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>>BIG IDEAS<<

12

12.1

How do electronic gadgets work?

How are magnetism and electricity linked?

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.

Electromagnetism and electronics

Fig 12.2 Complex electronic circuits look confusing, but they are often composed of standard electronic devices such as resistors, capacitors, diodes and transistors.

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.

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12.3 How do common electronic gadgets work?

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.

1 silicon is a semiconductor material. What types of materials are most metals? What about most non-metals?

2 have you seen the first Terminator movie? A silicon chip is fundamental in this movie. Brainstorm with your classmates why this is so.

3 how has the digital revolution affected your life? how important is digital technology to you?

12.2

Fig 12.3 Since its invention in 2001, the iPod has revolutionised personal audio and entertainment.

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.

1 can 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?

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.

12.1

Conventionalcurrent flow

+ –

N S

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.

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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.

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Using electricity to create magnetic fieldsIn 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

WireCard Iron filings

Current-carryingwire

–

+

Fig 12.4 The magnetic field around a straight current-carrying wire is circular.

Field

Current

Fig 12.5 The right-hand grip rule. The way the fingers point around the wire gives the direction of the magnetic field.

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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.

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2.1

Creating magnetic fieldsAimTo investigate the magnetic field around a single wire and around a solenoid when connected to direct current (dc) and alternating current (Ac)

EquipmentAc/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?

DiscussionWhy 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.•

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What do you know about using electricity to create magnetic fields?

1 how 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.

4 how could the strength of the magnetic field around a solenoid be increased?

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Current

Right hand

Force(out of palm)

Field

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.

Using magnetic force to create an electric motorIf 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.

practivity 12.1

Observing magnetic forceWhat you need: Power supply, strong horseshoe magnet, connecting wires

1 set up the equipment as shown in Figure 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?•

4 set 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 .

Fig 12.8

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.

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

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rotation. The commutator does this job

by connecting to the opposite brush

after each half-turn (180° rotation) of

the coil. Figure 12.10 shows the same

coil turned over 180°. The red side is

now on the left and the green side is

on the right.

The commutator has also rotated 180°

and now connects to the opposite

brush, which has the effect of reversing

the direction of the current in the coil.

The direction of rotation is,

therefore, maintained.

Most electric motors are more

complicated than this simplified

example. They often have several

sets of coils all at slightly

different angles to each other,

and electromagnets are often used

instead of permanent magnets.

Conventionalcurrent, /

Conventionalcurrent, /Commutator

+ –

Coil(armature)

BrushBrush

Current

Current

Field

Field

Force

Force

N S

Fig 12.9 How an electric motor works.

Conventionalcurrent, /

Conventionalcurrent, /Commutator

Coil(armature)

BrushBrush

Current

Current

Field

Field

Force

Force

N S

+ –

Fig 12.10 The coil has now turned over 180°.

What do you know about using magnetic force to create an electric motor?

1 draw 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.

2 draw 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.

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

e dc power supply.

4 Will the electric motor shown in Figure 12.12 rotate clockwise or anticlockwise? Justify your answer.

5 What energy conversion occurs in an electric motor?

6 in an electric motor, what is the job of the:

a split ring commutator? b brushes?

c armature?

(1)

(2)

(3)

(4)

(5)N S

+

Fig 12.12

Fig 12.11 A fuel-cell car has an electric motor.

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2.2

Building an electric motorAimTo build two electric motors—one using everyday objects and the other using a kit—and compare their similarities and differences

Equipmentdc motor kit

2 m of insulated copper wire

2 paper clips

1 d-size battery

Rubber band

Blu-tack

2 bar magnets

sticky tape

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.

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.

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.

Resultsdemonstrate your two motors to your teacher. Make any recommended adjustments to your motors.

DiscussionWhen 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.

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0

Moving wire

Induced current

G

S N

Fig 12.13 When the movement of a wire cuts through a magnetic field, an electric current is induced.

Generating electricityIf 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.

The voltage driving the current can be

increased by:

increasing the speed of the •

movement

cutting the field lines at right angles•

using a bundle of wires rather than a •

single wire.

The generatorA 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 currentWhat you need: solenoid, galvanometer, bar magnet

1 connect 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:

a a north end of a bar magnet is pushed into the coil

b a south end of a bar magnet is pushed into the coil

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

2 Try speeding up the movement and observe what happens.

explain why each of the above •produced the result it did.

Michael Faraday and the electric motorz

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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

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.

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

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.

Permanentmagnet

Permanentmagnet

Coil (armature)

Slip rings

Carbonbrushes

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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.

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.

a

a

b

Permanentmagnet

Permanentmagnet

Coil (armature)

Split ringcommutator

Carbonbrushes

SN

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.

a

b

b

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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.

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).

Transforming currentThe 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

were not connected electrically. However, the second current only lasts for a split second while the switch is being pressed on or off.

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

2 identify 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.

4 connect a galvanometer across the brushes that rub against the slip rings. Turn the handle slowly to verify that the output is Ac.

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.

Secondary coil

AC outputvoltage,Vs

AC inputvoltage, Vp

Primary coil

Iron core

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.

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

practivity 12.3

Creating electricity using a generator

What you need: Model generator, galvanometer, power supply, connecting wires

1 inspect the demonstration generator.

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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?

4 could the motors you built in experiment 12.2 be used to generate electricity? how?

5 is a mobile phone charger a step-up or step-down transformer? explain your answer.

6 describe the process that occurs inside a mobile phone charger when it is plugged in and connected to your phone.

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2.3

Build a simple transformerAimTo use coils of wire and an iron core and arrange them in order to produce an efficient transformer

EquipmentWide solenoid

Thin solenoid

iron core

Ac/dc power supply

6 v light globe

globe holder

connecting wires

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.

4 Record your arrangements in a results table with sketch diagrams and comments about the brightness of the light globe.

DiscussionWhich 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?

Extensionexamine 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.

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.

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How are magnetism and electricity linked?12.1

>>CONNECTING IDEAS<<

Remember1 copy and complete the following paragraph with the

most appropriate word or phrase.

A is able to attract objects made of iron or . A magnet has two

, north and . A current-carrying 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 .

Understand2 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.

Apply3 give the energy conversion that occurs in each of the

following:

a an electromagnet

b an alternator

c a dynamo

d an electric motor

Analyse 4 in 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?

Evaluate5 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?

Create6 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.

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?

Big Ideas

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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 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

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 light-dependent resistors, are variable and are called non-ohmic conductors. Other variable resistors are also available.

vce

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.

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.

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SKILLS LAB: Understanding resistor colour codes

carbon resistors typically have four colour-coded 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.

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.

next, read the other three bands. •The first band is blue, so it has a value of 6.

The second band is red, so it has a •value of 2. The number is now 62.

The third band is also red, so this •means 2 zeros need to be added to the number. The number is now 6200.

Resistor values are always coded in •ohms, so the value of this resistor is 6200 ohms.

1st digit

2nd digit

Multiplier

Tolerance

Fig 12.20 What is the values of this resistor?

Thermistors are temperature-dependent 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.

ohm’s LawGeorg 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

although it is more commonly written

as V = IR. The relationship can also be

expressed in a triangle.

I R

V

I

Fig 12.21 The Ohm’s law triangle.

Table 12.1 Resistor colour codes

Colour Value

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Grey 8

White 9

The triangle is a good memory tool to help you work out three formulas from the one diagram.

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MAThS LAB: Using Ohm’s Law to find resistance

EXAMPLE

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 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.2 Write the correct formula from the ohm’s Law triangle:

R = V

3 substitute the numbers:

R = 6

4 do 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?

ANSWERI

0.050

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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 RESISTANCE

disconnect the device you wish to •measure from its circuit.

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.

MEASURING DC CURRENT

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).

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.

9 v

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2.4

Investigating Ohm’s LawAimTo investigate the voltage drop across and the current flow through a resistor, and hence calculate an average value of the resistance

EquipmentPower supply

2 multimeters (or an ammeter and a voltmeter)

10 Ω resistor

Three other resistors with masking tape over their coloured bands

connecting wires

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.

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.

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?•

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.

AV

Power supplyswitch

10 Ω

Fig 12.22 Circuit set-up.

Voltage (V) Current (mA) Volts ÷ amps

What do you know about resistance?

1 What happens to the electrical energy carried by electrons as they flow around an electric circuit?

2 explain the difference between conventional current and electron flow.

3 Which quantity measures the electrical energy carried by each unit of charge in an electric circuit?

4 Write the three equations obtained from the ohm’s Law triangle.

5 calculate the current flowing through a 44 Ω resistor when it has a voltage drop of 11 v across it.

6 calculate the voltage drop across a 25 Ω resistor when a current of 50 mA flows through it.

7 calculate 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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practivity 12.4

Lighting up LEDsWhat you need: Power supply, red Led, 330 Ω resistor, 1 kΩ resistor

1 connect 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?

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?

3 draw a circuit diagram of your circuit when the Led is lit up.

Fig 12.23 The silver band on a diode matches the line on the circuit symbol.

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.

DiodesA 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.

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.

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,

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 diodesA 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.

What do you know about diodes?

1 What is the role of a resistor connected in series with a diode?

2 explain how to connect a diode in forward bias and reverse bias.

3 Why would an electrical device like a toaster need a rectifier?

4 draw a circuit diagram consisting of a 6 v dc power supply, a forward-biased 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 seven-segment 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.

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CapacitorsA 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

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).

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

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.

Transistors and integrated circuitsz

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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.

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.

Fig 12.27 Capacitors come in many different sizes.

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.

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2.5

Investigating capacitorsAimTo investigate the operation of charging and discharging a range of capacitors in conjunction with a range of resistors

EquipmentRange 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.

DiscussionWhat 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?

1 how 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.

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What happens in electronic circuits?12.2

>>CONNECTING IDEAS<<

Remember1 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

c ohm’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 ohm a diode that emits light

Understand2 What do each of the following stand for?

a Led b mF c kΩ

Apply3 What size resistor has the following coloured bands in

order?

a green, brown, black

b brown, yellow, red

4 What colour bands would a 7.9 MΩ resistor have?

5 design 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

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?

Evaluate 7 calculate the current flowing through a 30 Ω resistor

when it has a voltage drop of 12 v across it.

8 calculate the voltage drop across a 50 Ω resistor when a current of 25 mA flows through it.

9 calculate 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.

Create10 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.

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?

6 V AC

V

Fig 12.28

Big Ideas

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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 energyMaking sound using speakersThe 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

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

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.

Cylindricalmagnet Coil

Paper cone

NN

N

N

NN

N

N S

N

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.

b

a

What do you know about producing sound energy?

1 What is sound energy?

2 draw 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.

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practivity 12.5

Make a buzzerWhat 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

1 Assemble the apparatus.

2 set 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

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.

Fig 12.30 A tapping switch.

Using light energy

operating gadgets remotelyA 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

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 circuitsLight 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.

Fig 12.31 A television remote control uses an infrared LED to operate the television.

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Producing heat energyWe use electrical appliances every day to produce heat energy. Hairdryers, toasters, heaters, ovens and jugs are some of the most common.

HairdryersA 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.

ToastersOther 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

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

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?

2 how is pushing the volume up button on a television remote different from pushing the channel up button?

3 draw 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.

4 how does a printed circuit board work?

5 Why must the remote control be pointed at the television in order for it to operate correctly?

Fig 12.32 A toaster uses nichrome wire wrapped around mica to produce infrared radiation, which is used to toast bread.

practivity 12.6

Make your own electric jugWhat 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.

1 coil 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.

2 stand the beaker on the heatproof mat and add 50 mL water to the beaker, ensuring the nichrome coil is below the water level.

3 connect the straight sections of nichrome wire to a power supply set on 12 v dc and switch on the power.

4 Put the thermometer in the beaker and check the temperature. Leave the set-up for 5 or 10 minutes and then check the temperature again.

Questions to consider:

What advantage does a coiled •heating element have over a straight one?

Why must the two alligator clips not •be allowed to touch while the power is on?

Approximately how long did it take •for the water to get hot?

how could the speed of heating the •water be improved?

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What do you know about producing heat energy?

1 if 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 energyA 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.

MicrophonesThe 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.

Sound is a pressure wave. As the pressure hits the carbon granules it

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.

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.

In Unit 4 of VCE Physics you may study several different types of microphones in the detailed study Sound.

Mobile phonesThe 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

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.

iPodsAn 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.

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Fig 12.33 The internal components of a mobile phone.

What do you know about changing electrical energy?

1 Are the transformers that plug into a powerpoint step-up or step-down transformers? how do you know?

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?

4 Which components do a typical mobile phone and an iPod have in common?

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.

Fig 12.34 The internal components of an iPod.

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How do common electronic gadgets work?12.3

>>CONNECTING IDEAS<<

Remember1 name an electronic gadget that:

a converts electricity into sound

b communicates with a television

c converts electricity into heat

d converts sound into electricity.

Understand2 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

Apply3 a What sort of device is a mobile phone charger?

b is it a step-up or step-down device?

c draw 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?

Analyse5 Figure 12.35 shows an electric bell. explain how the

bell works when the switch is pressed.

Evaluate6 investigate 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.

Create7 draw a flow chart to show how a typical microphone

works (either a carbon microphone or a condenser microphone).

8 investigate 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.

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>>DIGGING DEEPER<<Research

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.

Review

Key words

gPS navigationglobal 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?

Airport security scannersMost 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?

Bluetooth headsetsconnecting things electronically is a complex task. Bluetooth headsets use a wireless, automatic connection method. how does this work? how does it create a connection?

RoboticsRobotics 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

MeWhat 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?•

how has your understanding of the electronic gadgets in your life changed?•

My worldWhy is it important to understand how electronic gadgets work?•

how has electronic technology made the world a smaller place?•

My futurein what ways do you think electronic gadgets will change your life in the future?•

What career paths could a study of electronics lead to?•

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

Test yourself

Log onto www.oxfordbigideas.com to do the student self-test and revision activities.