Electromagnetic
Induction
LESSON OBJECTIVES
At the end of the lesson you should be able to :
• Deduce from Faraday’s experiments on electromagnetic induction or other appropriate experiments:
• that a changing magnetic field can induce an e.m.f. in a circuit
• that the direction of the induced e.m.f. opposes the change producing it
• the factors affecting the magnitude of the induced e.m.f.
• Describe a simple form of a.c. generator
LESSON OBJECTIVES
At the end of the lesson you should be
able to :
• Describe the structure and principle of operation of a simple iron-cored transformer
• Recall and apply VP / VS = NP / NS and VPIP = VSIS
• Describe energy loss in cables and deduce the advantage of high voltage transmission
• Show an understanding of the use of a diode as a rectifier
Principle of
Electromagnetic
Induction
At the end of the lesson you should be able to :
Deduce from Faraday’s experiments on electromagnetic induction or other appropriate experiments:
• that a changing magnetic field can induce an e.m.f. in a circuit
• that the direction of the induced e.m.f. opposes the change producing it
When the
magnetic field
inside a coil (or
number of magnetic lines of force passing
through the coil)
changes
, an electromotive
force and hence a current is induced.
Principle of Electromagnetic
Induction
galvanometer
N S
Hollow cylinder
Magnet is stationary.
Magnetic field inside the coil remains
constant.
No induced e.m.f. and current.
galvanometer
N S
Hollow cylinder
stationary
Magnet moves into the coil
Magnetic field inside the coil increases. e.m.f. and current induced.
N S
Magnet moves further into the coil. Magnetic field inside the increases. e.m.f. and current induced.
N S
Magnet remains stationary in coil. Magnetic field inside the coil remains
unchanged.
No induced e.m.f. and current.
stationary
N S
Magnet moves out of the coil.
Magnetic field inside the coil decreases. e.m.f. and current induced.
N S
Faraday’s Law of EM Induction
The e.m.f. induced in a conductor
is
proportional
to
the rate of
change
of
magnetic
line
of
forces
linking the coil.
Key concept 1:
Lenz’s Law
The direction of the induced e.m.f.
and hence the induced current is
such
that
its
magnetic
effect
always
opposes
the
change
producing it.
Key concept 2:
When you move a magnet into or out of a
coil of wire, the magnet will experience a
• To oppose this motion, the induced current in the coil will make end A a North pole.
N S
A B
• Magnet moves into the coil.
• The galvanometer will deflect towards the right.
Becomes a North pole to oppose motion of magnet into coil
N S
A B
• To oppose this motion, the induced current in the coil will make end A a South pole.
• Magnet moves out of the coil.
• The galvanometer will deflect towards the left.
Factors affecting the magnitude of
the induced EMF and current
A larger E.M.F. is obtained when
• the magnet is moved at a
faster speed
in
and out of the coil.
• a
stronger
magnet is used.
THE
A.C
GENERATOR
At the end of the lesson you should be able to :
• Describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings. (where needed)
PRINCIPLES of a a.c.
generator
We have learned that with the presence of motion and magnetic field, we are able to produce electrical energy.
In this section, we are going to look at the principle of a simple A.C. generator, which
Resistor
Structure of a simple a.c. generator
N S
Permanent magnets
coil axle
Carbon brushes
Fleming’s Right-hand Rule
N
S First finger –
B – Field (B)
thuMb – Motion (M) seCond finger
N
S
Magnetic field Current Motion Magnetic field Current Motion A B D P C QCurrent flows from
A -> B -> C -> D-> Q -> P
N
S
P B
C
D
A
Q
Motion and magnetic field
are parallel
No induced current
N
S
Magnetic field Current Motion Magnetic field Current Motion P B C D A QCurrent flows from
D -> C -> B -> A-> P -> Q
N
S
P C
B
A
D
Q
Motion and magnetic field
are parallel
No induced current
Graph of induced e.m.f. against time
Induced e.m.f.
Time
Magnetic field
V
Time V -V Induced e.m.f. 0 -2V 2V T
Original number of turns in coil
Number of turns in coil doubled Number of
Time V -V Induced e.m.f. 0 -2V 2V T
T = T/2
TRANSFORMERS
At the end of the lesson you should be able to :
• Describe the structure and principle of operation of a simple iron-cored transformer
• A transformer is basically used to change the voltage of an alternating current. The figure below shows a basic iron-core transformer.
• A battery and a switch are connected to a
primary coil wound on the iron core. On the opposite end of the iron core, wound a
secondary coil that is connected to a lamp.
Soft iron core
Lamp (output)
Operation Principles of
Transformer
Primary coil
• When the switch is closed, the current in the primary coil increases from zero to maximum. • This produces an increasing magnetic field in
the primary coil.
• This changing magnetic field in turn induces an e.m.f in the secondary coil and lights up the lamp.
• Once the current in primary coil is steady, the magnetic field will remain constant and current is
not induced in the secondary coil. The lamp goes off.
• When the switch is opened, the current in the primary coil decreases from maximum to zero. This produces a decreasing magnetic field in the primary coil.
• This changing magnetic field in turn induces an
e.m.f. in the secondary coil and the lamp flashes again.
• To light up the lamp continuously, we can
use an alternating current supply instead
of battery.
• As the alternating current continually
changes
its
direction,
it
will
create
continually a
changing
magnetic field and
hence maintain an
induced
current in the
secondary coil.
Soft iron core
Primary coil Primary voltage or
Input voltage
Secondary coil
Lamp (output)
Secondary voltage or
Output voltage
A.C. supply
• This is done simply by having different number of turns in the primary and secondary coils of the transformer.
• Number of turns in secondary coil (NS) is more than number of turns in primary coil (NP) we have a step-up transformer.
• NS < NP, step-down transformer
Secondary output voltage
Primary input voltage
=
Number of turns in secondary coil
Number of turns in primary coil
=
VS
VP
NS
NP
NS
NP is known as turns ratio
Example 1
What is the output voltage if the turn ratio is 1/20 ?
=
VS
VP
NS
NP
=
VS
240
1
20
100 % efficient transformer :
Output Power = Input Power
VS IS = VP IP
=
IS
IP
VP
VS =
NP
NS
OUTPUT AND EFFICIENCY OF
TRANSFORMER
Example 2
A transformer is used to step-down an a.c. supply of “5 kW, 240V” to 12 V.
•What is the turn ratio of this transformer?
•What is the output power if the transformer is 100% efficient?
Example 2 SOLUTION
A transformer is used to step-down an a.c. supply of “5 kW, 240V” to 12 V.
•What is the turn ratio of this transformer?
Example 2
A transformer is used to step-down an a.c. supply of “5 kW, 240V” to 12 V.
•What is the output power if the transformer is 100% efficient?
Example 2
A transformer is used to step-down an a.c. supply of “5 kW, 240V” to 12 V.
•What is the corresponding output current?
100 % efficient transformer :
Output Power = Input Power
VS IS = 5000
12 (IS) = 5000
TRANSFORMERS
At the end of the lesson you should be able to :
Power Transmission
Power station Step-up transformer Underground cables Step-down transformers Factories, homesHigh voltage, high
current Higher voltage,
low current (lower power loss)
• The electricity generated at the power station is transmitted to the mains over long distance cable which will lose some energy during transmission due to cable resistance.
• Power loss in cable = I2R
• To reduce transmission loss, I and R have to be minimized.
• Resistance of cable can be minimized by using thick cables, but these are expensive and heavy.
• Step-up transformer can be used to produce
high voltage and low current transmission to reduce energy loss.
TRANSFORMERS
At the end of the lesson you should be able to :
• Show an understanding of the use of a diode as a rectifier
Converting A.C. to D.C.
Full-wave rectification
Full-wave rectification
Cathode Ray Oscilloscope
• The cathode ray oscilloscope is an electronic device that uses electron beam deflection to show how
Use of C.R.O.
Measuring Potential Difference
• Can be used for both a.c. and d.c. voltages
• The time base is
off.
• When d.c. voltage is applied, it either
deflects upwards or downwards.
Y-input
No supply connected to Y-input
Y-input
Y-gain is 2V / div
Time base is off
Y-input
Y-gain is 2V / div
Time base is off
1.5 V cell connected to Y-input in opposite
Y-input
Y-gain is 2V / div
Time base is off
3.0 V battery
Y-input
Y-gain is 5V / div
Time base is off
V
PP= 10 V
V
P= V
PP/ 2
= 5 V
VPP: peak-to-peak voltage
Y-input
Y-gain is 5V / div
Time base is 10 ms/div
V
PP= 10 V
V
P= 5V
T = 20 ms
f = 1 / T
Other Uses of C.R.O.
• Displaying Waveforms
