UNIT 4: ELECTROMAGNETIC INDUCTION

Magnet is a material or object that produces a magnetic field. This magnetic field is invisible
but is responsible for the most notable property of a magnet: a force that pulls on
other ferromagnetic materials, such as iron, and attracts or repels other magnets
A permanent magnet is an object made from a material that is magnetized and creates its own
persistent magnetic field.
MAGANETIS
Magnetism is one aspect of the combined electromagnetic force. It refers to physical phenomena
arising from the force caused by magnets, objects that produce fields that attract or repel other
objects.
A magnetic field exerts a force on particles in the field due to the Lorentz force. The motion of
electrically charged particles gives rise to magnetism. The force acting on an electrically charged
particle in a magnetic field depends on the magnitude of the charge, the velocity of the particle, and
the strength of the magnetic field.
All materials experience magnetism, some more strongly than others. Permanent magnets, made
from materials such as iron, experience the strongest effects, known as ferromagnetism. With rare
exception, this is the only form of magnetism strong enough to be felt by people.
OPPOSITES ATTRACT
Magnetic fields are generated by rotating electric charges. Electrons all have a property of angular
momentum, or spin. Most electrons tend to form pairs in which one of them is “spin up” and the
other is “spin down,” in accordance with the Pauli Exclusion Principle, which states that two
electrons cannot occupy the same energy state at the same time. In this case, their magnetic fields
are in opposite directions, so they cancel each other. However, some atoms contain one or more
unpaired electrons whose spin can produce a directional magnetic field. The direction of their spin
determines the direction of the magnetic field. When a significant majority of unpaired electrons are
aligned with their spins in the same direction, they combine to produce a magnetic field that is
strong enough to be felt on a macroscopic scale.
Magnetic field sources are dipolar, having a north and south magnetic pole. Opposite poles (N and
S) attract, and like poles (N and N, or S and S) repel. This creates a toroidal, or doughnut-shaped
field, as the direction of the field propagates outward from the North Pole and enters through the
South Pole.

The Earth itself is a giant magnet. The planet gets its magnetic field from circulating electric
currents within the molten metallic core. A compass points north because the small magnetic needle
in it is suspended so that it can spin freely inside its casing to align itself with the planet's magnetic
field. Paradoxically, what we call the Magnetic North Pole is actually a south magnetic pole because
it attracts the north magnetic poles of compass needles.

Ferromagnetism
If the alignment of unpaired electrons persists without the application of an external magnetic field
or electric current, it produces a permanent magnet. Permanent magnets are the result of
ferromagnetism. The prefix “Ferro” refers to iron because permanent magnetism was first observed
in a form of natural iron ore called magnetite, Fe3O4. Pieces of magnetite can be found scattered on
or near the surface of the earth, and occasionally, one will be magnetized. These naturally occurring
magnets are called lodestones. “We still are not certain as to their origin, but most scientists believe
that lodestone is magnetite that has been hit by lightning,” according to the University of Arizona.

People soon learned that they could magnetize an iron needle by stroking it with a lodestone,
causing a majority of the unpaired electrons in the needle to line up in one direction. According to
NASA, around A.D. 1000, the Chinese discovered that a magnet floating in a bowl of water always
lined up in the north-south direction. The magnetic compass thus became a tremendous aid to
navigation, particularly during the day and at night when the stars were hidden by clouds.

Other metals besides iron have been found to have ferromagnetic properties. These include nickel,
cobalt, and some rare earth metals such as samarium or neodymium which are used to make super-
strong Magnetizing ferromagnets.

Magnetizing ferromagnets
Ferromagnetic materials can be magnetized in the following ways:
1. Heating the object higher than its Curie temperature, allowing it to cool in a magnetic field
and hammering it as it cools. This is the most effective method and is similar to the
industrial processes used to create permanent magnets.
2. Placing the item in an external magnetic field will result in the item retaining some of the
magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials
aligned with the Earth's magnetic field that are subject to vibration (e.g., frame of a
conveyor) have been shown to acquire significant residual magnetism. Likewise, striking a
steel nail held by fingers in a N-S direction with a hammer will temporarily magnetize the
nail.
3. Stroking: An existing magnet is moved from one end of the item to the other repeatedly in
the same direction (single touch method) or two magnets are moved outwards from the
center of a third (double touch method).
 4. Electric Current: The magnetic field produced by passing an electric current through a coil
can get domains to line up. Once all of the domains are lined up, increasing the current will
not increase the magnetization.
Demagnetizing ferromagnets
Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:
1. Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of
the magnetic domains. This always removes all magnetization.
2. Placing the magnet in an alternating magnetic field with intensity above the materials
coercively and then either slowly drawing the magnet out or slowly decreasing the magnetic
field to zero. This is the principle used in commercial demagnetizers to demagnetize tools,
erase credit cards, hard disks, and degaussing coils used to demagnetize CRTs.
3. Some demagnetization or reverse magnetization will occur if any part of the magnet is
subjected to a reverse field above the magnetic material's coercively.
4. Demagnetization progressively occurs if the magnet is subjected to cyclic fields sufficient to
move the magnet away from the linear part on the second quadrant of the B-H curve of the
magnetic material (the demagnetization curve).
5. Hammering or jarring: mechanical disturbance tends to randomize the magnetic domains
and reduce magnetization of an object, but may cause unacceptable damage.
SOLENOID

Solenoid consists of a length of insulated wire coiled into a cylinder shape.

 Current in solenoid produces a stronger magnetic field inside the solenoid than outside.
The field lines in this region are parallel and closely spaced showing the field is highly
uniform in strength and direction.
 Field lines outside the solenoid are similar to that of a bar magnet, and it behaves in a
similar way – as if it had a north pole at one end and South Pole at the other end. Strength
of the field diminishes with distance from the solenoid.
 Strength of the magnetic field can be increased by:
1. Increasing the current in the coil.
2. Increasing the number of coils in the solenoid; and
3. Using a soft iron core within the solenoid.
 Reversing the direction of the current reverses the direction of the magnetic field.
 Right-hand rule can be used to find the direction of the magnetic field. In this case, point the
wrapped fingers (along the coil) in the direction of the conventional current. Then, the thumb
will point to the direction of magnetic field within the solenoid.

ELECTROMAGNETIC RELAY
Definition: Electromagnetic relays are those relay which operates on the principle of
electromagnetic attraction. It is a type of a magnetic switch which uses the magnet for creating a
magnetic field. The magnetic field then uses for opening and closing the switch and for
performing the mechanical operation

Types of an Electromagnetic Relay

By their working principle, the electromagnetic relay is mainly classified into two types. These
are

1. Electromagnetic Attraction Relay

2. Electromagnetic Induction Relay

1. Electromagnetic Attraction Relay

In this relay, the armature is attracted to the pole of a magnet. The electromagnetic force exerted
on the moving element is proportional to the square of the current flow through the coil. This
relay responds to both the alternating and direct current.

For AC quantity the electromagnetic force developed is given as

The above equation shows that the electromagnetic relay consists

two components, one constant independent of time and another dependent upon time and
pulsating at double supply frequency. This double supply frequency produces noise and hence
damage the relay contacts.

The difficulty of a double frequency supply is overcome by splitting the flux developing in the
electromagnetic relay. These fluxes were acting simultaneously but differ in time phase. Thus the
resulting deflecting force is always positive and constant. The splitting of fluxes is achieved by
using the electromagnet having a phase shifting networks or by putting shading rings on the
poles of an electromagnet.

The electromagnetic attraction relay is the simplest type of relay which includes a plunger (or
solenoid), hinged armature, rotating armature (or balanced) and moving iron polarized relay. All
these relays are shown below.
a. Balanced Beam Relay – In such type of relay two quantities are compared because the
electromagnetic force developed varies as the square of the ampere-turn. The ratio of an
operating current for such relay is low. If the relay is set for fast operation, then it will tend to
overreach on a fast operation.

b. Hinged armature
relay – The sensitivity of the relay can be increased for DC operation by adding the permanent
magnet. This relay is also known as the polarised moving relay.

2. Electromagnetic Induction Relay

The electromagnetic relay operates on the principle of a split-phase induction motor. The initial
force is developed on the moving element that may be disc or another form of the rotor of the
non-magnetic moving element. The force is developed by the interaction of electromagnetic
fluxes with eddy current, that is induced in the rotor by these fluxes

The different type of structure has been used for obtaining the phase difference in the fluxes.
These structures are
a. Shaded pole structure
b. Watt-hour meter or double winding structure
c. Induction cup structure.

a. Shaded pole structure

This coil is usually energised by current flowing in the single coil wound on a magnetic structure
containing an air gap. The air-gap fluxes produce by the initializing current is split into two flux
displace in time-space and by a shaded ring. The shaded ring is made up of the copper ring that
encircles the part of the pole face of each pole.

The disc is made

up of aluminium. The inertia of the aluminium disc is very less.. Hence they need less deflecting
torque for its movement. The two rings have the current induced in them by the alternating flux
of the electromagnetic. The magnetic field develops from the current produces the flux in the
portion of the iron ring surrounded by the ring to lag in phase by 40° to 50° behind the flux in the
unshaded portion of the pole.

b. Watt-hour Meter Structure

This structure consists E shape electromagnet and a U shape electromagnet with a disc-free to
rotate in between them. The phase displacement between the fluxes produced by the
electromagnet is obtained by the flux generated by the two magnets having different resistance
and inductance for the two circuits.
 The E-shaped electromagnet carries the two
windings the primary and the secondary. The primary current was carrying the relay current
I1 while the secondary winding is connected to the windings of the U-shaped electromagnet.

The primary winding carries relay current I1 while the secondary current induces the emf in the
secondary and so circulate the current I2 in it. The flux φ1 induces in the E shed magnet, and the
flux φ induces in the U-shaped magnet. These fluxes induced in the upper and lower magnetic
differs in phase by angle θ which will develop a driving torque on the disc proportional to φ1φ
sinθ.

The most important feature of the relay is that opening can control their operation or close the
secondary winding circuit. If the secondary winding is opened, then no torque will be developed,
and thus relay can be made inoperative.

c. Induction Cup Relay

The relay which works on the principle of electromagnetic induction is known as the induction
cup relay. The relay has two or more electromagnet which is energized by the relay coil. The
static iron core is placed between the electromagnet as shown in the figure below.
 The coil which is wound on the
electromagnet generates the rotating magnetic field. Because of the rotating magnetic field, the
current induces inside the cup. Thus, the cup starts rotating. The direction of rotation of the cup
is same as that of the current.

The more torque is produced in the induction cup relay as compared to the shaded and watt
meter type relay. The relay is fast in operation and their operating time is very less
approximately 0.01 sec.
HYSTERESIS

Definition: lagging of the magnetization of a ferromagnetic material, such as iron, behind

variations of the magnetizing field.

When ferromagnetic materials are placed within a coil of wire carrying an electric current, the
magnetizing field, or magnetic field strength H, caused by the current forces some or all of the
atomic magnets in the material to align with the field. The net effect of this alignment is to
increase the total magnetic field, or magnetic flux density B. The aligning process does not occur
simultaneously or in step with the magnetizing field but lags behind it.

If the intensity of the magnetizing field is gradually increased, the magnetic flux density B rises
to a maximum, or saturation, value at which all of the atomic magnets are aligned in the same
direction. When the magnetizing field is diminished, the magnetic flux density decreases, again
lagging behind the change in field strength H. In fact, when H has decreased to zero, B still has a
positive value called the remanence, residual induction, or retentivity, which has a high value
for permanent magnets. B itself does not become zero until H has reached a negative value. The
value of H for which B is zero is called the coercive force. A further increase in H (in the
negative direction) causes the flux density to reverse and finally to reach saturation again, when
all the atomic magnets are completely aligned in the opposite direction. The cycle may be
continued so that the graph of the flux density lagging behind the field strength appears as a
complete loop, known as a hysteresis loop. The energy lost as heat, which is known as the
hysteresis loss, in reversing the magnetization of the material is proportional to the area of the
hysteresis loop. Therefore, cores of transformers are made of materials with narrow hysteresis
loops so that little energy will be wasted in the form of heat.
ELECTROMAGNETIC INDUCTION

Definition: The induction of an electromotive force by the motion of a conductor across a magnetic
field or by a change in magnetic flux in a magnetic field.

This either happens when a conductor is set in a moving magnetic field (when utilizing AC power
source) or when a conductor is always moving in a stationary magnetic field.

Faraday’s law of Electromagnetic Induction

This law of electromagnetic induction was found by Michael Faraday. He organized a leading
wire according to the setup given underneath, connected to a gadget to gauge the voltage over the
circuit. So when a bar magnet passes through the snaking, the voltage is measured in the circuit. The
importance of this is a way of producing electrical energy in a circuit by using magnetic fields and
not just batteries anymore. The machines like generators, transformers also the motors work on the
principle of electromagnetic induction.

 First law: Whenever a conductor is placed in a varying magnetic field, EMF induces and this
EMF is called an induced EMF and if the conductor is a closed circuit than the induced
current flows through it.
 Second law: The magnitude of the induced EMF is equal to the rate of change of flux
linkages.
Based on his experiments we now have Faraday’s law of electromagnetic induction according to
which the amount of voltage induced in a coil is proportional to the number of turns and the
changing magnetic field of the coil.
So now, the induced voltage is as follows:
e = N × dΦdt
Where,
e is the induced voltage
N is the number of turns in the coil
Φ is the magnetic flux
t is the time
Lenz’s law of Electromagnetic Induction

Lenz law of electromagnetic induction states that, when an emf induces according to Faraday’s law,
the polarity (direction) of that induced emf is such that it opposes the cause of its production.

According to Lenz’s law

E = -N (dΦ/ dt) (volts)

Eddy currents

By Lenz law of electromagnetic induction, the current swirls in such a way as to create a magnetic
field opposing the change. Because of the tendency of eddy currents to oppose, eddy currents cause
a loss of energy. Eddy currents transform more useful forms of energy, such as kinetic energy, into
heat, which isn’t generally useful. In many applications, the loss of useful energy is not particularly
desirable, but there are some practical applications. Like:

 In the brakes of some trains. During braking, the brakes expose the metal wheels to a
magnetic field which generates eddy currents in the wheels. The magnetic interaction
between the applied field and the eddy currents slows the wheels down. The faster the wheels
spin, the stronger is the effect, meaning that as the train slows the braking force is reduces,
producing a smooth stopping motion.

 There are few galvanometers having a fixed core which are of nonmagnetic metallic material.
When the coil oscillates, the eddy currents that generate in the core oppose the motion and
bring the coil to rest.

 Induction furnace can be used to prepare alloys, by melting the metals. The eddy currents
generated in the metals produce high temperature enough to melt it.
Source: Geocities

Applications of Electromagnetic Induction

1. Electromagnetic induction in AC generator

2. Electrical Transformers

3. Magnetic Flow Meter

Electromagnetic induction in AC generator

One of the important application of electromagnetic induction is the generation of alternating

current.
Source: Physics365

The AC generator with an output capacity of 100 MV is a more evolved machine. As the coil
rotates in a magnetic field B, the effective area of the loop is A cosθ, where θ is the angle between A
and B. This is a method of producing a flux change is the principle of operation of a simple ac
generator. The axis of rotation coil is perpendicular to the direction of the magnetic field. The
rotation of the coil causes the magnetic flux through it to change, so an emf keeps inducing in the
coil.

Electrical Transformers

Another important application of electromagnetic induction is an electrical transformer. A

transformer is a device that changes ac electric power at one voltage level to another level through
the action of a magnetic field. A step-down transformer is the one in which the voltage is higher
in the primary than the secondary voltage. Whereas the one in which the secondary voltage has
more turns is a step-up transformer. Power companies use a step transformer to boost the voltage
to 100 kV, that reduces the current and minimizes the loss of power in transmission lines. On the
other end, household circuits use step-down transformers to decrease the voltage to the 120 or 240
V in them.

Solved Examples for You

Q. A straight wire length 0.20 m moves at a steady speed of 3.0 ms-1 at right angles to magnetic
field of flux density 0.10 T. e.m.f induced across ends of wire is

Answer: 0.06 V
INDUCTANCE
In electromagnetism and electronics, inductance is the tendency of an electrical conductor to
oppose a change in the electric current flowing through it. The flow of electric current through a
conductor creates a magnetic field around the conductor, whose strength depends on the
magnitude of the current. A change in current causes a change in the magnetic field.
From Faraday's law of induction, any change in magnetic field through a circuit induces
an electromotive force (EMF) in the conductors; this is known as electromagnetic induction. So
the changing current induces a voltage in the conductor. This induced voltage is in a direction
which tends to oppose the change in current (as stated by Lenz's law), so it is called a back EMF.
Inductance is defined as the ratio of the induced voltage to the rate of change of current causing
it. It is a proportionality factor that depends on the geometry of circuit conductors and
the magnetic permeability of nearby materials.[1] An electronic component designed to add
inductance to a circuit is called an inductor. It typically consists of a coil or helix of wire.
The term inductance was coined by Oliver Heaviside in 1886. It is customary to use the
symbol L for inductance, in honor of the physicist Heinrich Lenz. In the SI system, the unit of
inductance is the henry (H), which is the amount of inductance that causes a voltage of one volt,
when the current is changing at a rate of one ampere per second. It is named for Joseph Henry,
who discovered inductance independently of Faraday