NORTH
Pole
N
MAGNETIC
FIELD
MAGNET
SOUTH
Pole
S
Paper
N
Needle
S
Thumb Nail
Copper
Cable
MAGNETIC CIRCUIT,
ELECTROMAGNETISM AND
ELECTROMAGNETIC
INDUCTION
The end of lesson, students
should be ;







Understand magnetism
Understand the composite series magnetic
circuit
Understand the electrical and magnetic
quantities
Understand hysteresis
Understand electromagnetism
Determine the magnetic field direction.
Understand electromagnetic induction
iNTRoDUctION
MAGNET is
the material that have two poles NORTH
and SOUTH
NORTH
Pole
N
SOUTH
Pole
S
iNTRoDUctION
MAGNET can be define as
Needle
Material that can attract piece of
iron or metal
N
S
Thumb Nail
iNTRoDUctION
MATERIAL that ATTRACTED by the
MAGNET is known as
Needle
MAGNETIC SUBSTANCES
S
Thumb Nail
iNTRoDUctION
The ABILITY to ATTRACT the
MAGNETIC SUBSTANCES is known
Needle
as
MAGNETISM
S
Thumb Nail
iNTRoDUctION
MAGNETIC FIELD is
the force around the MAGNET which
can attract any MAGNETIC MATERIAL
around it.
FLUX MAGNET
is the line
around the MAGNET bar which form
MAGNETIC FIELD.
N
S
TYpEs of MAGNET
There are 2 types of
 PURE
MAGNET
MAGNET
 MANUFACTURE MAGNET
PURE MAGNET
Known as MAGNET
STONE
The stone ORIGINALY have the
NATURAL MAGNETIC
Basically the stone is found in the form
of IRON ORE
MANUFACTURE
MAGNET
There are 2 types of
MANUFACTURE MAGNET
 PERMANENT
MAGNET
 TEMPORARY MAGNET
PERMANENT MAGNET
The ABILITY of the MAGNET to
kept its MAGNETISM
The basic shape of PERMANENT
MAGNET
 U shape

horseshoe
 ROD
 Cylinder
 BAR
PERMANENT MAGNET
U shape
Horseshoe
Rod
Cylinder
Bar
PERMANENT MAGNET
 Permanent
magnet can be obtained
by:
◦ naturally or magnetic induction
( metal rub against natural
magnet)
◦ placing a magnet into the coil and
then supplied with a high
electrical current.
PERMANENT MAGNET
Permanent magnet used in small devices
such as:
speakers
meter
compass
TEMPORARY MAGNET
BECOME MAGNET only when
there is CURRENT SUPPLY to the
metal
It has magnetic properties when subjected
to magnetic force and it will be lost when
power is removed.
TEMPORARY MAGNET

Example :
relay
electric bells
CHARACTERISTICS OF MAGNETIC
FORCE LINES (FLUX).
 Magnetic
flux lines have direction
and pole.
 The direction of movement outside of
the magnetic field lines is from north
to south.
CHARACTERISTICS OF MAGNETIC
FORCE LINES (FLUX).
The strongest magnetic field are at the
magnetic poles .
DIFFERENT POLES ATTRACT each other
N
S
N
S
SAME MAGNETIC POLES will REPEL each
other
N
S
S
N
CHARACTERISTICS OF MAGNETIC
FORCE LINES (FLUX).
FLUX form a complete loop and never
intersect with each other.
 FLUX will try to form a loop as small as
possible.

N
S
MAGNETIC QUANTITY
CHARACTERISTICS
Magnetic Flux



Magnetic flux is the amount of
magnetic field produced by a
magnetic source.
The symbol for magnetic flux is .
The unit for magnetic flux is the
weber, Wb.
MAGNETIC QUANTITY
CHARACTERISTICS
Magnet Flux density



The symbol for magnetic flux
density is B.
The unit is tesla, T
the unit for area A is m2 where
1 T = 1 Wb/m.
MAGNETIC QUANTITY
CHARACTERISTICS
Magnet Flux density

Magnetic flux density is the
amount of flux passing
through a defined area
that is perpendicular to
the direction of flux
MAGNETIC QUANTITY
CHARACTERISTICS
Magnetic flux density
=
magnetic flux
area
Φ
B
A
Tesla
MAGNETIC QUANTITY
CHARACTERISTICS
Example 3
Flux, Φ
Area, A
A magnetic pole face has
rectangular section having
dimensions 200mm by 100mm. If
the total flux emerging from the
pole is 150Wb, calculate the flux
density.
B?
Φ
B
A
MAGNETIC QUANTITY
CHARACTERISTICS
Solution 3
Magnetic flux,  = 150 Wb = 150 x 10-6 Wb
Cross sectional area, A
= 200mm x 100mm
= 20 000 x 10-6 m2
Flux density,
6
Φ
150  10
B 
6
A 20000  10
= 7.5 mT
MAGNETOMOTIVE FORCE (MMF)
The force which creates the magnetic flux in a
magnetic circuit is called magnetomotive force
(mmf)
- The mmf is produced when a current passes
through a coil of wire. The mmf is the product of
the number of turns(N) and current (I) through
the coil.
Formula , Fm = N x I
Unit = Ampere Turns (A.T)
MAGNETIC FIELD STRENGTH,H
(MAGNETISING FORCE)

Defined as magnetomotive force, Fm per
metre length of measurement being
ampere-turn per metre.
number of turns
magnetomotive force
Current
Fm
NI
H 

l
l
average length of magnetic circuit
MAGNETIC FIELD STRENGTH,H
(MAGNETISING FORCE)
Example 1
Current, I
A current of 500mA is passed
through a 600 turn coil wound of a
toroid of mean diameter 10cm.
Turn, N Calculate the magnetic field
strength.
Fm NI Diameter, d
H

l
l
H?
MAGNETIC FIELD STRENGTH,H
(MAGNETISING FORCE)
Solution 1
I =
N=
l =
0.5A
600
 x 10 x 10-2m
NI
ampereturn / metre
l
600  0.5
H 
0.3142
H  954.81AT / m
H 
MAGNETIC FIELD STRENGTH,H
(MAGNETISING FORCE)
Example 2
An iron ring has a cross-sectional
area of 400 mm2. The coil resistance
is 474 Ω and the supply voltage is
240 V and a mean diameter of 25
cm. it is wound with 500 turns.
Calculate the magnetic field
strength, H
MAGNETIC FIELD STRENGTH,H
(MAGNETISING FORCE)
Solution 2
I = V/ R = 240 / 474 = 0.506 A
l = π D = π (25 x10-2) = 0.7854 m
H=
H=
NI
l
500  0.506
0.7854
H= 322.13 AT/m
PERMEABILITY
 For
air, or any other nonmagnetic medium, the ratio of
magnetic flux density to
magnetic field strength is
constant ,
 This constant is called the
permeability of free space and
is equal to 4 x 10-7 H/m.
B

H
µ0
PERMEABILITY

For any other non-magnetic medium, the
ratio

For all media other than free space
  r
B
 0 r
H
PERMEABILITY
r is the relative permeability and is
defined as
flux density in material
r 
flux density in vacuum
r varies with the type of magnetic
material.
PERMEABILITY
r for a vacuum is 1 is called the absolute
permeability.
The approximate range of values of
relative permeability r for some
common magnetic materials are :
 Cast iron
r = 100 – 250
 Mild steel
r = 200 – 800
 Cast steel
r = 300 – 900
PERMEABILITY
Flux density,
B
Example 4
A flux density of 1.2 T is
produced in a piece of cast
steel by a magnetizing force
of 1250 A/m. Find the
relative permeability of the
steel under these conditions.
B  0 r H
H
µr?
PERMEABILITY
Solution 4
B  0 r H
B
1.2
r 

7
0 H (4 10 )(1250)
 764
RELUCTANCE
Reluctance,S is the magnetic resistance of a
magnetic circuit to presence of magnetic flux.
Reluctance,
The unit for reluctance is 1/H or H-1 or A-T/Wb
Fm
Hl


S 




BA
(B / H ) A
0  r A
RELUCTANCE
S?
Example 5
Determine the reluctance of a piece of
metal of length 150mm and cross
sectional area is 1800mm2when the
relative permeability is 4 000. Find also
the absolute permeability of the metal.
Length, l
µ?
µr
RELUCTANCE
Solution 5
Reluctance,
S 

0 r A
150  10 3
=
(4  10 7 )( 4000)(1800  10 6 )
= 16 579 H-1
Absolute permeability,
=
  0 r
(4  10 )( 4000)
7
= 5.027 x 10-3 H/m
ELECTROMAGNET


Is a magnetic iron
core produced when
the current flowing
through the coil.
Thus, the magnetic
field can be produced
when there is a
current flow through a
conductor.
The direction of the magnetic field
can be determined using the
method:
1. Right Hand Grip Rules
2. Maxwell's screw Law.
3. Compass
Three rules may be used to indicate the
direction of the current and the flux
produced by current carrying
conductor.
Right Hand Grip Rule
 is
a physics
principle applied
to electric current
passing through a
solenoid,
resulting in a
magnetic field.
Right Hand Grip Rule
 When
you wrap your right hand
around the solenoid
your thumb points
in the direction of
the magnetic
north pole
your fingers in
the direction of
the conventional
current
Right Hand Grip Rule
 It
can also be applied to
electricity passing through a
straight wire
the thumb points in the
direction of the
conventional current
(from +ve to -ve)
the fingers point in the
direction of the magnetic
lines of flux.
MAXWELL’S SCREW LAW

Another way to determine the direction of
the flux and current in a conductor is to
use Maxwell's screw rule.
MAXWELL’S SCREW
LAW

a right-handed screw
is turned so that it
moves forward in the
same direction as the
current, its direction
of rotation will give
the direction of the
magnetic field.
Electromagnetic Effect
Direction of Current
going INside
Solenoid
Direction of Magnetic
Flux around Solenoid
Direction of Magnetic
Flux around Solenoid
Right Hand Grip
Rule
Direction of Current
going OUTside
Solenoid
Electromagnetic Effect
Direction of Current
going OUTside
Solenoid
Direction of Current
going INside
Solenoid
Maxwell Screw Law
Same Direction
Direction of Magnetic
Flux around Solenoid
Different Direction
Direction of Magnetic
Flux around Solenoid
Electromagnetic Effect
Factors that influence the strength of
the magnetic field of a solenoid
 The number of turns
 The value of current flow
 Types of conductors to produce coil
 The thickness of the conductor
ELECTROMAGNETIC INDUCTION



Definition : When a conductor is
moved across a magnetic field so
as to cut through the flux, an
electromagnetic force (emf) is
produced in the conductor.
This effect is known as
electromagnetic induction.
The effect of electromagnetic
induction will cause induced
current.
ELECTROMAGNETIC INDUCTION
2 laws of electromagnetic induction:
i. Faraday’s law
ii.Lenz’z Law
Faraday’s law
It is a relative movement of the magnetic
flux and the conductor then causes an
emf and thus the current to be induced in
the conductor.
 Induced emf on the conductor could be
produced by 2 methods
◦ flux cuts conductor or
◦ conductor cuts flux.

Faraday’s law
Faraday’s First Law : Flux cuts conductor
When the magnet is moved towards the coil,
a deflection is noted on the galvanometer
showing that a current has been produced in
the coil.
Faraday’s law
Faraday’s Second Law :Conductor cuts flux
When the conductor is moved through a
magnetic field . An emf is induced in the
conductor and thus a source of emf is created
between the ends of the conductor.
Faraday’s law
This induced electromagnetic field is given
by E = Blv volts
B=flux density, T
l =length of the conductor in the magnetic
field, m
v =conductor velocity, m/s
If the conductor moves at the angle  to
the magnetic field, then
E = Blv sin volts
Faraday’s law
Example
A conductor 300mm long moves at a
uniform speed of 4m/s at right-angles to
a uniform magnetic field of flux density
1.25T.
Determine the current flowing in the
conductor when :
a. its ends are open-circuited
b. its ends are connected to a load of 20 
resistance.
Faraday’s law
Solution
a. If the ends of the conductor are open
circuited no current will flow .
Faraday’s law
Solution
b. E.m.f. can only produce a current if there is a
closed circuit. When a conductor moves in a
magnetic field it will have an e.m.f. induced.
Induced e.m.f. , E = Blv
=(1.25)(0.3)(4)
= 1.5 v
From Ohm’s law
E
I 
R
1.5
I 
20
I  75mA
Lenz’z law

The direction of an induced emf is always
such that it tends to set up a current
opposing the motion or the change of flux
responsible for inducing that emf
Formula
MAGNETOMOTIVE FORCE (MMF), Fm = N x I
MAGNETIC FIELD STRENGTH
MAGNETIC FLUX DENSITY
PERMEABILITY
Fm NI
H

l
l
Φ
B
A
B
   0  r
H
S

0 r A
Fm
Hl





RELUCTANCE S 

BA
(B / H ) A
0  r A
Composite magnetic circuit
A series magnetic circuit that has parts of
different dimensions and material is called
composite magnetic circuit.
Each part will have its own reluctance. Total
reluctance is equal to the sum of reluctances
of individual parts.
Total reluctance
Comparison between magnetic
and electric circuit
Similarities & dissimilarities between
magnetic circuit and electric circuit
Similarities & dissimilarities between
magnetic circuit and electric circuit
Hysterisis and hysterisis loss
Figure 7.6