Chapter 22 - Electromagnetic Induction

Chapter 22
Electromagnetic Induction

Learning Outcomes
Deduce from Faraday’s experiments on
electromagnetic induction or other appropriate
experiments:
(i) that a changing magnetic ﬁeld can induce an e.m.f.
in a circuit;
(ii) that the direction of the induced e.m.f. opposes
the change producing it;
(iii) the factors affecting the magnitude of the induced
e.m.f.

Learning Outcomes
Describe a simple form of a.c. generator (rotating coil
or rotating magnet) and the use of slip rings (where
needed).
Sketch a graph of voltage output against time for a
simple a.c. generator.
Describe the structure and principle of operation of a
simple iron-cored transformer as used for voltage
transformation.

Learning Outcomes
Recall and apply the equations VP / VS = NP / NS and
VPIP = VSIS to new situations or to solve related
problems.
Describe the energy loss in cables and deduce the
advantages of high voltage transmission.

Quiz
In the previous chapter, you have learnt that a current can
produce a magnetic ﬁeld, is the reverse possible?

Yes, it is possible.
In this chapter you will learn that a magnetic ﬁeld can
produce an electrical current.

22.1 Electromagnetic Induction
Faraday’s Iron Ring Experiment


When switch S was closed or opened, Faradays
noticed that the compass needle deflected
momentarily. (The deflection of the compass needle
shows that there was a magnetic field present that was
caused by a current flowing in wire PQ)
However, the compass needle did not deflect when the
switch was left closed or opened.


From the experiment, it was concluded that when
the current in coil A was ,a
was caused to .
The current in B is called an .


the current in coil A was switched on or off , a

was caused to .



current was caused to
.



current was caused to ﬂow in coil B .




current was caused to ﬂow in coil B .

The current in B is called an induced current .


The induced current in coil B arose only when there
was a
.


The induced current in coil B arose only when there
change in the magnetic ﬁeld in the ring
was a
linking the coil B .

Faraday’s Solenoid Experiment

When the magnet is inserted into the solenoid, the
galvanometer needle
, indicating an
through the solenoid.


galvanometer needle deﬂected momentarily to

one direction , indicating an

through the solenoid.


galvanometer needle deﬂected momentarily to

one direction , indicating an induced current

ﬂowing in one direction through the solenoid.


When the magnet is withdrawn into the solenoid, the
galvanometer needle
, indicating an
through the
solenoid.


galvanometer needle deﬂected momentarily in

opposite direction , indicating an

through the
solenoid.


galvanometer needle deﬂected momentarily in

opposite direction , indicating an induced

current ﬂowing in opposite
direction through the
solenoid.


When the magnet is stationary (either inside or outside
the solenoid), there will be

.


When the magnet is stationary (either inside or outside
the solenoid), there will be no deﬂection in the

galvanometer indicating induced current
no
ﬂowing through the solenoid .

QUIZ
What do you think will happen if you
(i) change the number of turns of wire in the coil
(ii) change the strength of the magnet used
(iii) change the speed at which the magnet is moving

It was discovered that the magnitude of
the induced e.m.f. depends on:

(a) number of turns in the solenoid

(b) strength of magnet

(b) strength of magnet
(c) speed at which the magnet is
inserted or withdraw from the
solenoid

The Laws of Electromagnetic Induction
Faraday’s Law of Induction:
The e.m.f. (electromotive force) generated
in a conductor is

.

Faraday’s Law of Induction:
The e.m.f. (electromotive force) generated
in a conductor is to the
proportional
rate of change of magnetic lines of

force ﬂux) linking the
(magnetic
circuit .

Lenz’s Law:
The direction of the induced e.m.f is always
such that

.

Lenz’s Law:
The direction of the induced e.m.f is always
such that its magnetic effect

opposes the motion or change

producing it .

Demonstration of the two
laws of electromagnetism

Demonstration of the two laws
of electromagnetism
Apparatus:
Copper wire coil of about 20 turns, sensitive centre-zero
galvanometer, bar magnet.

of electromagnetism
Procedures:
1. Connect the ends of the coil to a sensitive centre-zero
galvanometer by means of long leads (i.e. connecting
wires).

of electromagnetism
Procedures:
2. Move the S-pole of a permanent bar magnet into the
coil and note any deﬂection on the galvanometer.

of electromagnetism
Procedures:
3. Once the bar magnet is inside the coil, hold it
stationary and gain note any deﬂection on the
a
galvanometer.

of electromagnetism
Procedures:
4. Next, move the S-pole of the magnet out of the coil
and note any deﬂection on the galvanometer.

of electromagnetism
Procedures:
5. Repeat steps 2 to 4 using the N-pole of the same bar
magnet.

of electromagnetism

S N

S-pole of magnet
moving into the solenoid.

of electromagnetism
1. Galvanometer shows a
deﬂection to the left -
induced current
S N
ﬂowing through the
circuit.

S-pole of magnet

of electromagnetism
induced current
S N
circuit.
2. The induced current
S-pole of magnet shows that an induced
moving into the solenoid. e.m.f. is generated in the
circuit.

of electromagnetism
3. The induced current on
the right-hand side of the
coil ﬂows in clockwise
S N
direction. By lenz’s law,
the direction of the
induced current opposes
the change producing it,
S-pole of magnet
hence a S-pole moving in
will induce a south pole
so as to try to repel it
away.

of electromagnetism
N S S N
S-pole of magnet
hence a S-pole moving in
will induce a south pole
so as to try to repel it
away.

of electromagnetism

S N

Magnet remains
stationary
in the solenoid.

of electromagnetism

S N 1. Galvanometer shows no
deﬂection - no current
circuit.
Magnet remains
stationary
in the solenoid.

of electromagnetism

S N

S-pole of magnet
S-pole of magnet
moving out the solenoid.

of electromagnetism
deﬂection to the right -
induced current
S N
circuit.

S-pole of magnet
S-pole of magnet

of electromagnetism
induced current
S N
circuit.
S-pole of magnet
moving out the solenoid. e.m.f. is generated in the
circuit.

of electromagnetism
coil ﬂows in anti-
S N
clockwise direction. By
lenz’s law, the direction of
the induced current
opposes the change
S-pole of magnet
S-pole of magnet
producing it, hence a S-
moving out the solenoid. pole moving out will
induce a north pole so
as to try to attract it.

of electromagnetism
S N S N
the induced current
opposes the change
S-pole of magnet
S-pole of magnet
producing it, hence a S-
moving out the solenoid. pole moving out will
induce a north pole so
as to try to attract it.

of electromagnetism

N S

S-pole of magnet
N-pole of magnet

of electromagnetism
induced current
N S
circuit.

S-pole of magnet
N-pole of magnet

of electromagnetism
induced current
N S
circuit.
N-pole of magnet
moving into the solenoid. e.m.f. is generated in the
circuit.

of electromagnetism
N S
the induced current
opposes the change
S-pole of magnet
N-pole of magnet producing it, hence a N-
moving into the solenoid. pole moving in will induce
a north pole so as to try
to repel it away.

of electromagnetism
S NN S
the induced current
opposes the change
S-pole of magnet
N-pole of magnet producing it, hence a N-
moving into the solenoid. pole moving in will induce
a north pole so as to try
to repel it away.

of electromagnetism

N S

Magnet remains
stationary
in the solenoid.

of electromagnetism

N S 1. Galvanometer shows no
deﬂection - no current
circuit.
Magnet remains
stationary
in the solenoid.

of electromagnetism

N S

S-pole of magnet
N-pole of magnet

of electromagnetism
induced current
N S
circuit.

S-pole of magnet
N-pole of magnet

of electromagnetism
induced current
N S
circuit.
S-pole of magnet
N-pole of magnet shows that an induced
moving out the solenoid. e.m.f. is generated in the
circuit.

of electromagnetism
N S
S-pole of magnet
N-pole of magnet hence a N-pole moving
moving out the solenoid. out will induce a south
pole so as to try to
attract it.

of electromagnetism
N S N S
S-pole of magnet
N-pole of magnet hence a N-pole moving
moving out the solenoid. out will induce a south
pole so as to try to
attract it.

Lenz’s Law and Conservation of Energy
• By Lenz’s law, the direction of the induced current is
such as to

the change causing it.
• When we move the magnet into the coil, we have to
do in overcoming the
between the like poles. Hence mechanical energy is
transformed into as indicated by
the induced current ﬂowing in the circuit.

such as to

oppose the change causing it.


such as to


do work in overcoming the


such as to


do work in overcoming the repulsion
force

such as to


do work in overcoming the repulsion
force
transformed into electrical energy as indicated by


• When we move the magnet out the coil, we have to
between the unlike poles. Hence mechanical energy is

do work in overcoming the


do work in overcoming the attraction force


do work in overcoming the attraction force

transformed into electrical energy as indicated by


22.2 Alternating Current Generators
• The a.c. generator is an electromagnetic device,
which transforms
into .
• The d.c. motor is an electromagnetic device, which
transforms into
.

which transforms kinetic energy (mechanical

energy) into
.
transforms into
.


energy) into
electrical energy .
transforms into
.


energy) into
electrical energy .
transforms electrical energy into

.


energy) into
electrical energy .
transforms electrical energy into

kinetic energy (mechanical energy) .

• An a.c. generator
consists of a
rectangular coil of
wire that is mounted
on an axle. By turning
the axle, the coil is
made to rotate
between the poles of
a permanent magnet.

• As the coil rotates,
the

and therefore

between the ends of
the coil.

the coil cuts the

magnetic ﬁeld

lines (magnetic

ﬁeld the
through
coil is changing)

and therefore

between the ends of
the coil.

the coil cuts the

magnetic ﬁeld

lines (magnetic

ﬁeld the
through
coil is changing)

and therefore
induced
current is
between the ends of
the coil.

• Note: the ends of the
coil must be
connected to an
external circuit with
an electrical load
such as a resistor for
the current to ﬂow.)

• The slip rings allow the transfer of the
induced in the rotating coil to the external
circuit.

D C C D
N S N S

A B B A
Slip rings

• The slip rings allow the transfer of the alternating

current induced in the rotating coil to the external

circuit.

D C C D
N S N S

A B B A
Slip rings

• The carbon brushes provides the contact between
the slip rings and the external circuit which prevents
wear and tear of the rings.

D C C D
N S N S

A B B A
Slip rings

• When the coil is horizontal,
• The rate at which the coil
.
• Hence the induced e.m.f. is the
.
B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

• The rate at which the coil the magnetic ﬁeld
cuts
lines is the greatest .

• Hence the induced e.m.f. is the
.
B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

• The rate at which the coil the magnetic ﬁeld
cuts
lines is the greatest .

• Hence the induced e.m.f. is the maximum induced

e.m.f. .

B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

• When the coil is vertical,
• The coil is moving parallel to the magnetic ﬁeld
lines, hence it is
.
• Hence the induced e.m.f. is the .
B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

lines, hence it is not cutting the magnetic ﬁeld

lines .
• Hence the induced e.m.f. is the .
B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

lines, hence it is not cutting the magnetic ﬁeld

lines .
• Hence the induced e.m.f. is the zero .

B

A
e.m.f./V

EO

0
time/s
-– EO

one revolution

• When the number of turns on the coil is doubled
without changing the frequency of the rotation of
the coil,
• The maximum output voltage will .
• The frequency of the changing of direction of
current ﬂow will .
e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

the coil,
• The maximum output voltage will doubled .

current ﬂow will .
e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

the coil,

current ﬂow will remains constant .

e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

• When the frequency of the rotation of the coil is
doubled without changing the number of turns
on the coil,
• The maximum output voltage will .
current ﬂow will . (i.e. Period is .)
e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

on the coil,

current ﬂow will . (i.e. Period is .)
e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

on the coil,

current ﬂow will doubled . (i.e. Period is halved .)

e.m.f./V

2EO
EO

0
– EO time/s
– 2EO

previous output
voltage

• The alternating voltage output of an a.c. generator
increases with

increases with
• an increase in the number of turns of coil
used.

increases with
used.
• an increase in the speed of rotation of the
coil.

increases with
used.
coil.
• a stronger magnet used.

increases with
used.
coil.
• a stronger magnet used.
• a soft iron core placed in the centre of the coil
of wire.

Other A.C. Generators (Practical design)
• To generate large currents (as in turbine in power
stations), it is more practical and advantageous to
keep the and to
.
• Hence instead of the coil cutting the magnetic ﬁeld
(discussed previously), now the magnetic ﬁeld cuts
the coil.
• Using this method, we can do away with the
which is prone to
wear & tear and not capable of carrying large currents.

keep the ﬁxed and to
coil
.
the coil.
which is prone to

keep the ﬁxed and to rotate the magnetic
coil
ﬁeld
(electromagnet/magnet) around the coil .

the coil.
which is prone to

keep the ﬁxed and to rotate the magnetic
coil
ﬁeld
(electromagnet/magnet) around the coil .

the coil.
• Using this method, we can do away with the slip
rings and the carbon brushes which is prone to


Fleming’s Right Hand Rule
• When a straight wire is moved inside a magnetic ﬁeld,
a current is induced in the wire.
!

Right!hand!

!

• Fleming’s Right Hand Rule (Dynamo rule) can be used
to determine the

in the wire.

Fleming’s Right Hand Rule
• When a straight wire is moved inside a magnetic ﬁeld,
a current is induced in the wire.
!

Right!hand!

!

• Fleming’s Right Hand Rule (Dynamo rule) can be used
to determine the

of the induced current
direction

in the wire.

22.3 Transformer
• A transformer is a device used to
of an a.c. supply.
• To change a at
to a at
or vice versa.

22.3 Transformer
• A transformer is a device used to vary the voltage

output of an a.c. supply.
• To change a at
to a at
or vice versa.

22.3 Transformer
• A transformer is a device used to vary the voltage

output of an a.c. supply.
• To change a low alternating voltage at

high current to a high alternating voltage at

low current or vice versa.

22.3 Transformer
• It is a useful electrical device that is essential for
• from power
stations to the consuming loads (households and
factories).
• for proper operation of
electrical appliances such as the mains-operated
television and record player.

22.3 Transformer
• electrical power transmission from power

factories).
• for proper operation of

22.3 Transformer
• electrical power transmission from power

factories).
• regulating voltages for proper operation of


Structure of a Closed-Core Transformer
Circuit Symbol

• It consists of a of wire and a
of wire wound round a
which consists of

(this is to reduce heat
loss due to induced eddy current).

Structure of a Closed-Core Transformer
Circuit Symbol

• It consists of a primary coil of wire and a
secondary coil of wire wound round a

laminated soft iron core which consists of

thin sheets of soft-iron insulated from each

other by a coat of
lacquer (this is to reduce heat
loss due to induced eddy current).

The Working Principle of Transformer

• The transformer is based on
, it transfer supplied
from the to the by
between the two coils.


• The transformer is based on Faraday’s iron ring

experiment , it transfer energy
electrical supplied
from the primary coil to the secondary coil by

electromagnetic induction between the two coils.


• At the primary coil, the
sets up a in the
secondary coil
. (Note: the frequency of the
alternating current in both the primary coil and the
secondary coil is .)


• At the primary coil, the applied alternating voltage

sets up a changing magnetic ﬁeld in the

secondary coil induces an e.m.f. the
which in
secondary coil . (Note: the frequency of the

alternating current in both the primary coil and the
secondary coil is same .)
the

• A step-up transformer is one where the e.m.f in the
primary coil is than the e.m.f. in the
secondary coil.
• The number of turns in the primary coil is
than the secondary coil
• A step-down transformer is one where the e.m.f in
the primary coil is than the e.m.f. in the
secondary coil.
than the secondary coil.

primary coil is smaller than the e.m.f. in the
secondary coil.
secondary coil.

secondary coil.
• The number of turns in the primary coil is lesser
secondary coil.

secondary coil.
the primary coil is larger than the e.m.f. in the
secondary coil.

secondary coil.
the primary coil is larger than the e.m.f. in the
secondary coil.
greater

• The equation that relates the number of turns of coil
and e.m.f. is shown in the formula below:

• The equation that relates the number of turns of coil
and e.m.f. is shown in the formula below:

NP = VP
NS VS
NP = No. of turns of coil in primary coil
NS = No. of turns of coil in secondary coil
VP = Applied e.m.f. in primary coil
VS = Induced e.m.f. in secondary coil

Power Transfer in a Transformer
• For an ideal transformer (100% efﬁcient), the power
supplied to the primary coil is fully transferred to the
secondary coil.
By principle of Conservation of Energy,

secondary coil.
Power in primary coil = Power in secondary coil

secondary coil.

VP IP = VS IS

secondary coil.

VP IP = VS IS
VP = Applied e.m.f. in primary coil
IP = Applied current in primary coil
VS = Induced e.m.f. in secondary coil
IS = Induced current in secondary coil

• However there is no ideal or 100% efﬁciency
transformer as
• in the form in the primary
coil, secondary coil and soft iron coil.
• There is a of . Not all the
produced by the primary coil is
to the secondary coil.

transformer as
• Energy is lost in the form of heat in the primary

• There is a of . Not all the
produced by the primary coil is
to the secondary coil.

transformer as
• Energy is lost in the form of heat in the primary

• There is a leakage of ﬂux . Not all the
magnetic
magnetic ﬂux produced by the primary coil is

linked to the secondary coil.

• In order to increase the efﬁciency of a transformer, it
should have
• for
primary and secondary coils so that the heating
effect is reduced.
• primary coil and secondary coils are
of the soft iron core to reduce
leakage of magnetic ﬂux

should have
• Low-resistance (thicker) copper wire for

effect is reduced.
• primary coil and secondary coils are
of the soft iron core to reduce

should have
• Low-resistance (thicker) copper wire for

effect is reduced.
• primary coil and secondary coils are wound on

the same part of the soft iron core to reduce


Example 1
An ideal transformer is used to step down the mains supply
from 240 V to 12 V to operate a lamp rated at 12 V, 60 W.
(a) Write down the turns ratio of the transformer.

(b) How many turns are on the secondary coil if the primary
coil has 2400 turns?

Example 1
(c) Calculate the current ﬂowing in the lamp when it is
working optimally.

(d) Find the current ﬂowing in the primary coil.

Example 1
(e) For such transformer, the secondary coil is usually made
of thick wire. Explain this.

Example 1
(e) For such transformer, the secondary coil is usually made
of thick wire. Explain this.
This is to reduce the heat loss as resistance is low
for a thick wire.

(Or to increase the efﬁciency of the transformer as
heat lost is reduced with a thick low resistance
wire.)

Example 2
A lamp labelled 5.0 V, 35 W is connected to a power
supply. When the lamp is in normal operation, its potential
difference is 5.0 V. A step down transformer is connected
between a 240 V power supply and the bulb.
Calculate the current ﬂowing in the 240 V power supply.

Transmission of Electrical Power
• Electrical energy generated in a power station is
transmitted to consumers using .
• However, is lost in the transmission cables in
the form of as the cables have .
• [The power loss in cables, P = .]
!
•
230 kV

25 kV 11 kV 230 V

Power Step up Network of power Step down
station transformer cables transformer

transmission cable
• However, is lost in the transmission cables in
the form of as the cables have .
!
•
230 kV

25 kV 11 kV 230 V


transmission cable
• However, energy is lost in the transmission cables in

the form of heat as the cables have .
resistance
!
•
230 kV

25 kV 11 kV 230 V


transmission cable
• However, energy is lost in the transmission cables in

the form of heat as the cables have .
resistance
• [The power loss in cables, P = I2 R .]
!
•
230 kV

25 kV 11 kV 230 V


• To minimize the lost of power due to heat, we can use
cables which reduces the resistance but we
will have to support very heavy cables and the
construction cost is high. Hence the other solution is
to in the transmission cables.

!

230 kV

25 kV 11 kV 230 V


thick cables which reduces the resistance but we
to in the transmission cables.

!

230 kV

25 kV 11 kV 230 V


thick cables which reduces the resistance but we
to transmit high voltage in the transmission cables.

!

230 kV

25 kV 11 kV 230 V


• The output a.c. voltage from the generator is
by a so that the
transmission current is .
• The very high voltage is then by a
before it is used in homes.
!

230 kV

25 kV 11 kV 230 V


step up by a
so that the
!

230 kV

25 kV 11 kV 230 V


step up by a step-up transformer so that the

!

230 kV

25 kV 11 kV 230 V



transmission current is kept low .
!

230 kV

25 kV 11 kV 230 V



• The very high voltage is then step down by a

!

230 kV

25 kV 11 kV 230 V



• The very high voltage is then step down by a

step-down transformer before it is used in homes.

!

230 kV

25 kV 11 kV 230 V


• Advantage of using a.c. voltage: It can be
easily using a
.(This is not possible by using d.c.)
• Advantage of high voltage transmission: It
in the transmission cables.
! •
230 kV

25 kV 11 kV 230 V


step-up & step-down easily using a

.(This is not possible by using d.c.)
! •
230 kV

25 kV 11 kV 230 V



transformer .(This is not possible by using d.c.)

! •
230 kV

25 kV 11 kV 230 V



transformer .(This is not possible by using d.c.)

reduces power loss in the transmission cables.

! •
230 kV

25 kV 11 kV 230 V


Example 3
The ﬁgure below shows a simple illustration of how
electrical power is transmitted from a power station to a
factory. !
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

The power station transmits power of 24 kW at 10 kV to
the sub-station. At the substation, the voltage is stepped
down to Vs = 2 kV before this power is transmitted to the
factory for use.

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(a) Calculate the current in the power cables between the
power station and the sub-station.

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(b) If the resistance of the power cables between the
power station and the sub-station is 0.05 Ω for every
1 km of cable, calculate the potential drop across the
power cables.

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(c) Hence or otherwise calculate the power loss from the
power station to the sub-station.

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(d) Find the primary voltage of the transformer at the sub-
station before it is stepped down to 2 kV.
(e)

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(e) What is the turns ratio of the transformer at the sub-
station in order for the transformer to step-down the
voltage to 2 kV?
(f)

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(f) Explain why electricity from the power station is
transmitted at a high voltage.

Example 3
!
(step-down
transformer)

24 kW 200 km Vs = 2 kV
10 kV

(f) Explain why electricity from the power station is
transmitted at a high voltage.

This is to reduce power loss

Chapter 22 - Electromagnetic Induction

More Related Content

What's hot (20)

Viewers also liked (6)

Similar to Chapter 22 - Electromagnetic Induction (20)

Recently uploaded (20)

Chapter 22 - Electromagnetic Induction

Editor's Notes