MODULE mod7A mod7B mod7C mod7D mod7E mod7F mod7G mod7H mod7I mod7J mod7K mod7L mod8A mod8B mod8C mod8D mod8E mod8F mod8G mod8H mod8I mod8J mod8K mod8L mod9A mod9B mod9C mod9D mod9E mod9F mod9G mod9H mod9I mod9J mod9K mod9L 8ja1
8Ja1 Magnets and magnetism
Name _____________________________ Class ____________
1 Write these materials in the correct columns in the table.
wood iron plastic aluminium copper steel nickel paper
Magnetic materials
Non-magnetic materials
2 a What does attract mean when we are talking about magnets?
______________________________________________________________________
b What does repel mean?
3 Write true or false next to each of these sentences.
a Magnets only attract magnetic materials. _________________
b Magnets can attract and repel other magnets. _________________
c Magnets repel non-magnetic materials. _________________
d Magnetism goes through paper. _________________
e Magnetism goes through iron. _________________
f The two ends of a magnet are called the east and the west poles. _________________
4 a If you put two north poles together, will they attract or repel? _________________
b If you put two south poles together, will they attract or repel? _________________
c If you put a north and a south pole together, will they attract
or repel? _________________
[ knowledge ]
8Ja2 Wordsearch - with a difference!
B
P
Q
U
T
S
O
Z
Y
C
E
L
F
N
G
D
R
M
A
V
H
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I
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1 There are eight words connected with magnetism in this wordsearch.
Find the words, and write them on the lines below. (Hint: The words all go left to right or top to bottom.)
2 Now write a clue for each word.
Word Clue
a ______________________ ___________________________________________________
b ______________________ ___________________________________________________
c ______________________ ___________________________________________________
d ______________________ ___________________________________________________
e ______________________ ___________________________________________________
f ______________________ ___________________________________________________
g ______________________ ___________________________________________________
h ______________________ ___________________________________________________
[ literacy, knowledge ]
8Ja3 What is a magnet?
Magnetism
Magnetism is a property of the atoms in iron, nickel and cobalt. Little groups of atoms act like tiny magnets. Each little group of atoms is called a domain.
A normal piece of iron does not seem to be magnetic, because all the domains are magnetised in different directions. They cancel each other out.
When a piece of iron is magnetised all the domains line up. They do not cancel each other out any more.
Making magnets
You can magnetise a piece of iron by stroking it. You always have to stroke in the same direction so that all the domains line up. You can also magnetise a piece of iron by leaving it next to another magnet for a
while. Eventually, the domains move around so they are lined up.
Removing magnetism
You can't stop a magnet being magnetic by cutting it up! If you break a magnet in two, the two halves would both be magnets, each with a north pole and a south pole. This is because all the domains are still lined up.
To destroy a magnet you have to get the domains to point in different directions. There are two ways of doing this.
- If you hammer a magnet, the vibrations let some of the atoms move around a little inside the metal. The domains can move around, and end up pointing in different directions.
- You can also remove magnetism by heating the magnet.
1 Which materials can magnets be made from?
2 What is special about the atoms in these materials?
3 What is a domain?
4 Why doesn't a normal piece of iron act like a magnet?
5 Describe two ways of making a magnet.
6 Can you ever have just a north pole of a magnet on its own? Explain your answer.
7 Explain why heating a magnet can remove its magnetism. (Hint: You will need to think what happens to atoms in any solid material when the material is heated.)
8 Why should you not drop magnets on the floor?
8Jb1 Compasses
Usman and Sally were making compasses. First of all they tried to magnetise a needle.
Usman tried it like this:
Sally tried it like this:
1 Whose method will work best? _________________________________________________
Explain your answer. ________________________________________________________
__________________________________________________________________________
2 Fill in the gaps in these sentences using words from the box.
One end of a ________________ will point north if the magnet is free to ________________.
This end of the magnet is called the ________________ ________________ pole, or just the
north ________________ . The other end of the magnet is called the ________________
pole, or the ________________ ________________ pole.
magnet north-seeking pole south south-seeking turn
3 Usman decided to stick his magnetised needle to a cork floating in a bowl of water. He had a steel bowl and a glass bowl.
a Why did Usman need to float his needle in a bowl of water?
b Should he use the steel bowl or the glass bowl?
Explain your answer. ____________________________________________________
8Jb2 Magnetic pigeons
Some people keep homing pigeons for sport. Birds are taken hundreds of miles away from their home in a lorry, and then they are all released together. The owner of the first bird to fly back to its own loft wins the pigeon race.
How do all these birds find their way? Some scientists think that the birds use the angle of the Sun to tell them which way to go. Some people think that the birds use the Earth's magnetism.
German scientists tried an experiment to find out if pigeons used magnetism or the Sun. They hatched
three sets of pigeons. Group A grew up normally, and could see the Sun all day. The other two groups only saw the Sun in the afternoons. The birds were taken a long way from home, and released on a sunny morning.
- The birds in Group A flew straight home.
- Group B birds, which had never seen the angle of the Sun in the morning, also flew home normally.
- Group C birds had small magnets glued to their backs. These magnets were strong enough to stop the pigeons detecting the Earth's magnetism. The birds in this group could not find their way back home.
1 How does a pigeon race work?
2 There are two things that pigeons might use to find their way home. What are they?
3 The German scientists thought that the pigeons in Group B were not using the Sun to find their way home.
a Why did they think this?
b What could they have been using instead?
4 How did the Group C pigeons show that pigeons normally use the Earth's magnetism to find their way?
5 Some birds and other kinds of animals migrate, or travel long distances each year. Research the migrations of one of these animals:
- swallows - geese - whales - lemmings - salmon
- Monarch butterflies - wildebeest
Find out where the animals or birds live, when they migrate, and why they migrate. You could also try to find out how they know where to go.
[ literacy, knowledge, research ]
8Jb3 True north and magnetic north
Maps are usually drawn with true north at the top. True north is the direction that points towards the North Pole. A compass needle does not point in the same direction as north on the map. It points to the north magnetic pole. We say that the compass points to magnetic north. The angle between magnetic north and true north is called the magnetic variation.
The North Pole and the north magnetic pole are not in the same place.
If you are using a map and compass to walk in the countryside, you need to adjust the reading on your compass to allow for magnetic variation.
The north magnetic pole moves around. It moves by about 15 km every year.
Orienteering is a sport in which runners have to find their way from point to point as fast as possible. Orienteers use special maps which are printed with magnetic north at the top.
1 What is the difference between true north and magnetic north?
2 Why don't compass needles point to true north?
3 What does magnetic variation mean?
4 What would happen if walkers did not adjust their compass readings to account for magnetic variation?
5 a What is special about orienteering maps?
b Why do you think orienteering maps are printed this way?
6 Imagine that you have been orienteering for the first time. Write a letter to a friend telling them what you had to do.
8Jc1 Magnetic fields
A magnetic field is the space around a magnet where it has an effect. The magnetic field of a bar magnet will make compass needles point in different directions.
Prediction
1 Copy this diagram, and draw in the directions that the compass needles will point. One has been done for you.
Now test your prediction by plotting the magnetic field.
Apparatus
- Bar magnet - Plotting compass
- Sheet of paper - Pencil
Method
1 Put the magnet in the middle of the piece of paper and draw around it. Draw nine dots around the north end of the magnet, like this:
2 Do not move the magnet while you are doing this practical work.
Put a plotting compass on the paper so that its tail is over one of the dots. Draw a dot on the paper near to the point of the compass.
3 Join the two dots and mark the line with an arrow. Then move the compass so that its tail is over your new dot. Make another dot next to the point.
4 Repeat step 3 until you come to the edge of the paper.
5 Now repeat steps 2 to 4 for the other dots that you drew around one end of the magnet.
The arrows on your lines show which way a north pole would move. They point away from the north pole of the magnet. The magnetic field goes from north to south.
[ observing ]
8Jc2 More magnetic fields
When two magnets are close to each other, their magnetic fields affect each other. You can investigate the shape and direction of the fields using a plotting compass.
- Two bar magnets - Paper and pencil - Plotting compass
You are going to investigate this arrangement first.
A X
1 Copy the diagram into your book. Leave some space above and below it.
2 Predict what the field will look like between the two magnets. Remember that the direction of the magnetic field is the direction that a north pole would take. Draw lines on your diagram to show your prediction.
Now place the magnets on a piece of paper and draw around them. Draw some dots around the end of the magnet marked X on the diagram, and use the plotting compass to plot the magnetic field.
Results
3 Make another copy of the diagram in your book, and this time draw the results you obtained with the plotting compass.
4 How accurate was your prediction?
5 Now do the same thing with these arrangements. Try to predict what you will find each time, then check your prediction using a plotting compass.
[ predicting, observing ]
8Jc3 Magnetic field questions 1
1 This diagram shows half of the magnetic field of a bar magnet. Carefully draw in the other half of the field.
Which way does the magnetic field go? Tick the correct box.
From the north pole to the south pole of the magnet.
From the south pole to the north pole of the magnet.
2 Fill in the gaps in these sentences, using words from the box.
A magnetic _______________ is the space around a _______________ where it can affect
magnetic _______________ . You can use _______________ _______________ to find the
shape of a _______________ field.
If you put a _______________ in a magnetic field, it will point towards the
_______________ pole of the magnet.
The Earth has a magnetic field. If there are no bar _______________ near, compasses will
point _______________ .
compass field iron filings magnetic magnet
magnets materials north south
8Jc4 Magnetic field questions 2
This diagram shows the magnetic field of a bar magnet. The lines show the direction that the north pole of another magnet would move if it was near the magnet.
1 This diagram shows part of the magnetic field between two magnets if their north poles are close together.
a Copy the diagram, and finish drawing the magnetic field lines. (Hint: The dotted lines are lines of symmetry to help you to draw the rest of the magnetic field).
b Add arrows to your drawing to show which way a north pole would move.
c Why would a north pole move away from the magnet at point A?
d Why do you think the field line bends at B?
2 How would the magnetic field between two magnets be different if two south poles were close together?
3 This diagram shows part of the magnetic field between two magnets if a north and a south pole are close together.
a Copy the diagram, and finish drawing the magnetic field lines.
c Why do you think the field lines are a different shape to the ones you drew in question 1?
[ knowledge, numeracy ]
8Jc5 More about compasses
Dip
Did you know that if you bought a compass in Australia it would not be the same as one you could buy in Britain?
The Earth's magnetic field is not parallel to the surface of the Earth. The angle between the horizontal and the direction of the magnetic field is called the dip.
A magnet suspended in the Earth's magnetic field would line up with the field.
Compass needles are just small magnets. Compasses that you can buy in the UK are weighted at one end of the needle so that the needle does not tilt.
1 What is dip?
2 Will the dip be the same everywhere in the world? Explain your answer.
3 Which end of a British compass needle will be weighted? Explain your answer.
4 How will an Australian compass be different to a British one? Use diagrams to help you to explain your answer.
Deviation
Any magnetic material will have an effect on the Earth's magnetic field. A compass used near a piece of magnetic material might not point north. The difference between magnetic north and the direction the compass is pointing is called deviation.
Deviation is not usually a problem for people using compasses for walking or orienteering, unless they are standing close to something made from iron or steel. Deviation can be a problem in ships. Sometimes deviation is corrected by placing small magnets or pieces of steel around the compass to cancel out the magnetic effects of the rest of the ship. Sometimes the navigator has a table of corrections to be made to the compass readings to allow for the effects of magnetic materials near the compass.
5 What is deviation?
6 a Why don't walkers usually have a problem with deviation?
b When might deviation be a problem for a walker trying to use a compass?
7 Why can deviation be a problem in ships?
8 Describe two ways of correcting for deviation in a ship.
9 What is the difference between deviation and variation? (Hint: You may need to look at your answers to worksheet 8Jb/3.)
[ knowledge, literacy ]
8Jd1 Making strong electromagnets 1
How can you make a strong electromagnet?
There are three things that you could change to alter the strength of the electromagnet:
- the number of turns of wire
- the current flowing
- the core
I think that if I have ______________ (more/fewer) turns of wire the electromagnet will be
______________ (stronger/weaker).
I think that if there is ______________ (more/less) current flowing the electromagnet will be
I think the electromagnet will be stronger if the core is made from ______________ .
This is because __________________________________________ .
Here is a diagram of the apparatus you will use. Label the diagram using words from the
apparatus list.
- Insulated wire
- Connecting wires
- Power pack
- Two crocodile clips
- Paper clips
- Iron rod or nail
- Glass rod
- Wooden rod or pencil
Test A: Number of turns of wire
To make this a fair test I must keep the _______________ and the _______________ the same.
1 Make an electromagnet with 10 turns of wire and an iron core.
2 Switch on the electricity and see how many paper clips the electromagnet picks up. Switch off again and write your result in the table.
3 Repeat steps 1 and 2 for 20, 30, 40 and 50 turns of wire. Keep the same setting on the power pack each time.
Recording your results
Number of turns
Number of paper clips picked up
10
20
30
40
50
Considering your results/conclusion
When there were more turns of wire, the electromagnet picked up _______________ (more/less)
paper clips. An electromagnet is _______________ (stronger/weaker) with a lot of turns of wire.
My prediction was _______________ (right/wrong)
Test B: Current
You can change the current by changing the voltage. A higher voltage will give you a higher current. Your teacher will tell you which voltages to use.
1 Make an electromagnet with 20 turns of wire and an iron core.
2 Switch the voltage to 1 V and switch on the power pack.
3 Find out how many paper clips the electromagnet will pick up. Switch off again and write your results in the table.
4 Repeat steps 2 and 3 for the other voltages.
Voltage
The electromagnet is strongest when the voltage and current are _______________ (highest/lowest).
My prediction was_______________ (right/wrong).
Test C: Core material
Planning
1 In your book, describe how you would investigate what happens if you change the material the core is made from.
2 Write down how you will make sure your test is fair.
3 Show your plan to your teacher before you start.
4 Draw a table in your book to record your results.
Considering your results/conclusions
5 Write down what you have found out.
6 Describe how you could make the strongest electromagnet possible from school equipment.
[ predicting, observing, considering ]
8Jd2 A reed relay circuit
You are going to build this circuit to see how a reed relay works.
- Reed switch
- Switch
- Crocodile clips
- Cell
- Bulb
- Pencil
1 Make a coil from the insulated wire by winding it round a pencil. Take the pencil out of the coil.
2 Connect the coil of wire to the cell and the switch, like this:
3 Put the reed switch into the middle of the coil of wire.
4 Connect the reed switch to a bulb and a power pack. Set the power pack to 12 volts.
5 Switch the power pack on. The bulb should not light up.
6 Now press the switch connected to the coil of wire (switch A). The bulb should come on.
1 If switch A is not being pressed, why doesn't the bulb light up when you turn the power pack on? (Hint: Look carefully at the circuit with the power pack in it.)
2 What happens to the coil of wire when switch A is pressed?
3 What happens to the reed switch when switch A is pressed?
4 Explain why the bulb lights up when you press switch A.
[ knowledge, observing ]
8Jd3 Making strong electromagnets 2
Which factors affect the strength of an electromagnet?
(You may not need all of this apparatus - it depends what you decide to investigate.)
- Insulated wire - Connecting wires
- Power pack - Two crocodile clips
- Paper clips - Iron rod or nail
- Glass rod - Wooden rod or pencil
1 Which factors could affect the strength of an electromagnet?
2 Which will you investigate?
3 How will you find out how strong your electromagnet is?
4 What will you have to keep the same to make your test fair?
5 How can you make sure your results are reliable? (Hint: How many times do you need to do each part of the experiment?)
6 Write a method to describe how you will carry out your investigation. Show your plan to your teacher before you start.
7 Write down what you think will happen. Explain why you think so.
8 Make a table to record your results.
9 Show your plan to your teacher, then carry out your investigation.
10 Plot a graph to show your results.
11 What have you found out from your investigation?
12 Are there any results that do not fit the pattern? If so, what do you think went wrong?
Evaluation
13 If you could do this investigation again, could you improve your method?
If you have time, investigate one of the other variables that you wrote about in question 1.
[ planning, observing, presenting, considering, evaluating ]
8Jd4 Changeover relays
Here is another kind of reed switch which has two contacts inside it. Reed switches can be used to make relays that are operated by magnets.
This reed switch can be used in two different ways. If a and c are connected it is a normally open relay. The relay closes and switches the light on if a magnet is held next to the relay.
If b and c are connected it is a normally closed relay. The bulb is normally on, and goes off if a magnet is put near the relay.
A coil of wire with a current flowing through it can be used instead of the magnet to move the iron reed.
1 Choose one of these problems:
A You need a burglar alarm to make a noise if someone opens a door or window.
B You want to turn on the light inside a car when the car door is opened.
C You want to make a buzzer that will let a blind person know when they have filled a kettle full of water.
D You need to switch a tumble dryer off automatically when the door is opened (you can use a small motor or a buzzer to represent the tumble dryer).
2 Now design a circuit to solve the problem you have chosen.
- Write a list of the apparatus you will need.
- Draw a circuit diagram and explain how your circuit works.
- When you have designed your circuit, show your plan to your teacher before you build your circuit and test it.
3 If you have time, try solving one of the other problems.
[ knowledge, planning ]
8Jd5 Relays help sheet
A - Burglar alarm
- You could start by making a model door from card and sticky tape.
- You will need a magnet fastened to the door, and a changeover relay fastened to the door frame.
- When the door is closed, the magnet should be next to the relay.
- You must connect the relay so that the buzzer is off when the door is closed.
You may need this apparatus: power pack or cells, connecting wires, crocodile clips, magnet, changeover reed switch, buzzer, card, scissors, sticky tape.
B - Car door
- You must connect the relay so that the light is off when the door is closed.
You may need this apparatus: power pack or cells, connecting wires, crocodile clips, magnet, changeover reed switch, bulb, card, scissors, sticky tape.
C - Water level buzzer
- You could use a beaker as a kettle, and fasten a relay to the outside of the beaker.
- You could switch the relay on by floating a small magnet on the water in the beaker.
- You must connect the relay so that the buzzer is off when the water level is low, and on when the magnet reaches the relay.
- You can adjust the level at which the buzzer goes off by moving the relay.
You may need this apparatus: power pack or cells, connecting wires, crocodile clips, magnet, changeover reed switch, buzzer, card, scissors, sticky tape, piece of wood or polystyrene, beaker.
D - Tumble dryer switch
- You could start by making a model tumble dryer door from card and sticky tape.
- You must connect the relay so that the motor is on when the door is closed.
You may need this apparatus: power pack or cells, connecting wires, crocodile clips, magnet, changeover reed switch, motor or buzzer, card, scissors, sticky tape.
8Jd6 Electric bells
1 Use words from the box to label the diagram.
armature cell
contacts electromagnet
gong pivot switch
2 Use words from the box below to fill in the gaps in these sentences. You may need to use some words more than once.
When someone presses the _______________ , an electric current can flow around the
_______________. When the current flows in the _______________ ______
_______________ it becomes a magnet and attracts the _______________ .
When the armature moves towards the ______________________ the end of it hits the
_______________ and makes a _______________ . Now that the armature has
_______________, there is a _______________ between the contacts. This means that the
_______________ cannot flow around the circuit. The ______________________ stops
being magnetic, and the _______________ springs back again.
When the armature has sprung back the contacts _______________ . A current can flow again,
so the electromagnet attracts the_______________ again. The armature keeps springing
_______________and forwards as long as someone has their finger on the switch.
armature backwards circuit coil of wire
current electromagnet gap gong
moved sound switch touch
8Jd7 Circuit breakers
Sometimes if something goes wrong with a piece of electrical equipment, a very high current flows. This can be very dangerous.
This is a fuse from the main electricity box in a house. It has a piece of wire in it that melts if too much current flows through it. When the wire has melted there is a gap in the circuit, and no more current can flow. When the fault has been repaired, a new piece of fuse wire must be put in before a current can flow into the house again.
Circuit breakers can be used instead of fuses. Some circuit breakers look like this inside.
Circuit breakers are used in houses for safety.
Circuit breakers use a kind of relay. A circuit breaker can be used to switch off the current if it gets too high. When the fault has been repaired the circuit breaker can be reset by moving a switch. A circuit breaker is sometimes called a trip switch.
1 What is a fuse?
2 What happens to a fuse if too much electricity flows through it?
3 Why are fuses used?
4 Look at the diagram of the circuit breaker.
a What happens to the steel cylinder if a large current flows through the coil?
b What happens to the circuit?
5 How could you change the design so that the circuit breaker 'trips' at a lower current? (Hint: Think how to make an electromagnet stronger.)
6 What are the advantages of a circuit breaker compared to a fuse?
8Jd8 Safer circuits
Electricity is very useful, but it can also be dangerous if electrical equipment goes wrong. If something goes wrong inside a machine like an electric drill, the electricity may run through the person using it and give them an electric shock.
Circuit breakers are designed to switch off the current if too much current is flowing. If a high current flows through the person then a circuit breaker would switch it off. If the current flowing through the person is low, then the circuit breaker would not switch it off. This means that the person could still get an electric shock.
A residual current device switches the current off if only a little current is going the wrong way. The current to an electrical machine flows through the live wire to the machine, and flows back through the neutral wire.
When the machine is working properly, the current is the same in the live and neutral wires.
If something has gone wrong and some current is flowing through a person, there will be less current in the neutral wire than in the live wire.
A residual current device.
1 What do circuit breakers do?
2 Why could you still get an electric shock if there was a circuit breaker in the circuit?
3 What are the names of the two wires that the current flows through?
4 Look carefully at the diagram of the residual current device.
Describe the path the current takes, starting with the live wire.
5 What will happen to the two coils of wire when a current is flowing through them?
6 If the current is exactly the same in both coils, what will happen to the iron bar? Explain your answer.
7 If there is a fault in the drill, and some of the current starts to flow through the person instead of through the neutral wire, there will be less current flowing through the neutral coil than through the live coil.
What can you say about the sizes of the magnetic forces produced by the two coils now?
8 What will happen to the iron bar?
9 What will happen to the electricity supply to the drill?
8Je1 Key facts cards
____________________________________
_____________________________________
attract block magnetic materials
recycling sorting lifting doors
attract north poles repel south
field magnetic space direction
DO not make a double sided photocopy of this worksheet
compass Earth field north
coils core current
electromagnet stronger
armature bell contact
current electromagnet
relay safety high voltage
low voltage electromagnet
[ knowledge, revision ]
8Je2 Electromagnets 1
1 Use words from the box to label the diagram and fill in the gaps in the sentences. You may need to use some of the words more than once.
bar magnet
coil of wire
field iron
iron core magnetic
power pack
An electric ______________ flowing through a ______________ ___ ______________ makes
a ______________ field around the wire. A coil of wire with a ______________ flowing through
it is called an __________________ . An ______________ core inside the coil makes the
______________ the magnetic ______________ of an electromagnet is similar to the
______________ field of a ______________ .
2 a This diagram shows half of the magnetic field of an electromagnet. Carefully draw in the other half of the field.
b Write N and S next to the correct ends of the electromagnet.
3 You can make an electromagnet stronger by:
- using __________________ (fewer/more) coils of wire
- using a __________________ (higher/lower) electric current
- using a core made from a __________________ (non-magnetic/magnetic) material.
[ literacy, knowledge]
8Je3 Electromagnets 2
1 What is an electromagnet?
2 Describe three ways to make an electromagnet weaker.
3 Draw the shape of the magnetic field around an electromagnet.
This diagram shows a home made current detector. If part of a circuit is connected to the drawing pins, the compass needle will show whether or not a current is flowing.
4 Which way will the needle point if there is no current?
5 What will happen if there is a current in the wire?
6 How could you change the design of the current detector to detect smaller currents? Explain your answer. (Hint: Think about how to make an electromagnet stronger.)
8Je4 Recording sound 1
Many people listen to music that has been recorded onto cassettes.
A microphone converts sound into electrical signals.
The electrical signals are recorded on magnetic tape.
A loudspeaker converts the electrical signals back into sound.
The microphone converts sound energy into electrical energy. Different sounds are converted into different patterns of electrical signals. If the singer is in a concert, these signals can be sent to a loudspeaker so that everyone can hear them. The signals can also be recorded onto magnetic tape.
The tape inside the cassette is a thin strip of plastic coated with a thin layer of tiny magnetic particles. The head inside the recorder is a small electromagnet. The electrical signals go through the coil of wire in the electromagnet. The magnetic field from the electromagnet magnetises bits of the tape as it moves past.
When the music is played, the tape moves past the head. The electromagnet in the head detects the magnetism and converts it into electrical signals that are sent to the loudspeaker. The loudspeaker converts the electrical signals into sound.
Expensive tape recorders have separate heads for recording and playing sound.
1 What does a microphone do?
2 What does a loudspeaker do?
3 What is the tape in a cassette made from?
4 What is the head inside a cassette recorder?
5 What happens to the tape when music is being recorded?
6 Copy these sentences and fill in the gaps:
When you play a ______________ , a motor in the cassette player moves the tape past the
______________. There is an ______________ in the head, which detects the
______________ patterns on the tape. The head converts these ______________ into
electrical ______________ . The electrical signals are sent to a ______________ , which
converts them into sound.
7 Why should you never put a strong magnet near cassettes?
8Je5 Recording sound 2
A microphone converts sound energy into electrical energy. Different sounds are converted into different patterns of electrical signals. These signals can be recorded onto magnetic tape. When the tape is played the sounds are converted back into electrical signals, and a loudspeaker turns these signals into sound.
A magnetic tape is a thin layer of plastic with a coating of tiny magnetic particles. The motor in the cassette player moves the tape at a steady speed past an erase head, a recording head and a playback head.
The erase head is only used when music is being recorded on the tape. A strong alternating current flows through the head, and this causes a strong alternating magnetic field in the gap. Any magnetic patterns that were already on the tape are erased, and the tape is ready for new sounds to be recorded.
The tape then goes past the recording head. The electrical signals from a microphone flow through the coil. The particles on the tape are magnetised in different directions, making a magnetic pattern on the tape.
When someone is playing the tape, the erase and record heads are not used. The playback head is near the tape. The magnetic patterns on the tape magnetise the ring, and the changing magnetic
The heads are small electromagnets.
field in the coil of wire induces a current in the wire. The strength and direction of the current depends on the magnetic pattern on the tape. This electric current goes to a loudspeaker, which converts it into sound.
In cheap cassette players, one head can do the job of recording and playback.
1 What do microphones and loudspeakers do?
2 What is the tape in a cassette made from?
3 What happens to the tape when music is being recorded?
4 How are the magnetic patterns on the tape converted back into sound? (There are two things you should describe.)
5 Why should you never put a strong magnet near cassettes?
6 Stereo sound has two different lines of magnetic patterns recorded on the tape, one for each loudspeaker.
a How many sets of heads does a stereo player need? Explain your answer.
b How many tracks of sound are there on a stereo tape? (Remember that you can turn a tape over to play the other side.)
8Je6 Demagnetising magnets
You can magnetise a piece of iron by putting it inside the coil of an electromagnet. The magnetic field inside the coil makes all the domains line up, and the iron becomes magnetised.
This shows the iron before it was magnetised.
1 Draw a similar diagram to show the iron after it has been magnetised.
Power packs can be used to give two kinds of current. Direct current is what you normally use in school, when the current always flows in the same direction. Alternating current changes direction 100 times each second, so the current goes backwards and forwards. If you use alternating current in an electromagnet the direction of the magnetic field changes every time the direction of the current changes.
There are two ways of demagnetising a piece of iron, for example a nail, using an electromagnet and alternating current. One way is to put the nail in the coil, and
then gradually reduce the size of the current.
2 What will happen to the domains in the iron when the alternating current is at full strength?
3 What will happen when the current is smaller? (Hint: Do you think all the domains will be affected?)
4 Explain how this method can demagnetise a magnet.
The other method is to put the iron in the coil with the current switched on, then gradually pull the iron out while the alternating current is still flowing.
5 Explain how this method works. Use diagrams showing what happens to the domains to help you explain. (Hint: Think about what happens to the domains inside the coil and outside the coil when the iron is part way out.)
8J Summary Sheets
Magnets and electromagnets
Magnetism is a non-contact force. Magnets attract magnetic materials. Iron, nickel and cobalt are magnetic materials. Mixtures, like steel, that include a magnetic material will also be attracted to a magnet. Other metals, like aluminium, are not magnetic and will not be attracted to a magnet. Iron oxide is a compound that is a magnetic material. It is used to make video and music cassettes and computer discs. Magnetic materials can also block magnetism.
You can make a magnet from a piece of iron or steel.
- The two ends of a bar magnet are called the north seeking pole and the south seeking pole or north pole and south pole for short.
- A north pole and a south pole attract each other.
- Two north poles or two south poles will repel each other.
- The space around a magnet where it has an effect is called its magnetic field.
This is the shape of the magnetic field of a bar magnet.
You can find the shape of the magnetic field using iron filings or using a plotting compass.
The Earth has a magnetic field. A compass is a small magnet that always points north. But magnetic materials placed near a compass can change the direction that it points.
Magnets can be used to sort iron and aluminium cans for recycling. Only the iron cans are attracted to the magnet. Magnets can also be used for holding fridge doors shut, and in compasses that sailors or walkers use.
A wire with electricity flowing through it has a magnetic field around it. An electromagnet is a coil of wire with an electric current flowing through it.
You can make an electromagnet stronger by:
- increasing the number of coils of wire
- increasing the size of the current (by increasing the voltage)
- using an iron core.
Electromagnets can be used for lifting things. They are also used in electric bells, relays and in video and music recording.
Electromagnets are used to make bells work.
A reed switch has two thin pieces of iron inside it. If a magnet is held near the switch, the pieces of iron are magnetised and touch each other. A reed switch can also be switched on using an electromagnet. Any switch that is worked by electricity is called a relay.
Relays are used to make things safer. For example, the starter motor in a car uses a high current and needs thick wires for the current to flow through. A relay is used in a car so that the driver does not have to touch any part of the circuit that has a high current.
8J Target Sheet
Topic
Targets
Before the unit
I have learned this
I have revised this
8Ja
1
Know which materials are magnetic.
2
know some properties of magnetic materials.
3
Know which materials will block magnetism.
4
Know the rules for magnets attracting and repelling.
8Jb
Know the names of the two ends of a magnet.
Know which way a compass needle points.
Know what magnetic compasses are used for.
Know how to make a magnetic compass.
8Jc
Know what a magnetic field is.
Know the shape of a magnetic field around a bar magnet.
Know the direction of a magnetic field.
Know about the Earth's magnetic field.
8Jd
Know what an electromagnet is.
Know how to make an electromagnet stronger.
Know how an electric bell works.
Know why relays are used in car starter motor circuits.
8Je
Know what causes a magnetic field around a wire.
Know the shape of the magnetic field around an electromagnet.
Know why a core can make an electromagnet stronger.
Know the shape of the magnetic field around a straight wire.
8J Word Sheets
Word sheets that include new words from the 'Focus on:' pages are available on the Exploring Science website.
8Ja - Magnetic attraction
Word
Pronunciation
Meaning
attract
Two things pulling towards each other.
cobalt
cO-balt
A metal that is a magnetic material.
iron
magnet
Something that can attract magnetic materials.
magnetic materials
Materials that are attracted to a magnet; iron, cobalt, nickel and steel are all magnetic materials.
magnetism
A non-contact force.
nickel
north pole
One end of a magnet. This ends points north if the magnet can move.
repel
Push away.
south pole
One end of a magnet.
steel
A mixture made mainly from iron; it is a magnetic material.
8Jb - Pointing north
A straight magnet, shaped like a small bar.
compass
A magnetised piece of metal that can swing around - it points north.
north-seeking pole
The end of a magnet that points north if the magnet can move freely. Often just called the north pole.
south-seeking pole
The end of a magnet that points south if the magnet can move freely. Often just called the south pole.
8Jc - Force field
iron filings
f-eye-lings
Tiny pieces of iron that are sometimes used to find the shape of a magnetic
field.
magnetic field
The space around a magnet where it can affect magnetic materials or other
magnets.
north magnetic pole
The place on the Earth where compasses point (it is not in the same place as
the North Pole marked on maps).
plotting compass
A small compass used for finding the direction of a magnetic field.
8Jd - Sounds magnetic/Keep your distance
armature
The iron part of a relay that moves when electricity is flowing in the solenoid
(or electromagnet).
electromagnet
A coil of wire with electricity flowing in it. An electromagnet has a magnetic
field like a bar magnet.
make and break
The switch in an electric bell that opens and closes. It is operated by an
switch
electromagnet that breaks the circuit whenever it is switched on.
permanent magnet
A magnet that keeps its magnetism - it does not depend on electricity.
reed switch
A switch made from two thin pieces of metal, which closes when it is in a
magnetic field.
relay
A switch that is switched on and off by electricity.
solenoid
A coil of wire.
8Je - Coils and cores
core
A solid bar inside an electromagnet - usually made of iron.