Induction 1
EM Induction
Next Slide
Basic definitions
• Electromagnetic induction : generation of electricity
from magnetism
• Michael Faraday
Photo
• Michael Faraday’s experiment in 1831 Example
• Lenz’s law : Diagram
An induced current flows in a direction so
as to oppose the change producing it.
Induction 2
EM Induction
Next Slide
Fleming’s right hand rule
• Motion of a straight wire in a magnetic field can
produce an induced current.
• Fleming’s right hand rule
Diagram
• Moving-coil microphone Diagram
• Magnetic tape recording and playback Diagram
• Alternator Diagram
• Dynamo
Diagram
Transformer 1
EM Induction
Next Slide
Mutual Inductance
• Mutual inductance in two soft-iron C-cores Diagram
• Application : simple transformer (a.c. supply is used)
Diagram
• Example
Diagram
• Practical transformer Photo
• Advantage in using transformer and a.c. Example
Transformer 2
EM Induction
Next Slide
Electrical Power in HK
• Arrangement : generation, transmission and
distribution Diagram
• Discussion
END of EM Induction
Back to Induction 1
EM Induction
Click Back to
• Michael Faraday
Induction 1
EM Induction
Next Slide
• A coil is connected to a centre-zero galvanometer as shown
in the following diagram.
coil
N
S
magnet
galvanometer
• A bar magnet is pushed into the coil and left for a few
seconds. Then it is removed from the coil.
• Flow of current in the coil would be indicated by the
galvanometer.
Induction 1
EM Induction
Next Slide
• When the magnet is pushed into a coil, current flows in the
coil. This current is called induced current.
Coil becomes a magnet
S
N
direction of moving
N
I : induced current
S
Induction 1
EM Induction
Next Slide
• When the magnet is held motionless inside a coil, no
current flows in the coil.
N
S
The magnet remains at rest.
Induction 1
EM Induction
Next Slide
• When the magnet is pulled out of the coil, current flows in
the opposite direction.
Coil becomes a magnet
N
S
direction of moving
N
I : induced current
S
Back to Induction 1
EM Induction
Click Back to
• The experimental result is exactly the same except that
the direction of the induced current is reversed when the
polarity of the magnet is changed.
• Induced current flows if the coil moves instead of the
magnet.
• Conclusion : an electromotive force (e.m.f.) is produced when
there is relative motion between the coil and the magnet.
• Induced e.m.f. can be increased by :
(a) increasing the speed of the relative motion,
(b) increasing the number of turns in the coil, and
(c) using a stronger magnet
Induction 1
EM Induction
Next Slide
• Induced current changes the coil into a magnet such that a
repulsive force is produced between the magnet and coil to
oppose the motion of the magnet.
Coil becomes a magnet
S
N
direction of moving
N
I : induced current
S
Back to Induction 1
EM Induction
Click Back to
• Induced current changes the coil into a magnet such that
an attractive force is produced between the magnet and coil
to oppose the motion of the magnet.
Coil becomes a magnet
N
S
direction of moving
N
I : induced current
S
Induction 2
EM Induction
Next Slide
• Fleming’s right hand rule :
Motion (M)
The directions of current,
magnetic and the motion
of the conductor are
represented by thumb, the
first finger and the second
finger respectively, if they
are held perpendicular to
each other.
(IBM in short)
Magnetic
field (B)
Induced
current (I)
Back to Induction 2
EM Induction
Click Back to
• Application of Fleming’s right hand rule
magnetic
field
motion
S
N
induced current
Back to Induction 2
EM Induction
Click Back to
• Moving-coil microphone
Back to Induction 2
EM Induction
Click Back to
• Magnetic tape recording and playback
Induction 2
EM Induction
Next Slide
• In an alternator, a coil is made to rotate in a uniform
magnetic field. As it rotates, induced current would be
produced in the arms of the coil.
n
N
p
A
B
m
q
S
Induction 2
EM Induction
Next Slide
• Direction of induced current is shown.
B
N
S
A
Induction 2
EM Induction
Next Slide
Current (mA)
3/4
0
B
A
1/4
A
1/2
B
Time (no. of
revolutions)
1
A
B
B
A
B
A
Induction 2
EM Induction
Next Slide
• At t = 0, side A and B of the coil are parallel to the
magnetic field, no induced current is produced.
• From t = 0 to 1/4 no. of revolution, the induced e.m.f.
increases from zero, reaching a maximum value at t = 1/4
no. of revolution, when the coil is horizontal. According to
right-hand rule, the current flows from m  n  p  q
n
N
p
A
B
m
q
S
Back to Induction 2
EM Induction
Click Back to
• From t = 1/4 to 1/2 no. of revolution, the induced e.m.f.
decreases and the current still flows from m  n  p  q.
When t = 1/2 no. of revolution, the induced e.m.f. becomes 0.
• From t = 1/2 to 3/4 no. of revolution, the current becomes
reversed and flows in the direction q  p  n  m. The
induced e.m.f. increases from zero to maximum again when
t = 3/4 no. of revolution.
n
N
p
A
B
m
q
S
Induction 2
EM Induction
Next Slide
• In a dynamo (d.c. generator), the half-rings of the
commutator reverses the connection of the coil with the
circuit.
n
N
p
A
B
m
q
S
Induction 2
EM Induction
Next Slide
Current (mA)
0
B
A
1/4
A
1/2
B
A
3/4
B
B
Time (no. of
revolutions)
1
A
B
A
Back to Induction 2
EM Induction
Click Back to
• The induced e.m.f. and hence current, can be increased
in alternator and dynamo by :
(a) using a stronger magnet,
(b) increasing the number of turns in the coil,
(c) winding the coil on a soft-iron armature,
(d) rotating the coil at a higher speed.
Transformer 1
EM Induction
Next Slide
• A ring -shape soft iron core is used for the wiring of the
primary coil and the secondary coil are
• The primary coil is connected to a battery and a switch,
the secondary coil is connected to a galvanometer.
primary coil
soft-iron core
secondary coil
Transformer 1
EM Induction
Next Slide
• When the switch is closed, the galvanometer gives momentary
deflection.The rate of change in B-field is very great and hence
current is induced in the secondary coil.
B-field due to
soft-iron core
secondary coil
primary coil
B-field due to
primary coil
secondary coil
Transformer 1
EM Induction
Next Slide
• When the switch is kept closed, the galvanometer gives no
deflection. Although B-field exists, there is no change in B-field
and hence no current is induced in the secondary coil.
soft-iron core
primary coil
B-field due to
primary coil
secondary coil
Transformer 1
EM Induction
Next Slide
• When the switch is opened, the galvanometer gives momentary
deflection in the opposite direction as before.The rate of change in
B-field is still great and hence current is induced in the secondary
coil in opposite direction.
B-field due to
soft-iron core
secondary coil
primary coil
B-field due to
primary coil
secondary coil
Back to Transformer 1
EM Induction
Click Back to
• This effect is called mutual inductance.
• We can transmit current even there is no direction connection
between two circuits, provided that the current is not stable.
Transformer 1
EM Induction
Next Slide
•Now we connect an a.c. power supply to the primary coil so that
an unstable current is maintained. A lamp is connected to the
secondary coil. The lamp emits light since current is always
induced in the secondary coil. This device is called transformer.
primary coil
a.c.
supply
secondary coil
Transformer 1
EM Induction
Next Slide
• The relationship between the primary voltage and secondary
voltage is given as :
VS NS

VP N P
• The ratio of the secondary voltage to the primary voltage is
equal to the ratio of the no. of turns in the secondary coil to the
primary coil.
• It means that the a.c. voltage can be easily changed to any
desired value by using this kind of device. It is the advantage that
using a.c. instead of d.c. in domestic circuit.
Transformer 1
EM Induction
Next Slide
• If N P  NS , it is a step-up transformer that increases the
p.d. of the a.c.
• If NS  N P , it is a step-down transformer that decreases the
p.d. of the a.c.
Step-down
transformer’s
symbol
Step-up
transformer’s
symbol
Back to Transformer 1
EM Induction
Click Back to
• Transformer is also an energy transmission device. However, the
energy transmission is not 100% efficient.
power output in the secondary coil
Efficiency 
 100%
power input in the primary coil
Transformer 1
EM Induction
Next Slide
• A transformer has 3000 turns in its primary coil and is used to
operate a 12 V 24 W lamp from the 200 V a.c. mains as shown
in the following diagram. Assume the lamp is operated at the
correct rating.
(a) Find the number of
turns in the secondary coil.
200 V a.c.
NP = 3000
(b) Find the current flow
in the secondary coil.
12 V 24 W
(c) Find the current flow in the primary
coil if the effeciency is equal to (i) 100% (ii)
50%
Transformer 1
EM Induction
Next Slide
(a) Applying the transformer equation,
VS NS
VS
12 V

 NS 
 NP 
 3000  180
VP N P
VP
200 V
The secondary coil has 180 turns.
(b) The correct rating of the lamp is 12 V and 24 W,
24 W
P  VI  I 
 2A
12 V
The current rating of the lamp is 2 A and so the
secondary current is 2 A.
Back to Transformer 1
EM Induction
Click Back to
(c) (i) Power input = Power output
12 V
VP  I P  VS  IS  I P 
 2 A  0.12 A
200 V
The primary current is 0.12 A
(ii) Power input = 2  Power output,
12 V
VP  I P  2(VS  IS )  I P  2 
 2 A  0.24 A
200 V
The primary current is 0.24 A
Back to Transformer 1
EM Induction
Click Back to
• A practical transformer is shown in the following photo.
• Soft iron core is used to
trap the magnetic field.
• Resistance of coils
• Eddy currents in the core
• Magnetization and
demagnetization of core
• Leakage of field lines
Transformer 1
EM Induction
Next Slide
4 kW of power is supplied at the end of power cables of total
resistance 5 . Calculate the power loss in the cables if power is
transmitted (a) at 200 V, (b) at 4000 V.
Solution :
(a ) Apply P  VI, current through cables  4000 W  200 V  20 A
Power loss in cables  I 2 R  (20)2  5 W  2000 W
(b) Apply P  VI, current through cables  4000 W  4000 V  1 A
Power loss in cables  I 2 R  (1)2  5 W  5 W
• The power loss is greatly reduced by transmitting the power
at high voltage.
Back to Transformer 2
EM Induction
Click Back to
• Power generation, transmission and distribution
400 kV
overhead
cables
power
station
11 kV
factory
220 V
home
25 kV
step-up
transformer
network of
power cables
step-down
transformer