles of two magnets repel each other and the unlike poles attract each
other.
4.
If a magnet is repeatedly rubbed on an iron-bar, the later one also becomes a
magnet.
5.
If a magnet is hammered or dropped on a hard surface or suddenly cooled down
after heating it red-hot, its magnetism gets lost.
6.3. MOLECULAR THEORY OF MAGNETISM
Every
substance is made of tiny particles called molecules. The molecules of some
substances act as magnets. Normally, the molecules rest arranged in a random
order and form closed chains in a substance and hence they neutralize each
other's magnetism. Consequently, the substance has no magnetism in normal
condition. Now if the same substance is subjected to an external magnetic
field, its 'tiny-magnets' begin to set up in the direction of the magnetic
field and finally the substance becomes a magnet.
The
magnetic property of molecules of a substance is produced by the orbital and
spin motion of the electrons contained in their atoms. The phenomenon is known
as molecular theory of magnetism.
 |
| 6.1. Fig. Magnetised and non-magnetised states of iron |
Magnets
arc classified into following two main groups:
1.
Natural magnets
2.
Artificial magnets
1. Natural
Magnet.
A magnet found naturally on the earth's surface is called a natural magnet, It
has an irregular shape and poor magnetic strength. Now a days it is almost out
of use.
2.
Artificial Magnet. A magnet made of iron or other magnetic metals
artificially is called an artificial magnet. There are following two types of
artificial magnets:
(i) Permanent. A magnet
capable to retain its magnetism for many years is called a permanent magnet. It
is made of steel or an alloy of magnetic metals. ALNICO or ALCONEX is the trade
name of one such alloy. It consists of aluminum, nickel, iron, copper and
cobalt (in the ratio of 8 : 14 : 51 : 3 : 24). In the early stage of scientific
development, the permanent magnets were made by rubbing natural magnets
repeatedly on steel bars. Now a days, steel or alloy magnetic metal bars are
kept inside a powerful current carrying coil for some time for making permanent
magnets. They are made in flat or round bar and horse shoe shapes.
(ii)
Temporary.
An electromagnet is called temporary magnet because its magnetism lasts so long
as the current is flowing through its coil. It can be made in any shape as
required.
6.5. MAGNETIC KEEPERS
Permanent
magnets are normally made in the form of bar-magnets, horse-shoe magnets and
cylindrical magnets. When the magnets are out of use or they are in storage, a
piece of mild steel (soft iron) called a keeper is put across the two poles of
a magnet or of two magnets as shown in Fig. 6.2
A
magnetic keeper forms a closed magnetic circuit with the magnet/magnets. It
helps in retaining the strength of magnets due to induction from any external
magnetic field etc. and thereby prevents weakening of the same.
6.6. MAGNETIC NEEDLE
A
magnetic needle or magnetic compass consists of a very small magnet pivoted
inside a non-magnetic case with a glass top. The needle can move freely on its
pivot. The north pole of the needle is painted black so as to facilitate in
distinguishing it out of its two poles.
According
to the above stated properties of a magnet, the magnetic n. eedle always rests
in a North-South direction. This small instrument is used in determining the
direction of magnetic field of a magnet in the laboratory. Navigators use this
instrument in deciding the direction of the 'head' of a ship w.r.t. true north.
6.7. MAGNETIC FIELD
The
space around a magnet where the effect of its magnetism can be detected is
known as magnetic field. Magnetic needle placed at a point in the magnetic
field will rest in the direction or the field and not in the N-S direction.
The
magnetic field is composed of imaginary lines of force. A fine of force is
smooth curve and the tangent drawn at any point on the curve represents the
direction of magnetic field acting at that point.
Properties of
Lines of Force.
The principal properties of lines of force are as follows:
1. A line of force completes its circuit from
north-pole to south-pole
outside the magnet and from south-pole to
north-pole inside the magnet.
2. Two lines of force never intersect each other.
3.
The tangent drawn at any point on a line of force represents the direction of
magnetic field acting at that point.
6.8. MAGNETIC FLUX AND FLUX DENSITY
1. Magnetic
Flux.
The total number of lines of force passing through the plane perpendicular to
the line of force of a magnetic field is called magnetic flux. Its symbol is
(phi) and its unit is Weber (Wb). Its other unit is Maxwell and 1 Maxwell = 1
line of force and 1 Wb = 108 Maxwells.
2. Magnetic
Flux Density.
The number of lines of force passing through a plane of unit area,
perpendicular to the line of force of a magnetic field is called magnetic flux
density. Its symbol is B and its unit
is Wb/m2.
where,
B = magnetic flux density,
Weberslmetre2
= magnetic flux, Webers.
A = area of perpendicular
plane, Metre2
6.9. ELECTROMAGNETISM
Magnetic
effect is one out of the three principal effects of electric current. A current
carrying conductor is always surrounded by a magnetic field. As in the case of
a magnetic field of a permanent magnet, a magnetic needle or compass placed at
a point in the magnetic field of a current carrying conductor will rest in the
direction of the field acting at that point and not in the N-S direction. The
phenomenon is known as electromagnetism.
1. Magnetic
Field of a Straight Conductor. The magnetic field of a straight
current carrying conductor is composed of concentric magnetic lines of forces,
see Fig. 6.5. The centre of these lines of force lies on the axis of the
conductor. The above stated magnetic field is produced on each and every point
along the axis of conductor.
 |
| Fig 6.5 Magnetic Field of a Straight Conductor. |
The
following rules are applied for the determination of the direction of its
magnetic field:
(i) Cork Screw
Rule.
If a cork screw is rotated straight conductor such a way that its tip forwards
in the direction of current flowing through the conductor, the direction of rotation
of its handle will represent the direction of magnetic lines of force.
(ii) Right Hand Rule. If a current
carrying conductor is held by the right fist in such a way that the thumb
points to the direction of current flowing through the conductor, the fingers
will then represent the direction of magnetic lines of force.
2. Magnetic
Field of a Conductor Loop. The magnetic field of a current carrying
conductor loop is also composed of concentric, magnetic lines of force, but in
this case the lines of force get condensed inside the loop and rarified outside
the loop. The direction s of lines of force is determined by applying Right
Hand Rule.
3. Magnetic Field of a Solenoid.
The magnetic field of a current carrying solenoid is identical to the magnetic
field of a bar magnet. One end of the coil acts as north-pole and the other as
south-pole. The following
rules are applied for the determination of polarity of its ends:
(i) End Rule. Look at any
one end of the solenoid. If the flow of current is in clockwise direction then
the end under observation is south-pole and in the contrary case the same is
north-pole.
(ii) Helix
Rule.
Hold the coil with your right hand in such a way that the fingers represent the
direction of magnetic lines of force then the thumb will point to the
north-pole.
6.10. PERMEABILITY
The
magnetic flux density (B) produced
by a magnetising force (H) in a
substance in comparison to that produced in air or vacuum is called the permeability of that substance. It is
denoted by Greek letter µ (mu)
where
B = magnetic flux
density, Wb/m2.
H = magnetising
force, N/Wb.
Magnetic substance can be classified
into following three classes on the basis of permeability :
1.
Ferro
magnetic
2.
Para
magnetic
3.
Dia
magnetic
1. Ferro
magnetic. The
substances which get magnetised, when they are kept in a strong magnetic field,
are called ferro magnetic. The permeability of such substance in much greater
than unity (upto 1000). The examples of such substance are – iron, steel,
nickel, cobalt etc.
Ferrite. A core made
of iron oxide is called a ferrite. It is a ferro-magnetic material which possesses
a high and definite permeability. It contains low hystsersis loss too. It is
used as a ‘core’ in the coils working at high frequencies. In addition to it,
Ferrites are used in ‘Switching’ and ‘Memory’ devices of computers.
2. Para
magnetic. The
substance which do not get magnetised but the intensity of magnetic flux
passing through them is increased when they are kept in a strong magnetic field,
are called para magnetic. The permeability of such substances is little greater
than unity. The examples of such substances are – copper, aluminum, silver,
gold etc.
3. Dia
magnetic.
The substances which reduce the intensity of magnetic flux passing through
them, when they are kept in a strong magnetic field, are called dia magnetic.
The permeability of such substances is lesser than unity. The examples of such
substances are – antimony, bismuth.
6.11. MAGNETIC INDUCTION
When a magnet is brought near to an
iron bar or when iron bar is brought near to a magnet, a magnetism is produced
in the iron bar. The phenomenon is known as magnetic induction. Actually,
before attracting between unlike poles, magnet attracts the iron bar. The
magnet need not to touch the iron bar for magnetic induction.
In various electrical measuring
instruments, soft iron pole pieces are used along with bar magnets in order to
give the desired shape to the magnet used, such pole pieces work on the
principle of magnetic induction.
6.12 INTENSITY OF MAGNETIC FIELD
The force acting on a unit pole
placed in a magnetic field (attractive or repulsive force) is called the
intensity of magnetic field. It is denoted by letter H and its unit is Wb/m.
[Note: Unit Pole. If two poles are placed at one metre apart in
vacuum and they exert a force of one newton on each other, each of them is
known as a unit pole. ]
The intensity of magnetic field is
also known as magnetic strength. The same is called magnetising force in
connection with electromagnets. The its unit is ampere turns/metre (A-T/m).
6.13. INTENSITY OF MAGNETISATION
Under the influence of a magnetising
force, the pole strength developed in a magnetic material per unit area is a
called its intensity of magnetisation. It is denoted by letter I.
Where,
I = intensity of magnetisation, Wb/m2
m = pole strength developed
in a bar, Wb
A = cross-sectional area of
the bar, m2.
6.14. MAGNETIC SUSCEPTIBILITY
For a magnetic material, the ratio
of the intensity of magnetisation, I and magnetising force, H is called
magnetic susceptibility. It is denoted by letter K.
Where, K = magnetic
susceptibility, henrys/m
I = intensity of
magnetisation, Wh/m
H = magnetisation
force, A-T/m.
6.15 MAGNETO MOTIVE FORCE
The
complete path of a line of force from north-pole to south-pole outside the
magnet and from pole to north-pole inside the magnet is called ma, nelic
circuit. The force responsible for producing magnetic flux in the magnetic
circuit is called magneto motive force (M.M.F).
Like
resistance in electrical circuits, the opposition for the production of
magnetic flux in a magnetic circuit is called its reluctance. Its symbol is R. Thus
where,
R = reluctance,
A-T/Wb
M.M.F. = magneto
motive force, A-T
=
magnetic flux, Wb
[Note. The above formula is
identical to R = V/I used in electricity. Here M.M.F. represents V and
represents I.
6.16. RETENTIVITY
A
small quantity of magnetism is retained in magnetic materials even after
withdrawal of the magnetising force which is known as residual magnetism. The
property of a magnetic material of retaining the residual magnetism in it is
known as retentivity. The selection of materials for making permanent magnets
is done on the basis of their retentivity. The retentivity of steel is much
more than that of mild-steel.
6.17. HYSTERESIS
When
a magnetic material is placed in the magnetic field of a current carrying coil
then the magnitude of magnetism (B) is produced in the material depends on the
amount of current (I) (i,e., coercive
Al magnetising force, H). The magnitude of magnetism increases on increasing
the current to a certain amount from zero. But, the magnitude of magnetism does
not reduce to zero even after reducing the magnetising current to zero. Some
magnetism is retained in the material due to residual magnetism.
 |
| Fig 6.12 Hysteresis Curve |
Now,
if the direction of current is reversed then before an increase in the
magnetism in opposite direction, it is necessary to counterbalance the residual
magnetism. In this way, curve OABCDEF is obtained for one complete cycle of the
current, see Fig. 6.12. It is evident from the curve that — The magnetism produced in the material is
lagging behind the amount of current or the material has hysteresis in it.
The
curves obtained for different materials are different in shape. The study of
its shape decides the utility of the material in making magnets. The material
whose hysteresis curve has longest segment OC2 (see Fig- 6.12) will
be more suitable for making magnets. In this way, the hysteresis is the
important property of magnetic materials.
6.18. COERCIVITY
The
amount of magnetising force required to counterbalance the residual magnetism
of a magnetic material is called its coercivity. The segment OR2 in
Fig. 6.12 represents coercivity.
6.19. ELECTROMAGNET
1. Introduction. A soft iron
bar placed inside a coil can be converted into a magnet provided D.C. is passed
through the coil, It has following the magnet formed in this way is called an
electromagnet, see Fig. 6.13 It has following characteristics:
 |
| Fig 6.13 Electromagnet |
2.
Characteristics
(i)
Its magnetic strength can be varied by changing the magnitude of current.
(ii)
Its polarity can be changed by changing the direction of the current passed
through its coil.
(iii)
Its magnetic strength can be made thousands time greater in comparison to
permanent magnets.
(iv)In
many equipments and machines only electromagnets can be possibly used, e.g.,
magnetic lift or crane, doctor's instruments used for the removal of an iron
particle from a human eye.
3. Uses.
Electromagnets are used in electric bells, relays, motors, generators,
electrical measuring instruments etc.
6.20. ELECTRIC BELL
1.
Introduction.
A bell employing an electromagnet is called an electric bell. It may be made to
operate on D.C., A.C. or D.C./A.C. An A.C. operated buzzer also works like an
electric bell.
2.
Construction.
The construction of an electric bell is shown in Fig. 6.14. It consists of an
electromagnet, bell, reed, screw and a push button.
 |
| Fig 6.14 Electric Bell |
The
screw and the reed are adjusted in such a manner that the electric circuit is
completed through them.
3. Working. The flow of
current is started in the coil's circuit on pressing the push button. The electromagnet
gets energised and attracts the reed. The ball attached to the reed strikes the
bell. Meanwhile, the contact between reed and the screw gets open and the reed
is returned back to its normal position due to de-energisation of the
electromagnet. As the screw touches the reed, the flow of current starts again
in the circuit and the above action, is repeated. The whole action repeats at
the rate of 25 to 50 times per second. In this way, the iron ball strikes the
bell 25 to 50 times per second and produces a loud sound.
4. Uses. It is used as
a in houses, offices and hotels etc. and as an alarming device in wireless
equipments.
6.21 ELECTRO MAGNETIC RELAY
1.
Introduction.
An electro magnetic relay or relay is an electrormagnetic device which can make
`on' or 'off' a number of circuits at a time. Normally, it works on 3 to 24
volts D.C. but a relay for working at high voltage can be made as per requirement.
Relays working at low voltages are called miniature relays.
2.
Construction.
It consists of mainly an electromagnet, reed and NO and NC contacts (normally open
and normally contacts) see Fig. 6.15.
 |
| Fig 6.15 Magnetic relay |
3. Working. On applying a
D.C. signal to the electromagnet, it gets energised and attracts the reed. The
NO contacts attached to the reed put the desired circuit to 'on’ and the NC
contacts put the desired circuit to ‘off’.
4. Characteristics
(i)
By using a relay, a control circuit can be operated at low voltage thus causing
a reduction in electric consumption and possibilities of an electric shock to
the operator.
(ii)
It can work at a pre-decided voltage and current and not at less than that
(iii)
Remote control of an equipment or machinery is possible by using-
transistorised circuit with a relay.
(iv)
NO and NC contacts of a relay are made of silver or platinum which establish a
good electrical contact for an electric circuit.
5. Uses. Relays are
used in receivers, transmitters, refrigerators voltage stabilizers, telephone,
remote control circuits etc.
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