06/07 CE Physics Ch 17 Electromagnetic Induction
Chapter 17 Electromagnetic Induction
17.1 Induced voltage and induced current
 Magnet moving near coil
 Conducting wire moving in magnetic field
17.2 Generator
 Coil moving in magnetic field
 Simple a.c. generator
 Simple d.c. generator
 Bicycle alternator 單車發電機
17.3 Transformer 變壓器
 Voltage ratio and turns ratio
 Efficiency of transformer
17.4 Transmission of electrical energy
 Advantage of transmission of electrical energy with a.c. source and transformer
 Power transmission system
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06/07 CE Physics Ch 17 Electromagnetic Induction
17.1 Induced voltage and induced current
(p. 141)
1. Magnet moving near coil (p. 142)
A
Experiment 17B Electromagnetic induction
A. Induced current in a coil (p. 142)
(a) Experimental procedures: Fig. 17.1 (p. 142)
(i) A coil is connected to a light-beam
galvanometer.
(ii) Observe the deflection of the light-beam
galvanometer when a bar magnet is:
- pushed into the coil,
- pulled away from the coil,
- held stationary near the coil.
(iii)Repeat the above process by moving the
magnet at a faster speed, increasing the number
of turns of the coil and using a stronger magnet.
(b) Result and conclusion:
(i) When a magnet is moved towards the coil,
the light spot of the light-beam galvanometer
deflects in one direction.
(ii) This shows that a current is induced.
Experiment 17A Electromagnetic induction (data-logging)
A. Induced current in a coil (p. 143)
(c) Experimental procedures: Fig. 17.2(a) (p. 143)
(i) A bar magnet falls through the coil.
(ii) A current sensor is used to monitor the
current induced in the coil.
(iii)The respective current-time graph of the coil
is shown on the computer.
(d) Result and conclusion: Fig. 17.2(b) (p. 143)
(i) When a magnet is moving towards the coil,
a current is induced.
(ii) When the magnet is moving away from the
coil, a current is induced and the direction of it
is reversed from that in (i).
2. Induced voltage and induced current (p. 143)
Electromagnetic induction:
(a) An induced voltage is produced in a conductor
that experiences a change of magnetic field.
(b) An induced current flows through the
conductor if it is connected to a closed circuit.
3. Faraday’s law of electromagnetic induction (p. 143)
The law:
(i) When there is a change of magnetic field
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06/07 CE Physics Ch 17 Electromagnetic Induction
through a conductor, a voltage is induced.
(ii) The strength of the voltage is directly
proportional to the rate of change of magnetic
field through the conductor.
4. Direction of induced current – Lenz’s Law (p. 144)
(a) An induced voltage drives a current around a
circuit as the voltage of a battery does.
(b) The direction of the induced current can be
determined by Lenz’s law.
(c) The law:
The induced current always flows in a direction
such that it opposes the change producing it.
5. Magnet moving towards coil (p. 144)
Fig. 17.3 (p. 144)
When the N-pole of a magnet is moved towards
a coil:
(a) By Lenz’s law, a current should be induced in a
direction to repel the magnet.
(b) The induced current produces a magnetic field
in the coil with the N-pole facing the magnet.
(c) From the right-hand grip rule, we can find the
direction of the current flow.
6. Magnet moving away from coil (p. 144)
Fig. 17.4 (p. 144)
When the N-pole of a magnet is moved away
from a coil:
(a) A current should be induced in a direction to
attract the magnet.
(b) The induced current produces a magnetic field
in the coil with the S-pole facing the magnet.
(c) The direction of the induced current is reversed
from the case in (5).
7. Magnet at rest near coil (p. 145) Fig. 17.5 (p. 145)
When the magnet is stationary near the coil:
There is no induced current because the coil
does not experience any change of magnetic
field.
8. Factors affecting the magnitude of induced voltage
(p. 145)
The magnitude of the induced voltage can be
increased by:
(i) moving the magnet faster,
(ii) using a stronger magnet, and
(iii) increasing the number of turns of the coil.
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06/07 CE Physics Ch 17 Electromagnetic Induction
Example 1 (p. 146)
9. Conducting wire moving in magnetic field (p. 146)
Experiment 17B Electromagnetic induction
B. Induced current in a long wire (p. 146)
(a) Experimental procedures: Fig. 17.6 (p. 147)
(i) Move a long wire up and down between a
pair of slab-shaped magnets on an iron yoke.
(ii) Repeat the process by moving the wire at a
faster speed, increasing the number of turns of
the wire and using a stronger magnet.
(b) Result and conclusion:
A current flows through the wire if its ends are
connected to a closed circuit.
Reason:
The wire cuts the magnetic field lines between
the slab-shaped magnets.
Experiment 17B Electromagnetic induction (data-logging)
B. Induced current in a long wire (p. 147)
(c) Experimental procedures: Fig. 17.7(a) (p. 147)
(i) Move a long wire up and down between a
pair of slab-shaped magnets on an iron yoke.
(ii) The current sensor is used to monitor the
current induced in the long wire.
(iii)The respective current-time graphs for
upward and downward motions are shown on
the computer.
(d) Result and conclusion: Fig. 17.7(b)(c) (p. 147)
A current flows through the wire if its ends are
connected to a closed circuit.
10. Fleming’s right hand rule (p. 148) Fig. 17.9 (p. 148)
(a) Consider a straight wire moving at a right angle
to a magnetic field:
The direction of the induced current can be
determined by Fleming’s right hand rule.
(b) The rule: Fig. 17.8 (p. 148)
(i) Hold the thumb and the first two fingers of
the right hand at right angles to each other.
(ii) Point the first finger in the direction of the
magnetic field and the thumb in the direction of
the motion of the wire.
(iii)The second finger shows the direction of the
induced current.
(c) The direction of the magnetic field is from the
N-pole to the S-pole of the magnets.
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06/07 CE Physics Ch 17 Electromagnetic Induction
(d) No current is induced if the wire is moved
parallel to the magnetic field lines.
Fig. 17.10 (p. 149)
11. Factors affecting the magnitude of induced voltage
(p. 149) Fig. 17.11 (p. 149)
The magnitude of the induced voltage in the
wire increases if:
(i) the wire is moved faster,
(ii) more turns of wire are used, and
(iii)a stronger magnet is used.
Activity 1 Fleming’s right hand rule (p. 150),
Example 2 (p. 150), Class Practice 1 (p. 151),
STS Corner 1 Applications of induced current in coil
(p. 152)
17.2 Generator (p. 154)
12. Generator (p. 154)
(a) Generators convert kinetic energy into electrical
energy.
(b) Two types of generators:
(i) a.c. generator (alternator)
(ii) d.c. generator (d.c. dynamo)
13. Coil moving in magnetic field (p. 154)
Fig. 17.12 (p. 154)
(a) The motion of generators can be described by a
rectangular coil in a magnetic field.
(b) It will be turned clockwise at a constant speed
and a current will be induced in it.
(c) Four stages of induced current in the coil:
Fig. 17.13 (p. 155)
(Stage 1) The coil starts to turn clockwise
(plane of the coil // magnetic field lines):
(i) As arms PQ and RT are moving upwards
and downwards respectively, they cut the
magnetic field lines perpendicularly.
(ii) By Fleming’s right-hand rule, the induced
current flows through the coil in the direction of
PQRT.
(Stage 2) The coil is turned 90º from the
starting position:
(i) The coil passes through the vertical position.
(ii) The two arms of the coil are moving parallel
to the magnetic field lines.
(iii)Therefore, no current is induced in the coil.
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06/07 CE Physics Ch 17 Electromagnetic Induction
(Stage 3)
The coil is turned 180º from the
starting position:
(i) Arms PQ and RT are moving downwards
and upwards respectively.
(ii) The induced current flows in the direction of
TRQP. It is opposite to that in Stage 1.
(Stage 4) The coil is turned 270º from the
starting position:
(i) The coil is vertical again. There is no current
flowing through the coil as Stage 2.
(ii) After the coil has passed the vertical
position, the direction of the current is reversed
again.
(iii)This goes on until the plane of the coil is
parallel to the magnetic field lines as in Stage 1.
(d) The induced current is maximum when the
plane of the coil is horizontal.
Reason:
The rate of cutting the magnetic field lines is the
highest.
(e) Principle of an a.c. generator:
(i) The direction of the induced current is
reversed for each half revolution.
(ii) a.c. is produced when the coil is turned in
one direction continuously.
4. Simple a.c. generator (p. 156) Fig. 17.14 (p. 156)
A simple a.c. generator consists of:
(a) A rectangular coil:
It wounds on a frame. The frame is mounted on
an axle between the poles of a magnet.
(b) Slip rings:
(i) They are two copper rings connected to the
ends of the coil and rotate with it.
(ii) Prevent the wires from twisting when the
coil is rotated.
(c) Carbon brushes:
Pressed against the slip rings and connect the
coil to the external circuit.
(d) Fig. 17.15 shows the a.c. flowing through the
coil as the coil is rotated. Fig. 17.15 (p. 157)
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15. Simple d.c. generator (p. 157)
Experiment 17C Generator (p. 157)
(a) Experimental procedures: Fig. 17.16 (p. 157)
(i) A d.c. generator can be constructed by
replacing the slip rings in an a.c. generator by a
pair of half-rings (commutator).
(ii) The generator is connected to a
galvanometer and is rotated.
(iii)Observe the pointer of the galvanometer.
(iv)Repeat the above process by rotating the
armature at a faster speed, increasing the
number of turns of the coil and using a stronger
magnet.
(b) Result and conclusion (p. 158):
Fig. 17.17 (p. 158)
(i) Direct current is induced in the coil.
(ii) The direction of the induced current depends
on the direction of rotation of the coil.
Reason:
The commutator reverses the connections of the
coil with the external circuit every half turn of
the coil. Thus the current always flows through
the external circuit in one direction.
(c) The induced voltage can be increased by:
(i) rotating the coil at a higher speed,
(ii) increasing the number of turns of the coil,
(iii)winding the coil on a soft-iron core, and
(iv) using a stronger magnet.
(d) The higher the rotating speed of the coil, the
higher the frequency of the induced current.
16. Bicycle alternator (p. 159) Fig. 17.18 (p. 159)
(a) A bicycle alternator is a practical a.c.
generator.
(b) Uses:
It provides electrical energy to a small headlamp
for illumination purpose.
(c) Structure:
It consists of a coil wound on a U-shaped
soft-iron core and a cylindrical magnet (rotor)
joined to an axle.
(d) Working principle:
(i) When the wheel rotates, it drives the axle of
the generator and the magnet to rotate.
(ii) The coil experiences a changing magnetic
field, hence an alternating voltage is induced.
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06/07 CE Physics Ch 17 Electromagnetic Induction
(iii)The induced voltage causes an alternating
current to flow through the headlamp.
(e) Energy conversion:
Chemical energy of the cyclist
 Electrical energy in the coil
 Light and heat of the headlamp
(f) The waveform of the induced voltage on a CRO
shows it is alternating. Fig. 17.19 (p. 159)
Example 3, Class Practice 2 (p. 160),
STS Corner 2 Stealing electricity (p. 161)
17.3 Transformer (p. 162)
17. Transformer (p. 162) Fig. 17.20 (p. 162)
(a) A transformer is a device to change the
magnitude of a.c. voltage.
(b) It consists of a soft-iron core wound with two
coils.
(c) The two coils have been insulated properly so
that no current can flow from one coil to the
other.
18. Mutual induction (p. 163)
Fig. 17.21, Fig. 17.22 (p. 162)
(a) In a transformer,
(i) coil A is connected to a d.c. power supply
and a switch, and
(ii) coil B is connected to a galvanometer.
(b) The transformer is operated in three different
cases:
(Case 1) At the instant when the switch is
closed:
(i) A current flows in coil A and magnetizes the
soft-iron core.
(ii) The magnetic field lines are guided by the
core to coil B.
(iii)Then coil B experiences a change of the
magnetic field and a current is induced in it.
(iv)The pointer of the galvanometer deflects
momentarily.
(Case 2) When the switch remains closed
(p. 163):
(i) The current in coil A becomes steady and the
magnetic field in the core remains unchanged.
(ii) There is no induced current and the pointer
of the galvanometer returns to zero position.
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(Case 3) At the instant when the switch is open:
(i) The current in coil A and the magnetic field
in the core decrease.
(ii) A current flows in the opposite direction in
Case 1 is induced in coil B to oppose the
change.
(iii)Thus, the pointer of the galvanometer
deflects momentarily to the opposite direction.
(c) Result:
(i) When the switch closes and opens
alternately, the ponter vibrates about the zero
reading.
(ii) An a.c. is produced in coil B.
(iii)The phenomena of changing the current in a
coil to induce a voltage and current in another
coil is called mutual induction.
(d) Primary and secondary coils: Fig. 17.23 (p. 163)
(i) The coil connecting to an energy source is
called the primary coil.
(ii) The coil acting as the output of the
transformer is called the secondary coil.
19. Light bulbs connected to a transformer (p. 164)
Fig. 17.24 (p. 164)
(a) Connection of light bulbs:
(i) The primary coil is connected to an a.c.
power supply and a light bulb.
(ii) The secondary coil is connected to a light
bulb which glows continuously.
(b) Reason:
(i) The a.c. in the primary coil sets up an
alternating magnetic field in the core.
(ii) An alternating voltage and current of the
frequency of the power supply are induced in
the secondary coil continuously.
20. Voltage ratio and turns ratio (p. 164)
Experiment 17D Transformer (p. 164)
To study the relation between:
(i) the ratio of the number of turns of the
primary and secondary coils (turns ratio), and
(ii) the voltages across the primary and
secondary coils (voltage ratio).
(a) Experimental procedures: Fig. 17.25 (p. 164)
(i) The voltages across the primary and
secondary coils are shown in two CROs.
(ii) Vary the numbers of turns of coils to obtain
different sets of voltages across the coils.
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(b) Result and conclusion (p. 165):
For a transformer,
Vp N p

Vs N s
where
Vp is the input voltage (primary voltage),
Vs is the output voltage (secondary voltage),
Np is the number of turns of the primary coil,
Ns is the number of turns of the secondary coil.
21. Step-up and step-down transformers (p. 165)
Fig. 17.26, Fig. 17.27 (p. 166)
(a) Step-up transformer:
(i) Output voltage greater than input voltage.
(ii) Occurs when Ns > Np.
(iii)Example: With turns ratio of 1 : 10,
if Vp = 20 V, Vs = 200 V.
(b) Step-down transformer:
(i) Output voltage smaller than input voltage.
(ii) Occurs when Ns < Np.
(iii)Example: With turns ratio of 20 : 1,
if Vp = 20 V, Vs = 1 V.
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22. Efficiency of transformer (p. 166)
(a) Mathematically:
Efficiency of a transformer
Output electrical power
=
 100%
Input electrical power
e=
Vs I s
 100%
Vp I p
where Vp, Vs are the input and output voltages
respectively, and
Ip, Is are the input and output currents
respectively.
(b) For an ideal transformer:
(i) No energy is lost.
(ii) Output electrical power is equal to the input
electrical power.
(iii)The efficiency of the transformer is 100%.
(iv)
Vp I s N p
 
Vs I p N s
(v) Example:
If the output voltage is stepped up by 10 times,
the output current would be stepped down by 10
times.
(c) For a practical transformer (p. 167):
Fig. 17.28 (p. 167)
(i) Efficiency is less than 100%.
(ii) Some electrical energy is lost.
(iii)Most of this energy turns into heat which
could melt the transformer and cause a fire.
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23. Methods to improve efficiency of transformer
(p. 167)
(a) Minimize the heat loss:
(i) Reason:
As the coils have some resistance, the current
flowing through them heats them up.
(ii) Solution:
Use conducting wires of smaller resistance such
as copper or silver.
(b) Reduce the eddy currents:
(i) Reason:
- Currents are induced in the soft-iron core as
the magnetic field changes.
- These currents are called eddy currents,
which can heat up the core.
(ii) Solution:
Lamination of the core: Fig. 17.29 (p. 167)
Use a stack of thin metal slices that are insulated
from one another for making the core.
(c) Reduce heat loss by magnetization and
demagnetization:
(i) Reason:
The continuous magnetization and
demagnetization of the iron core, caused by the
a.c., heats up the core.
(ii) Solution:
Use soft-iron cores because they can be
magnetized and demagnetized easily.
Example 4, Class Practice 3 (p. 168)
17.4 Transmission of electrical energy
(p. 169)
STS Corner 3 Electricity supply (p. 169)
24. Transmission of electrical energy with d.c. source
(p. 170)
Experiment 17E Transmission of electrical energy (p. 170)
(a) Experimental procedures: Fig. 17.30 (p. 170)
(i) Use a d.c. power supply to represent a power
station.
(ii) Connect a light bulb to the power supply (A)
with the transmission cables.
(iii)At the other end of the transmission cables
(B), connect another identical light bulbs to
represent a home user.
(iv)Turn on the power supply and observe the
brightness of the bulbs.
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(b) Result and conclusion:
(i) The power loss of heat in the cables:
Plost = I 2R
where I is the current in cables and R is the
resistance of cables.
(ii) The power loss is so great that only a small
percent of the electrical power could be
transmitted to the light bulb.
(iii)Therefore, the light bulb is dim.
25. Transmission of electrical energy with a.c. source
and transformers (p. 171)
(a) Experimental procedures: Fig. 17.31 (p. 171)
Electricity is now transmitted from an a.c.
source to the light bulb through transformers.
(b) Result and conclusion:
(i) The voltage is at first increased by a step-up
transformer (A).
(ii) Is is greatly reduced.
(iii)The power loss in the cables (Is2 R) would be
much reduced.
(iv)The step-down transformer (B) restores the
voltage for the light bulb. The light bulb is
bright.
Example 5 (p. 172) , Class Practice 4 (p. 173)
26. Power transmission system (p. 174)
Fig. 17.32 (p. 174)
Transmission system in Hong Kong:
(i) Electricity is transmitted at an extra high
voltage to the zone substations.
(ii) At the zone substations, the high voltage is
stepped down by transformers and then
transmitted to the local substations.
(iii)In the local substations, electricity is further
stepped down for customer uses.
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