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>>BIG IDEAS<<
12
Electromagnetism
and electronics
How do electronic gadgets work?
†† Fig 12.2 Complex electronic circuits look confusing, but
12.1 How are magnetism and electricity linked?
†† Fig 12.1 A fuel-cell car has an
electric motor, which uses a
combination of magnetism and
electricity in order to produce
rotation and generate power.
The link between magnetism and
electricity was a chance discovery.
Since then, much has been uncovered
about electricity and magnetism. For
example, light is made up of a
combination of electric and magnetic
fields and is an electromagnetic
phenomenon. The electromagnetic
spectrum is a range of different types
of light. A further link between
electricity and magnetism involves
force. When a magnetic field and
wires carrying an electric current
are arranged correctly, the
electromagnetic force generated can
make an electric motor rotate. Electric
motors can operate on household 240
volts alternating current (AC) or on
direct current (DC) from a battery. In
fact, electric motors can be
disconnected from a power supply
and spun by hand—or by the wind in
the case of a wind turbine—and
electricity is produced.
1 What devices do you know of that
use electric motors? List as many as
you can.
2 Wind power is one method of
generating electricity. What other
methods do you know of?
3 Magnetism isn’t always associated
with electricity. The Earth’s magnetism
isn’t due to electric currents flowing
inside the Earth. What is the Earth’s
magnetism due to?
4 What do we use magnetism for?
Brainstorm as many applications as
you can with your classmates.
they are often composed of standard electronic devices
such as resistors, capacitors, diodes and transistors.
Do you, your friends or someone in your family love electronic gadgets, like iPods, mobile
phones, headphones, computers, cameras, battery chargers or even calculators? How do they
work? Understanding complex devices such as these is not easy but there are some fundamental
electronic components that are easier to master individually and in simpler combinations. We will
examine some of these components, as well as take a look at some modern electronic gadgets.
12.3 How do common electronic
gadgets work?
12.2 What happens in
electronic circuits?
Everyday devices, such as the television
remote control or the calculator you use in
mathematics classes, are electronic devices
that often contain complex electronic circuits.
Yet the devices that make up those circuits
are reasonably simple. Understanding how the
separate components work is fundamental to
understanding how complex devices, such as
computers and televisions, work. An electric
circuit in itself is not very complex. However,
once components are incorporated into it, the
final device can be complex. Some
components control the amount of electricity
flowing in different parts of the circuit, some
amplify the current, some store
the electric charge and release it slowly over a
period of time, some only allow the current to
flow through them in one direction, some
convert the electricity to light and some
receive light and convert it to electricity. Each
component may be relatively simple, yet
combined the whole device can become very
complex, such as a television set.
1Can you remember the fundamental
requirements of an electric circuit? What are
they?
2 When you operate a television remote control
to change channels, what energy conversion
happens in the remote control? What energy
conversion happens at the television?
3 What is an electric current?
4 What are the two types of electric current?
Electronics is a modern area of science and, as technology has
developed over recent years, the individual electronic components
themselves have become smaller and smaller until we are where we
are today with miniature electronic circuits that contain many
thousands of such devices in a tiny space. This technology has
enabled computers to be vastly reduced in size from the first
prototype that occupied a whole building! Mobile phones have
benefited from miniature electronics and are much more compact
than the first models. A modern iPod has replaced the juke box or
CD collection because it can store music files through the use of
electronics. Electronics deals with devices that use electrons,
mainly through semiconductor materials, such as silicon. Silicon
is used extensively to manufacture silicon chips, which are also
known as microchips or integrated circuits. The invention of the
integrated circuit revolutionised electronics as the individual
components were no longer physically joined together, but rather
were printed as one ‘integrated’ unit. Everything in our modern
lives—computing, communications (including the Internet) and
entertainment devices—depends on integrated circuits.
Many people believe that the digital revolution is one
of the most important events in the history of humans.
1Silicon is a semiconductor material.
What types of materials are most
metals? What about most nonmetals?
2Have you seen the first Terminator
movie? A silicon chip is
fundamental in this movie.
Brainstorm with your classmates
why this is so.
3How has the digital revolution
affected your life? How
important is digital
technology to you?
†† Fig 12.3 Since its invention in
2001, the iPod has revolutionised
personal audio and entertainment.
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12.1
How are magnetism and
electricity linked?
In 1820, the Danish physicist Hans Christian Øersted observed that a tiny
compass changed direction when a nearby electric current was turned
on. This chance discovery demonstrated the relationship between
electricity and magnetism and led to the development of many new ideas.
In this section we will investigate some of these ideas. In particular, we will
look at how different magnetic fields can be obtained from different arrangements
of current-carrying wires, how electric motors operate and how to generate electricity.
Using electricity to
create magnetic
fields
In Chapter 9 of Big Ideas Science Book 1
you learnt that a magnetic field exists in
the space surrounding a magnet. When
another magnet or a piece of iron or
steel comes into the field, it experiences
a force. This force can be either
attraction (a pull) or repulsion (a push).
The force is stronger closer to the
magnet. The shape of the magnetic field
can be made visible if iron filings are
sprinkled around the magnet. When a
–
small compass is placed in the field, its
needle shows the direction of the
magnetic field.
Magnets are not the only things that
create magnetic fields. When Danish
physicist Hans Christian Øersted
discovered that a wire carrying an
electric current caused a compass
needle to move when the current was
switched on, he concluded that
electricity could cause magnetism. A
single current-carrying wire creates a
circular magnetic field that gets weaker
as the distance from the wire increases.
To predict the direction of the magnetic
field around a single current-carrying
wire, we can use the right-hand grip
rule (also called the right hand curl
rule). The right thumb is pointed in the
direction of the current and the way the
fingers curl gives the circular direction
of the magnetic field.
To create a stronger, straighter magnetic
field, a long single current-carrying wire
can be looped into coils. Such a coil of
loops is known as a solenoid. The
magnetic field produced by this
arrangement is very similar to that of a
bar magnet. To determine the direction
of the magnetic field in this case, the
curled fingers of the right hand follow
the direction of the conventional current
flow around the loops and the right
thumb gives the magnetic field direction
through the centre of the solenoid.
The conventional current flow points
Current-carrying
wire
Current
Field
Wire
+
†† Fig 12.4 The magnetic field around a straight
current-carrying wire is circular.
S
+
Iron filings
Card
N
†† Fig 12.5 The right-hand grip rule. The way the
fingers point around the wire gives the direction of
the magnetic field.
–
Conventional
current flow
†† Fig 12.6 For a solenoid, the curled fingers follow
the conventional current flow and the right thumb
points in the direction of the magnetic field. The
left-hand end of the solenoid will be the north pole.
to the north (N) end (or pole) of the
solenoid.
To create an even stronger magnetic
field, a soft iron core can be added
inside the solenoid. Soft iron is pure
iron. Pure iron is easily magnetised.
If the current is switched on, the core
becomes magnetised and strengthens
the magnetic effect of the solenoid. If
the current is switched off, the
magnetic field is reduced. This is an
example of an electromagnet, which
is a type of magnet that can be turned
on or off. The versatile nature of
electromagnets has enabled many
devices to be invented that use the
fact that their magnetism can be
turned on and off.
E XPE RIME NT 12 .1
Creating magnetic fields
Aim
To investigate the magnetic field around a single wire
and around a solenoid when connected to direct current
(DC) and alternating current (AC)
Equipment
• AC/DC 12 V power supply
• Solenoid
• Iron core
• Connecting wires
• Plotting compass
• Retort stand
Method
1 Sit the solenoid on the retort stand base.
2 Connect the solenoid to the power supply. Use the
DC connections on the power supply and turn the
knob to 12 V. Before switching on the power supply,
position the plotting compass under one of the
connecting wires so that its needle is parallel to the
wire.
2 Switch the power supply on and observe the
compass needle. Move the compass above the
connecting wire and observe the needle again. Test
the compass with the other connecting wire in a
similar way. Record your observations.
3 Insert the iron core into the solenoid. What do you
notice during this process? Does the iron core get
hot after a while? Try to pull the iron core out of the
solenoid while the power is still on and after the
power is switched off. Was there a difference? Move
the solenoid off the retort stand base and again try
to remove the iron core while the power is on. Was
there a difference?
4 Remove the iron core and change the power supply
connections to AC. Reinsert the iron core. Is there
any evidence that the magnetic field is vibrating?
Does the iron core get hot after a while?
Discussion
• Why is the magnetic field around the solenoid
stronger than that around a single wire?
• Explain the difference in pulling the iron core out
of the solenoid with the power on and with the
power off.
• Comment on the effect of the retort stand base.
• Compare the effects of DC and AC on the iron core.
• Why did the iron core get hot?
• Write a suitable conclusion for this experiment.
What do you know about using electricity to create magnetic fields?
1How could two bar magnets be
arranged to produce:
a attraction?
b repulsion?
2 What happens to the strength of the
magnetic field as you come closer to a
current-carrying wire?
3 An electromagnet made by a student
will pick up three paper clips, but it
is not strong enough to pick up four
paper clips. Give two ways the student
could modify the electromagnet so it
could pick up four paper clips.
4How could the strength of the
magnetic field around a solenoid be
increased?
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Using magnetic
force to create an
electric motor
If a current-carrying wire is placed at
right angles to another magnetic field,
such as that provided by a strong
magnet, the two magnetic fields can
interact. The two fields reinforce each
other in some places and cancel each
other out in other places. This results
in an unbalanced magnetic field that
exerts a force on the electrons moving
inside the wire, making the wire move.
The right-hand slap rule is used to
predict the direction of the force on the
wire. The right thumb matches the
current direction, the outstretched
fingers follow the magnetic field of the
strong magnet (from north to south),
and the palm of the hand pushes in
the direction of the force.
Force
(out of palm)
Current
Field
Right hand
†† Fig 12.7 The right-hand slap rule. The thumb
represents the current, the fingers represent the
magnetic field and the palm pushes in the direction
of the force.
How does an electric
motor work?
The force on a single wire is not
particularly useful. To create a more
effective type of force, the single wire
can be looped into coils, similar to a
solenoid. If this coil is then placed in
another magnetic field and a current is
passed through the coil, the forces on
the coil cause it to rotate. Such a device
is an electric motor.
practivity 12.1
Observing magnetic force
What you need: Power supply, strong horseshoe magnet, connecting wires
1Set up the equipment as shown in Figure 12.8.
†† Fig 12.8
2 Turn the power on. The wire should ‘jump’ out of the magnetic field.
• Why did this happen?
3 Predict what will happen if you change the positions of the wire and the magnet.
Set up the equipment to match your predictions and observe what happens.
• Can you explain your observations?
4Set up the equipment so that the current is parallel to the magnetic field and observe
what happens.
• Can you explain your observations?
Questions to consider ...
• How is the force dependent on the angle between the current and the magnetic
field?
• Complete the following: When the angle is zero, the force is
. When
the angle is 90°, the force is
.
It is possible that you’ve used an
electric motor already today. A
hairdryer uses an electric motor to
drive the fan that blows the hot air
over your hair. Electric motors
attached to fans are also used in most
heaters and air conditioners to blow
warm or cool air around a room or
inside a car. A washing machine, a
clothes dryer, a blender and a CD
player all use electric motors to create
rotation.
Figure 12.9 shows how an electric
motor works. The coil of wire, called
an armature, usually consists of
many turns but is shown here as a
single loop for clarity. The pivots at
each end of the armature are omitted
for clarity.
The coil is connected to the DC power
supply using brushes and a split ring
commutator (SRC). The direction of the
conventional current is shown by the
arrows. The right-hand slap rules on
each side of the diagram show the
direction of the forces on the sides of
the coil. The downward force on the
left side and the upward force on the
right side create an anticlockwise
rotation. Once the coil rotates past the
vertical, though, these forces need to
be reversed to maintain smooth
Current
Force
Current
Force
Field
Field
Field
Field
Coil
(armature)
Coil
(armature)
Current
Force
N
N
S
Brush
Conventional
current, /
+
Current
Force
S
Brush
Brush
Commutator
–
Conventional
current, /
Conventional
current, /
+
rotation. The commutator does this job
The direction of rotation is,
by connecting to the opposite brush
therefore, maintained.
after each half-turn (180° rotation) of
Most electric motors are more
the coil. Figure 12.10 shows the same
complicated than this simplified
coil turned over 180°. The red side is
example. They often have several
now on the left and the green side is
sets of coils all at slightly
on the right.
different angles to each other,
The commutator has also rotated 180°
and electromagnets are often used
and now connects to the opposite
instead of permanent magnets.
the direction of the current in the coil.
Conventional
current, /
Commutator
–
†† Fig 12.10 The coil has now turned over 180°.
†† Fig 12.9 How an electric motor works.
brush, which has the effect of reversing
Brush
†† Fig 12.11 A fuel-cell car has an electric motor.
What do you know about using magnetic force to create an
electric motor?
1Draw a diagram that shows the best arrangement of a single
current-carrying wire and a strong magnet in order to produce
the maximum force on the wire.
2Draw a diagram that shows the arrangement of a single
current-carrying wire and a strong magnet in order to produce
zero force on the wire when the current is flowing.
5 What energy conversion occurs in an electric motor?
6In an electric motor, what is the job of the:
a split ring commutator?
c armature?
(4)
3 Figure 12.12 shows the major components of an electric
motor labelled 1–5. Match each number to the correct label
below.
a permanent magnet
b armature coil
c split ring commutator
d brush
eDC power supply.
4 Will the electric motor shown in Figure 12.12 rotate clockwise
or anticlockwise? Justify your answer.
b brushes?
(1)
N
S
(5)
†† Fig 12.12
+
(3)
(2)
chapter twelve: electromagnetism and electronics
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Building an electric motor
E XPE RIME NT 12 . 2
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296
Aim
To build two electric motors—one using everyday
objects and the other using a kit—and compare their
similarities and differences
Equipment
DC motor kit
2 m of insulated copper wire
2 paper clips
1 D-size battery
Rubber band
Blu-tack
2 bar magnets
Sticky tape
6 Wind the copper wire into coils, leaving the ends
sticking straight out. These ends will fit into the
loops of the paper clips. When you are happy with
your coil and you’ve checked that it sits easily in the
paper clip loops, tape up the coil to hold it together.
Sit it in the loops ready for start-up.
7 To start your motor, bring the north and south poles
of the two bar magnets close to the sides of the coil.
The coil may need a kick-start to get it running.
Method
1 Assemble the kit motor according to the instructions.
Connect it to a DC power supply on the correct
voltage and switch it on.
2 To assemble a motor from the everyday objects,
strip the ends of the length of copper wire.
3 Unwind the paper clips to create two roughly straight
pieces with small loops in them.
Results
Demonstrate your two motors to your teacher. Make any
recommended adjustments to your motors.
4 Hold the straight parts of the two paper clips at
either end of the battery and place the rubber
band around them to hold them against the battery
terminals.
5 Place the battery on the bench and secure it with
a piece of Blu-tack on either side. The paper clips
should point straight up and the loops should be
approximately level with each other.
Discussion
• When looking for similarities between the two
motors, look at what both motors have in common in
terms of any parts or design features. For example,
both motors need a power source but both motors
use different power sources. So this can be a
similarity and a difference. Is there an advantage of
one source over the other?
• When looking for differences, look beyond what you
actually see. Consider how each motor is turned on
or off. Is there an advantage of one method over the
other? Think of as many other differences as you
can. As a suggestion, think about the number of
coils and the number of turns in each coil, the type
of magnets used, reversal of operation, noise, sparks
and stability.
• Also consider any difficulties you might have had in
building your motors or in getting them to run.
• Construct a table to compare the similarities and
differences.
Michael Faraday and the electric motor
Albert Einstein rated Michael Faraday
as equal in genius to Galileo and
Newton, yet Faraday was a poorly
educated bookbinder who developed an
interest in science by reading the books
he was working on. Gaining
employment with Sir Humphrey Davy,
an eminent English chemist, his
scientific work was mainly in the areas
of chemistry and physics. In chemistry,
he discovered the chemical benzene,
which is nowadays used in plastics,
insecticides, medical drugs and
In 1831, Faraday began a series of
crucial experiments that have had a
far-reaching impact on our modern
lives. In one experiment, he wrapped
two insulated coils of wire around an
iron ring and found that, upon passing
a current through one coil, a momentary
current was induced in the other coil.
He also experimented with various
Generating
electricity
If a wire is connected to a sensitive
ammeter, called a galvanometer, and
the wire is moved rapidly up and down
between the poles of a strong horseshoe
magnet, a current will flow in the wire
and will be registered on the
galvanometer. This effect happens even
when no power is supplied to the
circuit. It doesn’t require electricity
because it generates, or induces,
electricity. This process is known
as electromagnetic induction.
This effect happens because the
magnetic field exerts a force on the
moving electrons inside the wire that
pushes the electrons along the wire.
This flow of electrons constitutes an
electric current. When the wire is
pushed in the opposite direction, the
electrons are pushed along the wire
in the other direction and the current
is, therefore, reversed. This
reversing current is known as an
alternating current (AC). The same
effect is achieved if the wire is held
still and the magnet is moved up and
down. Michael Faraday discovered
that the motion must result in a
‘cutting’ of the imaginary magnetic
field lines that run from the north
pole to the south pole of the magnet.
ways of producing electricity from
magnetism and with designs of electric
motors, and in the area of electrolysis.
His main discovery, which eventually
became known as Faraday’s Law, states
that a magnetic field changing in time
creates a proportional electromotive
force. Faraday’s inventions have formed
the foundation of the electric motor
technology we use today. Due to both
his discoveries and inventiveness, the
many uses of electricity have become
major features of our modern-day lives.
detergents. He also worked on an early
version of the well-known Bunsen
burner. His best work, however, was in
the physics area of electromagnetism.
The voltage driving the current can be
increased by:
Moving wire
S
N
• increasing the speed of the
movement
• cutting the field lines at right angles
• using a bundle of wires rather than a
single wire.
0
G
Induced current
†† Fig 12.13 When the movement of a wire cuts
through a magnetic field, an electric current is
induced.
The generator
A more efficient way of generating
electricity is to wrap one long wire into
a coil and to rotate it in a magnetic field.
This is the reverse operation to an
electric motor. In fact, if a simple motor
is disconnected from the power and
practivity 12.2
Generating alternating current
What you need: Solenoid, galvanometer,
bar magnet
b a south end of a bar magnet is
pushed into the coil
1Connect a solenoid coil to a
galvanometer. The galvanometer has
a needle in the centre. The needle can
swing either to the left or the right to
indicate the alternating direction of
current flow in the circuit. Try each
of the following and observe in which
direction the needle moves:
c the magnet is held stationary
inside the coil
d the magnet is vibrated up and
down inside the coil
e the coil is vibrated up and down
over the magnet
a a north end of a bar magnet is
pushed into the coil
2 Try speeding up the movement and
observe what happens.
• Explain why each of the above
produced the result it did.
chapter twelve: electromagnetism and electronics
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PART three: energy
298
Permanent
magnet
Coil (armature)
Permanent
magnet
N
S
Carbon
brushes
Split ring
commutator
a
a
b
†† Fig 12.14 Both (a) an electric fan and (b) a wind generator use a coil and
magnetism. In the electric fan, electricity makes the coil spin, which spins the
fan blades. In the wind generator, the wind makes the fan blades spin, which
generates electricity.
Permanent
magnet
b
†† Fig 12.15 (a) A dynamo has a split ring commutator and its output is DC. (b) A
bicycle dynamo uses the same generating principle but a magnet usually rotates
while the coil remains stationary.
Coil (armature)
Permanent
magnet
N
S
Carbon
brushes
a
Slip
rings
b
†† Fig 12.16 (a) An alternator has slip rings and its output is AC. (b) An alternator generates electricity to charge a car’s battery and keep the electrical systems of a car running.
practivity 12.3
2Identify the coil of wire, the magnets, the brushes, the split ring
commutator and the two slip rings.
• Can you see that as the coil rotates, it cuts up and down
across the magnetic field lines between the poles of the
magnets?
3 The coil generates AC. The split ring commutator converts the
AC in the coil to a DC output. Connect a galvanometer across
the brushes that rub against the commutator. Turn the handle
slowly to verify that the output is DC.
4Connect a galvanometer across the brushes that rub against the
slip rings. Turn the handle slowly to verify that the output is AC.
Creating electricity using
a generator
What you need: Model generator, galvanometer, power
supply, connecting wires
1Inspect the demonstration generator.
5 Remove the galvanometer and turn the handle rapidly so that
the light globe glows.
• What energy conversion occurs in a generator?
6 Remove the drive belt and connect the DC terminals of a power
supply to the brushes that rub against the commutator. Turn
on the power. The coil should spin. This shows that the device
is now acting as an electric motor. Try to increase the speed of
rotation.
made to spin, it generates electricity.
The faster the coil is spun and the
greater the number of turns in the coil,
the greater the voltage that is generated.
were not connected electrically.
However, the second current only lasts
for a split second while the switch is
being pressed on or off.
Such a device is known as a generator,
although the names dynamo and
alternator are also used. A dynamo
generates direct current (DC), which
flows in one direction only. An
alternator generates AC. The difference
between the two is in the connections
from the coil. The coil itself generates
AC as it spins. If a split ring commutator
is attached to the ends of the coil, the
output will be DC (Fig 12.15a). If two slip
rings are attached to the ends of the coil,
the output will still be AC (Fig 12.16a).
To continually change the current,
AC can be used in the first, or primary,
coil. This will generate, or induce,
an AC in the secondary coil. The AC
flowing in the primary coil has a
vibrating magnetic field due to the
Transforming current
The movement of a wire or coil in a
magnetic field or vice versa is not the
only way to generate electricity. One
of Michael Faraday’s experiments
involved the current being turned on
or off in one coil that was linked to a
second coil via an iron core. A
momentary current flowed in the
second coil even though the two coils
regular reversal of the current
direction. This vibrating magnetic
field is carried through the iron core to
where it can vibrate across the turns of
the secondary coil, thus generating AC.
If the number of turns in each of the
coils is different, the current can be
increased or decreased.
You probably use a device like this
every day. It is called a transformer
Iron core
AC input
voltage, Vp
Primary coil
AC
output
voltage,
Vs
Secondary coil
†† Fig 12.17 A transformer consists of primary and
secondary coils, wrapped around the same iron
core. The AC input voltage is supplied to the
primary coil and an AC output voltage is induced
in the secondary coil.
†† Fig 12.18 A laptop computer has a transformer
in its power cord.
chapter twelve: electromagnetism and electronics
299
and it transforms, or changes, the
current and voltage to different values.
Your mobile phone charger, for
example, plugs into 240 volts and
converts this voltage to the lower
amount needed to recharge the lithium
battery in your phone. If the primary
coil has a greater number of turns, the
voltage is lower in the secondary coil
and it is called a step-down
transformer. If the secondary coil has a
greater number of turns, the voltage is
greater in the secondary coil and it is
called a step-up transformer.
A lot of electrical devices operate on
less than 240 volts but it is convenient
to plug them into a powerpoint. If the
cord has a box as part of it, that’s the
transformer. It is often labelled as an
AC adaptor. Often the transformer
also converts AC to DC by using a
rectifier circuit in addition to the
transformer. Rectifiers will be looked
at in section 12.2.
Build a simple transformer
E XPE RIME NT 12 . 3
PART three: energy
300
Aim
To use coils of wire and an iron core and arrange them
in order to produce an efficient transformer
4 Record your arrangements in a results table
with sketch diagrams and comments about the
brightness of the light globe.
Equipment
Wide solenoid
Thin solenoid
Iron core
AC/DC power supply
6 V light globe
Globe holder
Connecting wires
Discussion
• Which arrangement of the two coils worked the
best? Why?
• Which voltage type, AC or DC, worked the best?
Why?
• You may have noticed that there is no electricity
going directly into the light globe. How, then, does
the light globe get the electricity it needs to shine?
Method
1 Connect the thin coil to the power supply set on
6 V AC.
2 Connect the wide coil to the light globe.
3 Try at least four different arrangements of the two
coils with and without the iron core and connected
to the DC terminals and the AC terminals.
Extension
Examine your mobile phone charger transformer or any
other transformer you have at home. What information
does it have written on it? Copy this into your workbook
and highlight any information that relates to the current
or voltage.
What do you know about generating electricity?
1 What energy conversion occurs in a generator?
2 Which of the following will generate electricity?
a a bar magnet is moved into a coil
b a bar magnet is moved away from a coil
c a bar magnet is held still inside a coil
d a coil is lowered over an upright bar magnet
e a current is turned on in a coil that is above another coil
f an iron core is inserted into a coil
3 What are the similarities and differences between a wind
generator and an electric fan?
4Could the motors you built in Experiment 12.2 be used to
generate electricity? How?
5Is a mobile phone charger a step-up or step-down
transformer? Explain your answer.
6Describe the process that occurs inside a mobile
phone charger when it is plugged in and connected to
your phone.
Big Ideas
12.1
How are magnetism and electricity linked?
Remember
Analyse
1Copy and complete the following paragraph with the
most appropriate word or phrase.
4In Faraday’s double coil experiment, why didn’t the current flow in the second coil when the switch of the first coil was left on?
A
of iron or
is able to attract objects made
. A magnet has two
, north and
. A currentcarrying wire has a
magnetic
around it. The direction of the
field is given by the
rule. In an
electromagnet, many
of wire are
wrapped around an iron
.
Understand
2 Are the following statements true or false? If the
answer is false, rewrite the statement to make it true.
a The direction of a magnetic field is the way a south
pole of a compass will point.
b The fingers of the right-hand grip rule indicate the
magnetic field direction for a solenoid.
c The split ring commutator in a DC electric motor
reverses the current direction in the armature every
half-turn to keep it rotating in the same direction.
d A changing magnetic field generates no current.
e An alternator is used to generate a current that
flows in one direction only.
f
A step-down transformer has more turns in the
primary coil than in the secondary coil.
Evaluate
5 The Synchrotron (see Chapter 13) is a huge scientific
instrument that accelerates electrons to very high
speeds. The electrons are forced to move in a
circular path by large electromagnets. The direction
of travel of an electron is the reverse to the direction
of conventional current given by the right-hand slap
rule. Work out the arrangement of the north and south
magnetic poles and the direction of the electron
beam if the electrons are to be pushed to the right.
Research this phenomenon to see if your arrangement
is correct. If you were incorrect, what error(s) of
judgement did you make?
Create
6 The amount of electricity generated from spinning
a dynamo depends on the magnetic field strength;
the size of the coil and the rotation speed. Design
an experiment to investigate each of these three
variables. Write an aim, list of equipment, hypothesis
and method. You don’t need to carry out the
experiment. Carefully explain in your method section
how each variable is tested, one at a time, while the
other variables remain constant.
Apply
3Give the energy conversion that occurs in each of the
following:
a an electromagnet
b an alternator
c a dynamo
d an electric motor
>>CONNECTING IDEAS<<
7 Both electric motors and analogue meters, such as voltmeters, ammeters and galvanometers, operate on the motor effect.
Current flow in a coil in a magnetic field produces force but in a meter the needle moves and stops rather than spinning. How
might the internal workings of such a meter be similar to, and how might they differ from, that of an electric motor?
chapter twelve: electromagnetism and electronics
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PART three: energy
302
12.2
What happens in electronic
circuits?
In Chapter 8 of Big Ideas Science Book 2, you were introduced to some
basic electronic components, such as the light globe, resistor, battery,
switch and fuse, as well as measuring devices, such as the ammeter
and voltmeter. Can you recall what each device does in an electric circuit?
In this section we will investigate many more devices and how they are
combined in circuits to perform particular functions.
What is resistance?
You may recall from earlier study of
electricity that electrical circuits are
all about energy. A source of energy,
such as a battery or power supply, gives
electrons electrical energy and, as the
electrons flow around a circuit, this
electrical energy is converted into
other forms of energy, such as light in
a light globe or heat in a toaster or an
oven. The flow of electrons is called
the current, although we always refer
to conventional current, which
historically flows in the opposite
direction to the flow of electrons. The
electrical energy possessed by each
unit of charge (remember, electrons are
negatively charged) is the electrical
potential, which is more commonly
called voltage.
how difficult it is for charged particles
to move through it. Electrons collide
with the atoms in the wires and the
various other components of a circuit
and some of their electrical energy is
converted into heat. Most connecting
wires are thick and made from good
conductors. Consequently, they have
very low resistance and hardly any
energy is lost by the electrons.
However, in special wire like that
used in a toaster, a lot of energy is lost
by the electrons and converted into
heat—so much that the wire glows red
hot and browns our toast.
Resistors are placed deliberately in
circuits to control or reduce the size
of the current. Most of the standard
resistors you will use are made up of
a ceramic–carbon mixture inside
moulded plastic cases with coloured
bands to identify their value.
Resistance is measured in SI units
called ohms. The symbol for an ohm
is Ω (Greek capital letter omega). A
kilo-ohm (kΩ) is 1000 ohms and a
megaohm (MΩ) is 1 000 000 ohms.
Resistors that obey Ohm’s Law and
always maintain a constant resistance
are known as ohmic conductors. Other
types of resistors, such as lightdependent resistors, are variable and
are called non-ohmic conductors. Other
variable resistors are also available.
How much current flows in a circuit is
determined by the resistance of the
circuit. As you learnt in Chapter 8 of
Big Ideas Science Book 2, the electrical
resistance of a material is a measure of
vce
In Unit 1 of VCE Physics you will study
electric circuits in the topic Electricity.
In Unit 3 of VCE Physics you will study
electronic components in detail in the
topic Electronics and Photonics.
a
b
†† Fig 12.19 Many types of resistors are available. (a) The resistance of carbon resistors is indicated by the coloured
bands on their plastic case. (b) The resistance of a light-dependent resistor (LDR) varies depending on the
brightness of the light shining on it. This makes LDRs useful in sunset sensors that control automatic lighting
circuits, like street lights or security lights.
Carbon resistors typically have four colourcoded bands on their case. These bands
are part of a code that allows you to work
out their approximate value and tolerance.
The fourth band is the tolerance band,
which gives you an indication of how
accurate the resistor is. A gold band as the
fourth band means a 5% accuracy, a silver
band means 10% accuracy and no fourth
band means 20% accuracy. The lower the
percentage, the more accurate the resistor
should be.
To read the three other bands, start at
the other end to the tolerance band. The
first two bands form a two digit number
according to their colour (see Table 12.1).
The third band tells you how many zeros
to put after the number.
• The third band is also red, so this
means 2 zeros need to be added to
the number. The number is now 6200.
†† Table 12.1 Resistor colour codes
• Resistor values are always coded in
ohms, so the value of this resistor is
6200 ohms.
Black
0
Brown
1
Red
2
Orange
3
2nd digit
Yellow
4
Green
5
Blue
6
Violet
7
Grey
8
White
9
Multiplier
Tolerance
• Next, read the other three bands.
The first band is blue, so it has a
value of 6.
Thermistors are temperaturedependent resistors. They are useful in
thermostats for controlling cooling or
refrigeration units. A rheostat is also a
variable resistor. It has a slider on the
top, which is moved along to change
the resistance. A potentiometer is a
type of variable resistor with a dial
that rotates. A light dimmer is a
potentiometer, as is the temperature
control on an oven. Nichrome wire is
an alloy of nickel and chromium. Its
high melting point and high resistivity
make it suitable as a heating element,
such as in hair dryers, electric ovens
and toasters.
Value
1st digit
Look at the resistor in Figure 12.20.
What does its code mean?
• First, read the tolerance band. As
this is gold, the resistor has a 5%
accuracy.
• The second band is red, so it has a
value of 2. The number is now 62.
Colour
†† Fig 12.20 What is the values of this resistor?
Ohm’s Law
Georg Ohm, a German physicist,
discovered the connection between
voltage, current and resistance. Ohm
found that the voltage drop across a
fixed value resistor will always be
directly proportional to the current
through the resistor. This relationship
is known as Ohm’s Law and is written
as:
R=V
I
although it is more commonly written
as V = IR. The relationship can also be
expressed in a triangle.
The triangle is a good memory tool to help
you work out three formulas from the one
diagram.
V
I
R
†† Fig 12.21 The Ohm’s law triangle.
chapter twelve: electromagnetism and electronics
SKILLS LAB: Understanding resistor colour codes
TIP
SKILLS LAB
303
SKILLS LAB
SKILLS LAB
Maths Lab: Using Ohm’s Law to find resistance
EXAMPLE
3Substitute the numbers:
Find the value of a resistor that has a voltage drop of 6 V
across it when a current of 50 mA flows through it.
1
2
First, check the units: 6 V is in volts and so can be
used as is; 50 mA (milliamps) needs to be converted
to amps. ‘Milli’ means 10−3 or 0.001, so 50 mA =
50 × 10−3 or 0.050 A.
Write the correct formula from the Ohm’s Law triangle:
R=V
I
R=
6
0.050
4Do the calculation: 6 ÷ 0.050 = 120 Ω.
YOUR TURN
This Law can also be used to work out the voltage drop or
the current. What is the voltage drop across a resistor with
a value of 180 Ω and a current of 50 mA?
ANSWER
9V
PART three: energy
304
SKILLS LAB: Using a multimeter
Most multimeters can measure DC and
AC voltages and currents as well as
resistance. Multimeters usually have
a central circular dial that is turned to
indicate the quantity to be measured.
Remember to turn it off once you are
finished because otherwise the battery
will go flat!
The multimeter usually comes with
two ‘test leads’ that are plugged into
the sockets. There is generally a 10 A
socket (which you probably won’t use),
one for most other measurements and
a ‘COM’ socket, standing for common.
Use the last two sockets for your
measurements.
Measuring DC voltages
• Turn the dial to the V section and
start with a high range (i.e. the high
numbers), typically 500 or 200.
• Touch the test leads to the ends of
the device you wish to measure.
(Note: Voltmeters are connected
across a device.)
• Then go down to lower numbers to
give an appropriate reading. If a ‘1’
appears on the screen, you have
gone too low, so go up to a higher
scale.
• Take your reading and switch the
circuit off again. Don’t waste power!
Measuring DC current
Measuring resistance
• Turn the dial to the A section and
start with a high range again,
typically 10 or 200 m (m for
milliamps, 200 mA = 0.2 A).
• Disconnect the device you wish to
measure from its circuit.
• Turn the power off in your circuit.
Break (disconnect) the circuit at an
appropriate place and connect the
multimeter. (Note: Ammeters are
connected in series, or in line, with
a device.)
• Switch the circuit back on. Move
down through the current settings
until you obtain a reading. Take your
reading and switch the multimeter
off again.
• With the dial on the highest
resistance range, typically 20 M
(M for mega), touch the test leads
to the ends of the device and move
down through the ranges until you
obtain an appropriate reading.
• Remember to check the scale. If you
are on the 20 K range and the screen
says 18.6, this means 18.6 kΩ or
18 600 Ω.
Ask your teacher if you are unsure
of any readings or how to obtain a
reading.
E XPE RIME NT 12 .4
Investigating Ohm’s Law
Aim
To investigate the voltage drop across and the current
flow through a resistor, and hence calculate an average
value of the resistance
Equipment
Power supply
2 multimeters (or an ammeter and a voltmeter)
10 Ω resistor
Three other resistors with masking tape over their
coloured bands
Connecting wires
3 Switch on the power supply, take the readings on
the two multimeters and switch the power off again.
4 Change the dial on the power supply to 4 V and
repeat step 3. Then change the dial to 6 V and
repeat.
5 Copy and complete the results table below.
Voltage (V)
Current (mA)
Volts ÷ amps
Method
1 Identify the 10 Ω resistor. It should be colour-coded
brown, black, black.
2 Connect the circuit as shown in Figure 12.22.
Use the DC terminals of the power supply and start
with the dial on 2 V. Turn the dials on the multimeters
to the most correct setting. Remember, one acts as
an ammeter and the other acts as a voltmeter.
6 Repeat the experiment for the other three resistors,
without reading their coloured bands.
7 Complete a results table for each of the three
masked resistors and calculate their resistance.
Remove the masking tape and determine their
resistance values from their coloured bands.
Discussion
• From your results table, what can you say about the
values in the last column?
• What quantity does the last column measure?
Power supply
switch
V
A
10 Ω
• For the three masked resistors, how close were
the values you obtained to those marked on their
coloured bands? Can you write the difference as a
percentage of the average?
• Which value—the one obtained by reading the
coloured bands or the one obtained from the volts
divided by amps value—gives the most accurate
measure of a resistor’s resistance? Explain your
answer.
†† Fig 12.22 Circuit set-up.
What do you know about resistance?
1 What happens to the electrical energy
carried by electrons as they flow
around an electric circuit?
4 Write the three equations obtained
from the Ohm’s Law triangle.
2Explain the difference between
conventional current and electron flow.
5Calculate the current flowing through
a 44 Ω resistor when it has a voltage
drop of 11 V across it.
3 Which quantity measures the electrical
energy carried by each unit of charge
in an electric circuit?
6Calculate the voltage drop across a
25 Ω resistor when a current of 50 mA
flows through it.
7Calculate the value of a resistor that
has a voltage drop of 8 V across it when
a current of 0.4 A flows through it.
8 What is the value of a resistor that has
three coloured bands of:
a red, white, black?
b yellow, green, red?
c brown, blue, orange?
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306
Diodes
A diode is a semiconductor device that
allows current to flow in one direction
only. Most diodes are made of specially
treated silicon. The circuit symbol of a
diode is shown in Figure 12.22. We can
think of the triangle as an arrow that
shows the direction that the diode
allows the conventional current to flow.
When a diode is connected correctly, it
allows current to be conducted through
it. This is called forward-biased
conduction. However, when the diode
is not connected correctly, it does not
conduct. This is called reverse-biased
conduction.
but most hairdryers contain a rectifier
circuit that converts the AC to DC
before it flows to the heating elements
and the fan motor.
Light-emitting diodes
A light-emitting diode (LED) is a special
type of diode that not only restricts
current flow to one direction only but it
also emits light of a particular colour.
Typically, the light from LEDs is either
one of the visible colours (commonly
red, yellow or green), infrared (IR) or
ultraviolet (UV). The remote controls of
televisions, VCRs and DVDs send their
messages via infrared LEDs. Red LEDs
are also widely used as indicator lights
on electrical equipment to show that
the power is on or to indicate a particular
setting. They are also finding increasing
applications in torches, garden and
vehicle lighting. In traffic lights, they
are replacing incandescent globes and
appear as dots of coloured light. LED
televisions are also being produced.
practivity 12.4
Lighting up LEDs
What you need: Power supply, red LED,
330 Ω resistor, 1 kΩ resistor
Diodes can only carry small currents,
much less than 1 amp. Bigger currents
produce too much heat, which would
destroy the diode, so diodes are almost
always connected in series with a
resistor.
1Connect a power supply set on 8 V
DC in series with a red LED and a
330 Ω resistor (orange, orange,
brown). If the LED doesn’t light up,
reverse its connections.
• Which leg of the LED (one leg is
longer than the other) needs to be
connected to the positive side of
the circuit for it to light up?
Silicon diodes are useful for converting
AC to DC. Such a device is called a
rectifier. A lot of electrical equipment
operates on DC instead of AC, but it is
convenient to plug them into the AC
powerpoints at home or school. A
hairdryer, for example, plugs into AC,
2 Try a larger resistor, such as 1 kΩ
(brown, black, red), in the circuit and
observe what effect it has.
• Why must a resistor be used in
this circuit?
†† Fig 12.24 As LEDs are more efficient, longer lasting
and use less power than light globes, their role is
shifting from being used as indicator lamps to other
wide-ranging applications.
3Draw a circuit diagram of your circuit
when the LED is lit up.
What do you know about diodes?
1 What is the role of a resistor
connected in series with a diode?
2Explain how to connect a diode in
forward bias and reverse bias.
3 Why would an electrical device like a
toaster need a rectifier?
†† Fig 12.23 The silver band on a diode matches the
line on the circuit symbol.
4Draw a circuit diagram consisting of
a 6 V DC power supply, a forwardbiased LED and a 100 Ω resistor all
connected in series. Add a voltmeter
to measure the voltage drop across
the LED.
5 A television remote control usually
has an infrared LED that converts
electrical energy into infrared light
energy. What sort of device must the
television set have to communicate
with the remote?
6 Many digital clocks use a sevensegment display. The seven sections
are lit by LEDs and can be individually
switched on or off to indicate the
digits 0–9. Look at a digital clock
and draw how the digits 0–9 can be
displayed using seven LEDs.
Transistors and integrated circuits
The invention of the transistor in 1947 heralded the dawn of
the electronic age. A transistor is similar in operation to two
diodes and it is made from the same material—silicon. The
legs of a transistor are known as the collector, base and emitter.
A transistor has two main functions. It can act as a switch,
even though it has no moving parts. In this role it can control
the functioning of many electronic circuits, including
computers. Its other function is to act as an amplifier. When a
†† Fig 12.25 Transistors come in different shapes and sizes. The miniaturisation
of transistors has revolutionised electronics and computing.
Capacitors
A capacitor is a device that can store
electric charges for short periods of
time. It usually consists of two metal
plates separated by an insulator. The
sheets can be rolled up into a compact
cylinder, which gives capacitors their
distinctive mini drink can shape.
The charging process causes negative
charge (electrons) to flow onto one of
the plates while it is taken away from
the other plate. This leaves the first
plate negatively charged and the other
plate positively charged. The stored
charge eventually leaks away, so it can
only be stored for a short period. It can
be discharged through a device such as
an LED.
The capacitance of a capacitor is
usually determined by the size of its
plates. The larger the plates, the more
small current flows into the base, a much larger current flows
from the collector to the emitter, giving an amplification of
the base signal. The transistor replaced larger devices, called
vacuum tubes or valves, that were used to amplify radio
signals. This made ‘transistor radios’ much more portable.
These days, many millions of semiconductor devices can be
printed onto wafers of silicon, called silicon chips. The
finished device is called an integrated circuit or microchip.
†† Fig 12.26 When it was introduced in March 1998, this operational amplifier,
which contains 50 transistors, was the world’s smallest integrated circuit. It
is used in phones, games and many other electronic devices. Even smaller
integrated circuits are now available.
charge it can store. This capacitance is
measured in SI units called farads
(after Michael Faraday), with the
symbol F. A millifarad (mF) is 0.001 F
(or 10−3 F) and a microfarad (µF) is
0.000 001 F (or 10−6 F).
in conjunction with diodes. The diodes
convert AC to DC and the capacitor
helps to smooth out the DC signal to
a reasonably constant amount.
Capacitors come in different forms. An
electrolytic capacitor must be
connected the right way around in a
circuit. One leg will be marked with a
plus (+) sign and must be connected to
the side of the DC circuit that goes back
to the positive terminal of the battery or
power supply.
Since capacitors take time to fill with
charge and to discharge, they are often
used in timing circuits, such as
toasters. They are also used to separate
an AC signal from a DC carrier signal as
they block DC signals but allow AC
signals to flow in a circuit. Their other
main job is as part of a rectifier circuit
†† Fig 12.27 Capacitors come in many different sizes.
chapter twelve: electromagnetism and electronics
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307
Investigating capacitors
E XPE RIME NT 12 . 5
PART three: energy
308
Aim
To investigate the operation of charging and discharging
a range of capacitors in conjunction with a range of
resistors
Equipment
Range of capacitors
Range of resistors
0–12 V power supply
Red LED
Connecting leads
Alligator clips
Method
1 Choose a capacitor and resistor and record their
values in a results table.
2 Connect the negative leg of the LED to the resistor.
Attach leads with alligator clips to the positive leg
of the LED and the other end of the resistor. This
is where the charged capacitor will be connected
shortly to form the discharge circuit.
3 Charge the capacitor by correctly connecting it for a
short time to the power supply set on 8 V DC.
4 Quickly disconnect the capacitor from the power
supply and connect it to the waiting discharge
circuit. Ensure that the positive leg of the capacitor
is connected to the positive leg of the LED. Observe
what happens when the connections are made.
5 Repeat the charging and discharging process for
different capacitors and resistors, changing one
component at a time.
Discussion
• What effect does a larger capacitor have on the
LED?
• What effect does a smaller resistor have on the LED?
• Why must step 4 of the method be performed
quickly?
• Write a suitable conclusion for this experiment and
have it checked by your teacher.
What do you know about
capacitors?
1How is a capacitor different from a diode?
2 Why does a capacitor have an insulator between its
plates?
3 A battery is connected to a capacitor. Explain what
happens to the electrons in the circuit over the period of
time it takes to fully charge the capacitor.
4 A camera uses a battery and a capacitor in conjunction
with the flash mechanism. Explain how these
components might work together.
5 Write 400 microfarads in farads.
6 Most modern cars have a time delay between when you
get in and close the car doors to when the interior light
fades and goes out. Draw a circuit diagram to show the
circuit that might control the interior light.
Big Ideas
12.2
What happens in electronic circuits?
Remember
1 Match each word with its meaning.
a resistance
the SI unit of capacitance
b thermistor
a device that allows current
to flow through it in one
direction only
cOhm’s Law
the SI unit of resistance
d potentiometer
a device that stores electric
charge
e diode
describes the relationship
between voltage and current
f light-emitting diode a variable resistor controlled
with a rotating dial
g capacitor
the ratio of voltage drop to
current flow through a device
h farad
a device whose resistance
varies with temperature
i
a diode that emits light
ohm
Understand
2 What do each of the following stand for?
a LED
b mF
c kΩ
Apply
3 What size resistor has the following coloured bands in
order?
a green, brown, black
b brown, yellow, red
capacitor is charged and switch A is off, this part of
the circuit will provide power to the LED when switch
B is pressed.
Analyse
6 What is the most likely purpose of the circuit in
Figure 12.28?
6 V AC
V
†† Fig 12.28
Evaluate
7Calculate the current flowing through a 30 Ω resistor
when it has a voltage drop of 12 V across it.
8Calculate the voltage drop across a 50 Ω resistor
when a current of 25 mA flows through it.
9Calculate the value of a resistor that has a voltage
drop of 18 V across it when a current of 0.3 A flows
through it.
4 What colour bands would a 7.9 MΩ resistor have?
Create
5Design a circuit using a DC power supply, resistor,
capacitor and switch A. The circuit is to charge up
the capacitor when switch A is pressed. Add to
your circuit an LED, resistor and switch B. When the
10 Create a poster that shows the circuit symbols of
all the electronic devices you have studied so far.
Include information on each device explaining what
it is used for.
>>CONNECTING IDEAS<<
11 Power lines carry electricity from power stations to the cities and towns. They experience a voltage loss along the lines
according to Ohm’s Law. How should the quantities of I and R change to minimise this voltage loss? How can this be done in
real life? What changes would have to happen to the power line system to achieve this?
chapter twelve: electromagnetism and electronics
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310
12.3
How do common electronic
gadgets work?
Electronic devices that we use every day incorporate various electronic
components combined into complex circuits. Some devices, such as
earphones, produce sound. Some produce light or use light to analyse
something, such as a barcode scanner. Others produce heat to cook our
food or dry and style our hair. Still others don’t do any of these functions but
help us in other ways to recharge the batteries that power our devices or store
or process information, such as in computers.
Producing sound
energy
Paper cone
Cylindrical
magnet
Making sound using
speakers
The most common type of speaker is
a moving coil or dynamic speaker. It
consists of a stationary permanent
magnet attached to the speaker frame.
The speaker cone, which is usually
made from paper, is attached to a coil
of wire, called the voice coil. The
electric current supplied to the speaker
varies in size and direction, following
the pattern of music or a person’s voice.
This electric current makes the voice
coil become an electromagnet. The
magnetism of the coil interacts with
the permanent magnet, sometimes
attracting and sometimes repelling.
This causes vibration and, since the
coil is attached to the speaker cone,
it too vibrates, sending out pressure
waves into the air, which we hear
as sound.
Speakers come in a range of sizes, from
the tiny earphones that come with
iPods or MP3 players to the huge
speaker systems used at concerts.
Earphones are simply a pair of tiny
speakers that connect to an audio
source. The music files stored on an
N
Coil
N
N
N S N
N
N
N N
break inside their plastic coating, the
speakers will stop working, so look
after the wires and don’t wrap them up
too tightly.
A mobile phone also uses a speaker to
produce the sound of a person’s voice or
the various ring tones and beeps that a
phone makes. Home phones use a
speaker too, as does a television, CD
system, radio and many other devices.
a
What do you know
about producing
sound energy?
1 What is sound energy?
b
†† Fig 12.29 (a) A speaker uses electromagnetism to
create sound waves. (b) The vibration of the cone in
the air produces the sound waves that we hear.
iPod are converted into electric
current, which is why an iPod needs a
battery. The wires carry the electric
current and the tiny speakers convert
the electrical energy into sound
energy. If one or both of the wires
2Draw a flow chart to illustrate how
a speaker converts electricity into
sound.
3 Make a list of all the things at home
and school that produce sound
energy. Do all of them use a
speaker?
4 If you have an old pair of
earphones that maybe don’t work
anymore, carefully take them
apart to see what’s inside. Draw
a labelled diagram to show your
results.
practivity 12.5
Make a buzzer
What you need: Power supply, tapping switch, narrow solenoid
coil, leg of a tripod (most unscrew), 1 hole rubber stopper to
fit over tripod leg and inside solenoid neck, empty can, tripod,
connecting wires, alligator clips
symbols. Use a short buzz for a dot and a longer buzz for
a dash. Leave a gap between letters. At the end of a word,
leave a longer gap before the next word.
1 Assemble the apparatus.
2Set the power supply on 12 V AC. Adjust the height of the
tripod leg above the closed end of the can until buzzing is
clearly heard when the switch is pressed.
• How is the sound produced?
• How is this demonstration similar to the operation of a
speaker? How is it different?
3 Find a copy of the Morse code on the Internet or in a book.
Try sending a word via Morse code using the tapping switch.
Let your partner listen to the message and record the
Using light energy
Operating gadgets
remotely
A television remote control uses light
energy to communicate with the
television set. In fact, most remote
controls use infrared light, which is the
invisible type of light usually associated
with heat. The remote control sends a
pulse of infrared light from an infrared
LED. This pulse represents a particular
code that corresponds to a command,
such as to change the channel or
†† Fig 12.30 A tapping switch.
increase the volume. An infrared
photodetector on the television receives
the light signal and converts it back into
electricity. The television’s
microprocessor then interprets the
signal and carries out the command.
Inside a remote control, an integrated
chip detects when a button is pressed
on the keyboard and converts it into a
digital code, a bit like Morse code. Each
button has a different code sequence.
The signal is amplified by a transistor
and sent along the conducting pathways
on the printed circuit board. The
printed circuit board is usually a thin
piece of green fibreglass that has
conducting paths etched onto it by
machines, in a similar way to a printer
printing ink onto a page. Each button
has a contact point underneath it. When
the button is pressed, it is like flicking a
switch, so the contacts join to complete
the circuit. The electrical signal is sent
to the LED and light is sent to the
television.
Other types of remotes, such as garage
door remotes and Bluetooth headsets,
use radio frequency signals. Instead of
sending out light, these remotes send
out a coded radio wave. A receiver on
the device picks up the signal in a
similar way to the television. Walkie
talkies, cordless home phones and
mobile phones also transmit using
radio waves.
Sensor circuits
†† Fig 12.31 A television remote control uses an
infrared LED to operate the television.
Light is also used in sensor circuits,
such as automatic doors and burglar
alarms. For automatic doors that use
light, the box above the doors sends out
a signal. When someone stands in front
of the doors, the signal is disturbed and
this opens the doors. In an alarm
system, the sensor activates the alarm
when the pattern of light in the room is
broken or disturbed.
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What do you know
about using light
energy?
1 What is the main difference
between a television remote
control and a garage door remote
control?
2How is pushing the volume up
button on a television remote
different from pushing the channel
up button?
3Draw a flow chart to show the
stages in operating a television
remote control, starting with
pushing a particular button and
ending with the television carrying
out the command.
4How does a printed circuit board
work?
5 Why must the remote control be
pointed at the television in order
for it to operate correctly?
Producing heat
energy
We use electrical appliances every day
to produce heat energy. Hairdryers,
toasters, heaters, ovens and jugs are
some of the most common.
Hairdryers
A hairdryer has two basic components:
a motor-driven fan and a heating element.
When plugged in and switched on,
current is supplied to the heating
element, which is usually bare nichrome
wire. The current also makes the fan
motor spin. The air flow from the fan is
directed over the heating element,
generating warm air, which flows out
of the barrel of the hairdryer. The motor
speed is determined by the current flow.
Low current will produce a low speed of
rotation and less air is pushed through
the hairdryer. With more current, the
motor speeds up.
The nichrome wire of the heating
element is an alloy of nickel and
chromium. It has a high resistance
that allows it to heat up, and it doesn’t
oxidise when heated, which makes it
very useful in toasters too. If a hairdryer
has different heat settings, flicking the
switch to low cuts off part of the circuit
supplying current to the heating element,
producing less available heat.
Toasters
Other heating devices, such as toasters,
also commonly use nichrome wire to
convert electrical energy into heat energy.
The nichrome wire creates infrared
radiation, which toasts the bread.
The toast is held down in a toaster by an
electromagnet. When the toast is pushed
†† Fig 12.32 A toaster uses nichrome wire wrapped
around mica to produce infrared radiation, which is
used to toast bread.
down using a bar, the bar presses a pair
of contacts on the circuit board—which
is usually made up of transistors,
capacitors and resistors—and current is
supplied to the nichrome wire to start
toasting the bread. The metal in the bar
is attracted by an electromagnet, so the
toast is held down.
The circuit acts as a timer. The capacitor
charges up through the resistor and
when it reaches a high enough voltage
across its plates, it cuts off the current to
the electromagnet, which releases the
toast. If a toaster has a variable heat
setting, it is most likely a variable
practivity 12.6
Make your own electric jug
What you need: Power supply,
approximately 70 cm of nichrome wire,
pencil, thermometer, 250 mL beaker,
heatproof mat, 2 connecting wires,
alligator clips
Do not allow the two alligator clips to
touch while the power is on.
1Coil the nichrome wire around the
pencil, leaving a 10 cm straight
section of wire at each end. Check
that the coil will fit into the beaker.
2Stand the beaker on the heatproof mat
and add 50 mL water to the beaker,
ensuring the nichrome coil is below
the water level.
Questions to consider:
3Connect the straight sections of
nichrome wire to a power supply set
on 12 V DC and switch on the power.
• Why must the two alligator clips not
be allowed to touch while the power
is on?
4 Put the thermometer in the beaker and
check the temperature. Leave the setup for 5 or 10 minutes and then check
the temperature again.
• Approximately how long did it take
for the water to get hot?
• What advantage does a coiled
heating element have over a straight
one?
• How could the speed of heating the
water be improved?
resistor. A larger resistance will mean
the capacitor takes longer to charge up
and the toast will be held down for
longer before being released.
compresses them, changing the
resistance. By connecting the carbon to
a power supply, the changing resistance
changes the current flowing through the
carbon, with the result that the current
matches the sound wave.
What do you know
about producing
heat energy?
Another common type of microphone
uses two metal plates that form a
capacitor (also known as a condenser).
Sound pressure waves cause one plate
of the capacitor to vibrate, which varies
the separation of the plates and hence
the capacity of the capacitor. This causes
charge to flow on and off the plates,
creating an alternating current that
matches the sound wave.
1If a hairdryer has a DC fan motor,
what other electrical circuitry must
it contain?
2 What are two properties of
nichrome wire that make it suitable
as a heating element?
3 Which electronic component might
control a variable heat setting on a
hairdryer?
4 List the electronic components
commonly found in toasters.
5 Draw a flow chart to illustrate the
operation of a toaster, starting with
when the toast is pushed down to
when it pops up.
Changing electrical
energy
An iPod is similar to a mobile phone in
that it has a very complex circuit board
with microelectronic components and
several chips, a liquid crystal display,
a touch-sensitive click wheel, a
rechargeable lithium battery and, of
course, a sophisticated hard drive for
file storage.
In Unit 4 of VCE Physics you may study several
different types of microphones in the detailed
study Sound.
Mobile phones
The inside of a mobile phone contains
several electronic devices: a
microphone, a speaker, a rechargeable
lithium battery, a keyboard with buttons
that work like those on a television
remote control, an antenna, a liquid
†† Fig 12.34 The internal components of an iPod.
What do you know
about changing
electrical energy?
1 Are the transformers that plug into
a powerpoint step-up or stepdown transformers? How do you
know?
Microphones
Sound is a pressure wave. As the
pressure hits the carbon granules it
iPods
vce
A lot of electronic devices don’t produce
sound or heat and don’t use light.
Instead, they convert the voltage and
current of the electrical energy to
a higher or lower amount. Transformers
have already been discussed on page 299.
A transformer converts AC to DC in a
mobile phone charger to recharge the
battery, to power a laptop computer and
to run a DC motor in a hairdryer.
The microphone in a mobile phone
converts the sound energy from our
voice into electrical energy, which can
then be coded and sent as a radio wave
signal. Different types of microphones
are available but the oldest and simplest
type uses carbon granules.
crystal display, a motor that causes
vibration, and a very complex circuit
board containing several silicon chips
that gives even an average mobile
phone an amazing processing and
storage ability.
2 A transformer will not work if DC is
connected to the primary coil. Why
not?
3 Compression of carbon granules in
a microphone is likely to reduce the
resistance. Why is this?
†† Fig 12.33 The
internal components
of a mobile phone.
4 Which components do a typical
mobile phone and an iPod have in
common?
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Big Ideas
12.3
How do common electronic gadgets work?
Remember
Analyse
1Name an electronic gadget that:
5 Figure 12.35 shows an electric bell. Explain how the
bell works when the switch is pressed.
a converts electricity into sound
b communicates with a television
c converts electricity into heat
d converts sound into electricity.
Switch
Pivot
Understand
Electromagnet
2 What is meant by each of the following terms?
a voice coil
b printed circuit board
c nichrome wire
d amperage
e photodetector
f infrared light
Spring
Contacts
Apply
3 a What sort of device is a mobile phone charger?
bIs it a step-up or step-down device?
cDraw a sketch diagram to show two sets of coils
and an iron core like those inside a mobile phone
charger. Illustrate which set would connect to the
powerpoint and which set would connect to your
phone.
d What other electronic component must a mobile
phone charger have?
4 A radio controlled car works with a remote control.
How do you think this remote control unit works?
Which electronic components is it likely to have inside
it? Which electronic components is the toy car likely to
have inside it?
Bell
Striker
†† Fig 12.35
Evaluate
6Investigate an electronic device you have at home
or in the classroom, such as a toaster or television.
Record all the voltage, current and power values
printed on its casing.
Create
7 Draw a flow chart to show how a typical microphone
works (either a carbon microphone or a condenser
microphone).
>>CONNECTING IDEAS<<
8Investigate how a computer mouse with a ball underneath it, called a track-ball mouse, works. The How Stuff Works website
at http://www.howstuffworks.com has a good article that is easy to understand and is a good place to start.
>>DIGGING DEEPER<<
Research
Review
Choose one of the following topics for a research project. A few guiding questions
have been provided but you should add more questions that you wish to investigate.
Present your report in a format of your own choosing.
GPS navigation
Bluetooth headsets
Global positioning system (GPS)
technology has produced satellite
navigation gadgets for use in cars and
boats. In a car, a GPS communicates
with satellites orbiting the Earth in order
to locate the driver’s car on the road.
The unit can give directions to other
locations stored in its database. What
electronic circuitry is needed for one of
these gadgets? How does the car unit
communicate with the satellites?
Connecting things electronically is
a complex task. Bluetooth headsets
use a wireless, automatic connection
method. How does this work? How
does it create a connection?
Airport security scanners
Most of us have walked through
metal detectors at the airport. These
devices work on the principal of
electromagnetic induction, but how do
they detect metal? And while you are
walking through one of these, what
happens to your hand luggage? How
does the X-ray scanner work?
Robotics
Robotics engineers are experts in
electronics, mechanics and computer
software. They design robots for
various applications, from industrial
robots that work on car assembly lines
to human-like versions, such as Asimo,
the humanoid robot made by Honda.
How is the control of a robot achieved?
What role do electronic sensors play?
What are the actuators that make the
robot move?
Reflect
Me
• What new science skills have you learnt in this chapter?
• What was the most surprising thing you found out about electronic gadgets?
• What was the most difficult aspect of this topic?
Key words
alternating
current (AC)
alternator
armature
brushes
capacitance
capacitor
condenser
cone
current
diode
direct current
(DC)
dynamo
electric motor
electrical
potential
electrical
resistance
electromagnet
electromagnetic
induction
farads
galvanometer
generator
heating element
infrared light
integrated circuit
light-emitting
diode (LED)
magnetic field
microchip
microphone
nichrome
non-ohmic
conductors
ohm
Ohm’s Law
ohmic conductors
photodetector
potentiometer
primary coil
printed circuit
board
radio frequency
rectifier
resistor
rheostat
right-hand grip
rule
right-hand slap
rule
secondary coil
silicon chip
slip ring
solenoid
speaker
split ring
commutator
step-down
transformer
step-up
transformer
thermistor
transformer
voice coil
voltage
• How has your understanding of the electronic gadgets in your life changed?
My world
• Why is it important to understand how electronic gadgets work?
• How has electronic technology made the world a smaller place?
My future
Test yourself
• In what ways do you think electronic gadgets will change your life in the future?
• What career paths could a study of electronics lead to?
Log onto www.oxfordbigideas.com
to do the student self-test and
revision activities.
chapter twelve: electromagnetism and electronics
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