BENE 1113
PRINCIPLES OF ELECTRICAL AND
ELECTRONICS
CHAPTER 2:
MAGNETIC AND
ELECTROMAGNETIC
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TOPICS COVERED:
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MAGNETIC UNITS
POLES OF MAGNET
MAGNETIC FIELD
ELECTROMAGNETISM
ELECTROMAGNETIC INDUCTION
RIGHT HAND RULE
FLEMING LEFT HAND RULE
FARADAY’S LAW
LENZ’S LAW
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INTRODUCTION
• The first experience with magnetism occurred when
pieces of stone were found to have the ability to attract
iron or similar materials. These stones were called
magnets.
• Magnetic forces are refer to the force that acts between
magnets and magnetic materials:
-There are two types of magnetic poles,
conventionally called North and South.
-Like poles repel, and opposite poles attract.
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INTRODUCTION
• Magnetic fields are described by drawing flux lines that
represent the magnetic field.
• Each of magnetic flux line travels from the north pole to
the south pole thro space.
• The line returns to the north pole thro the magnet itself.
Where lines are close
together, the flux
density is higher.
Where lines are further
apart, the flux density is
lower.
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INTRODUCTION
• Magnetic flux lines are invisible, but the effects can be
visualized with iron filings sprinkled in a magnetic field.
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LAWS OF MAGNETISM
1. Like poles repel each other
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LAWS OF MAGNETISM
2. Unlike poles attract each other
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LAWS OF MAGNETISM
• 3. The attractive/repelling force increases as the
distance between the magnet decreases.
– To demonstrate this law, one bar magnet is placed on the table.
By slowly sliding one pole of a second bar magnet toward the
opposite pole of the first bar that resting on the table.
– When the two magnets become closer enough, the magnet on
the table will be drawn to the second magnet.
– When the attractive force gains enough strength to overcome the
force of friction that holding the first magnet to the table, the first
magnet slides toward the second.
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Magnetic attraction and
repulsion
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Lines of force tend to take the path of least magnetic
resistance. This fact introduces two features:
- The lines tend to take the shortest possible
path between the north and south poles when
this path is through materials that cannot be
magnetized.
- When a material that can be magnetized is
placed within the magnetic field, the path of
some of the lines of force is distorted in order
to pass through this material.
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NON MAGNETIC MATERIALS
• Materials that have no obvious magnetic
properties.
• Magnetic fields of the individual atoms
are randomly aligned and thus tend to
cancel out.
• Non magnetic material such as paper,
glass, wood and plastic.
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PERMANENT MAGNET
• Magnets made of steel alloys hold their
magnetism for a long period of time. That is
called permanent magnet.
• Magnetic fields of the individual atom are
aligned in one preferred direction, giving rise
to a net magnetic field.
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MAGNETIC MATERIALS
• Materials respond differently to the force of a magnetic
field.
– A magnet strongly attract Ferromagnetic materials
– A magnet weakly attract Paramagnetic materials
– A magnet weakly repel Diamagnetic materials
• The orientation of the spin of the electrons in an atom,
the orientation of the atoms in a molecule or all ability of
domains of atoms or molecules to line up are the factors
that how a material responds to a magnetic field.
•  the responds to magnetic field,  that substance
become magnetized (to become a magnet)
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MAGNETIC
MATERIAL
MAGNETIC MATERIALS
• Ferromagnetic Material – A material easy to
magnetize. (i.e., Iron Steel, Cobalt, Perm-alloy,
and Alnico)
• Paramagnetic Material- A material that can be
slightly magnetized.
• Diamagnetic Material – A material that is difficult
to magnetize.
FERROMAGNETIC
MATERIALS
• There are domains in which the magnetic
fields of the individual atoms align, but the
orientation of the magnetic fields of the
domains is random.
• This offer no net magnetic field.
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• A useful property of ferromagnets is
that when an external magnetic
field is applied to them, the
magnetic fields of the individual
domains tend to line up in the
direction of this external field, due
to the nature of the magnetic forces.
• This cause the external magnetic
field to be enhanced.
• Ferromagnet material such as iron,
nickel and cobalt.
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MAGNETIC MATERIALS
• Paramagnetic materials
– Weakly attracted to magnetic field.
– Aluminum and copper
– These materials can be a magnet, but their attractive
force can only be measured with sensitive
instruments.
– The force of a ferromagnetic magnet is about a million
times that of a force made with a paramagnetic force.
– Sometimes, this materials are typically considered as
non magnetic materials.
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MAGNETIC MATERIALS
• Diamagnetic material
– Means that when they are located at the
strong magnetic field, they induce a weak
magnetic force in the opposite direction.
– In other words, they weakly repel a strong
magnet.
– Bismuth and carbon graphite are the
strongest diamagnetic, followed by mercury,
silver, water, diamonds, wood and living
tissues.
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Magnetic Flux
• The unit of magnetic flux is weber (Wb)
• One weber equals 1x108 lines of magnetic
flux.
• The weber is a very large unit thus
microweber (μWb) is used.
• 1 μWb equals 100 lines of magnetic flux.
MAGNETIC UNITS
1. Flux Density:
• Is the amount of flux per unit area
• Symbolized by B
• Unit: tesla (T) or Wb/ m2
• 1 Wb/m2 = tesla

B
A
where
 is the flux (group of 1x108 lines of force)
A is the cross-sectional area in m2
The many invisible lines of magnetic
force surrounding a magnet are called
the magnetic flux. The strength of a
magnetic field can be determined by
the flux density.
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Example 1
• Compare the flux and the flux density in the two
magnetic cores shown in Figure below. The diagram
represents the cross section of a magnetized material.
Assume that each dot represents 100 lines or 1 μWb.
Example 2
• What happens to the flux density if the same flux
shown in the first figure is in a core of 5.0cm x
5.0cm?
• If the flux density in a certain magnetic material
is 0.23T and the area of the material is 0.38in2,
what is the flux through the material?
• Calculate B if A = 0.05 in2 and Φ = 1000μWb
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ELECTROMAGNETISM
• Electromagnetism is related to the magnetic field
generated around a conductor when current is passed
through it.
• When electricity passed through a wire, a magnetic field
is created around the wire in a specific direction. The
magnetic field disappears when the current flow stop
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Visible affects of an electromagnetic field.
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ELECTROMAGNETISM
Right Hand Rule
• To find the direction of the magnetic
field.
• The field strength is not uniform
throughout the magnetic field; the
further away from the conductor, the
weaker the field intensity.
• The magnetic field strength and flux
density can be increased by increasing
the no of turn or current or adding an
iron core
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Electromagnetic Properties
1. Permeability (μ):
– The ability of a material to establish a magnetic field
– The higher the permeability, the more easily a
magnetic field can be established.
– The permeability of a vacuum (μ0) is 4πX10-7
Wb/At·m (Webers/ampere-turn.meter).
– The relative permeability (μr) of a material is the ratio
of its absolute permeability to the permeability of a
vacuum.

r 
0
Unit is dimensionless
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Electromagnetic Properties
2.Reluctance ( ):
– The opposition to the establishment of a
magnetic field in a material.
– The value of reluctance is directly proportional
to the length (l) of the magnetic path and
inversely proportional to the permeability (μ)
and to the cross-sectional area (A) of the
material, as expressed by the following
equation:
l

A
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Unit is At/Wb
Example 3
• Calculate the reluctance of a torus (a
doughnut shaped core) made of low
carbon steel. The inner radius of the torus
is 1.75cm and the outer radius of the
torus is 2.25 cm. Assume the permeability
of low carbon steel is 2 x 10-4 Wb/At.m.
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Example 4
• Mild steel has a relative permeability of 800.
Calculate the reluctance of a mild steel core that
has a length of 10 cm and has a cross section of
1.0cm x 1.2 cm.
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Example 5
• A square magnetic core has a dimension
of 8cm x 8cm x 3cm(its thickness) with
0.5cm air gap at one of its side. It has a
relative permeability of 6000.The cross
section is 2cm by 3cm.Calculate the
reluctance of the core and the gap.
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Electromagnetic Properties
3. Magnetomotive Force (mmf):
– Current in a conductor produces a magnetic field.
The cause of the magnetic field is called the
magnetomotive force (mmf).
– The unit of mmf is ampere-turn (At).
– The formula for mmf is
Fm  NI
where Fm is the magnetomotive force, N is the number
of turns of wire and I is the current in amperes.
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Electromagnetic Properties
Figure above illustrates that a
number of turns of wire
carrying a current around a
magnetic material creates a
force that sets up flux lines
through the magnetic path.
The amount of flux depends on
the magnitude of the mmf and
on the reluctance of the
material as expressed by:
This equation is known
as Ohm’s Law for
magnetic circuits since
Φ is analogous to
current, the mmf (Fm) is
analogous to voltage,
and reluctance (R) is
analogous to resistance
Fm


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OHM’S LAW APPLIED TO
MAGNETIC CIRCUITS
•
Ohm’s Law, when applied to electrical circuits, gave the formula:
I=V/R
(2.1)
where: I = current flow in amperes
V = electromotive force/voltage
R = current flow opposition/resistance
•
A similar version can be applied to magnetic circuits, that is:
Fm


where:
(2.2)
 = magnetic flow of lines of force (webers)
Fm = magnetomitive force (ampere-turns)
R = magnetic reluctance (NI/ Wb)
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MAGNETIC UNITS
•
•
•
From Eq. 2.2, it can be seen that increasing either the current or
turns of a solenoid will increase the flux.
A decreases in R would also increases the flux.
Ohm’s Law for magnetic circuits may be given in three
variations to suit particular problems:

NI
( webers)
Rm
NI
( At / Wb )

NI  Rm ( Am pere turns)
Rm 
•
For Electrical Equivalent;
V
R
V
R 
I
V  IR
I 
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MAGNETIC UNIT
1. Magnetizing Force (Magnetic Field Intensity), H
• The m.m.f. required to magnetize a unit length of a
magnetic path.
• The unit is expressed in ampere-turns per metre (At
/ m) and the symbol is H.
Fm NI
H

l
l
where
H = magnetizing force or magnetic field intensity
NI = Ampere turns
l = length between poles of the coil
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RELATIONSHIP BETWEEN
B&H
• When the magnetomotive force, Fm increases, the
magnetizing force, H increases.
Fm
• At the same time, the flux increases since  

• The flux density also increases as

B
A
• In other word, B is also proportional to H
BH
B / H = Constant = 
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RELATIONSHIP BETWEEN
B&H
•
The ratio B/H in a material is always constant and is
equal to the absolute permeability,  of the material.
(= o r)
•
Obviously,
B = orH (in medium)
B = oH (in air)
RECALL:
μ = relative ability
of substance to conduct
magnetic lines of force
as compared with air.
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Example 5
• How much flux is established in the
magnetic path of Figure below if the
reluctance of the material is 2.8 x 105
At/Wb?
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Figure 10-12
Example 6
• There is 85mA of current through a coil
with 500 turns.
– What is mmf?
– What is the reluctance of the circuit in the flux
is 500 μWb?
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EXERCISE
1.
An electromagnet has 600 turns and the total
reluctance of the magnetic core is 800 units. Calculate
the flux produced when 10 A flows through the coil.
2.
A contractor coil has 7200 turns, which are wound on
iron core, rectangular in section, and having crosssectional dimension of 20mm x 30 mm. If the flux
density in the magnetic circuit is 1.2 Tesla, find the
reluctance of the magnetic core. The current drawn is
0.1 A.
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POLES OF A MAGNET
•
The points about the poles of a magnet:
(i) The poles of a magnet cannot be separated.
If a bar magnet is broken into two parts, each part
will be a complete magnet with poles at its ends. No
matter how many times a magnet is broken, each
piece will contain n-pole at one end and s-pole at
the other.
(ii) The two poles of a magnet are equal in
strength
- The force between two magnetic poles is directly
proportional to the product of their poles strengths
and inversely proportional to the square of
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distance between them
ELECTROMAGNETIC INDUCTION
• When there is a relative motion between a conductor
and a magnetic field, a voltage is produced across the
conductor.
• This principal is known as electromagnetic induction
and the resulting voltage is induced voltage.
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ELECTROMAGNETIC INDUCTION
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ELECTROMAGNETIC INDUCTION
• When a current carrying conductor is placed at right
angles to a magnetic field, it experiences a mechanical
force F, given by;
F  BIl
where: B = flux density in wb/m2
I = current through conductor in ampere
l = length of conductor metres
• The direction of this force can be found by Fleming’s
Left-hand rule.
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ELECTROMAGNETIC INDUCTION
Faraday’s Law
1.
The amount of voltage induced in a coil is directly
proportional to the rate of change of the
magnetic field with respect to the coil.
(a)
(b)
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2.
The amount of voltage induced in a coil is directly
proportional to the number of turns of wire in the
coil (N).
(a)
(b)
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ELECTROMAGNETIC INDUCTION
Faraday’s Law
Faraday’s Law state that:
The voltage induced across a coil of wire equals the
number of turns in the coil times the rate of
change of the magnetic flux.
In mathematic,
vind
 d 
 N

 dt 
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ELECTROMAGNETIC INDUCTION
Lenz’s Law
•
Lenz’s Law is used to find the direction of induced
e.m.f and hence current in a conductor or coil.
•
Lenz’s Law is stated as follows:
The direction of the induced current is such as to
oppose the change causing it.
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ELECTROMAGNETIC INDUCTION
• LENZ’S LAW:
• The diagram shows the north
pole of a bar magnet
approaching a solenoid.
• According to Lenz's law, the
current which is generated in the
coil must opposes the
approaching magnetic field.
• This is achieved if the direction
of the induced current creates a
north pole at the end of the
solenoid closest to the
approaching magnet, as the
induced north pole tends to repel
the approaching north pole.
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ELECTROMAGNETIC INDUCTION
Lenz’s Law
• The diagram shows the north pole
of a bar magnet withdrawing from
a solenoid.
• According to Lenz's law, the
current which is generated in the
coil must oppose the departing
magnetic field.
• This is achieved if the direction of
the induced current creates a
south pole at the end of the
solenoid closest to the departing
magnet, as the induced south pole
tends to attract the departing
north pole.
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THANK
YOU
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