Previously
From Page 220 Forces appear whenever two objects interact.
From Page 225 Unbalanced forces cause the motion of a body
to change.
Unit 6
Magnets
How do magnets work?
Magnets are interesting things – they can attract
or repel each other without touching.
It is very difficult to explain how magnets work,
but you can learn a lot about what magnets can do
without knowing a lot of advanced scientific ideas.
We use magnets every day without really thinking
about it. They are great for holding letters and
notes to metal fridge doors – but did you know
there is a magnet inside the fridge that helps keep
the door closed tightly?
In this Unit, you will learn:
about magnetic and non-magnetic materials;
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how magnets attract and repel each other;
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how we represent magnetic fields;
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how electromagnets work.
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Key words
attracts
compass
core
electromagnet
magnet
magnetic field
magnetic field lines
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magnetic material
magnetic pole
north pole
permanent magnet
repels
solenoid
south pole
Magnets
Which materials are magnetic?
A bar magnet is an example of a permanent magnet. It will remain magnetic for a
long time.
A bar magnet can attract pieces of iron and steel. Any material that is
attracted by a magnet is called a magnetic material. The table below
gives some examples.
How could you show that a
magnet’s attraction can work
through a non-magnetic material
– for example, through paper?
S
N
S
Magnetic materials
Non-magnetic materials
iron
copper
steel (most types)
stainless steel
nickel
aluminium
ferrite (used for fridge magnets)
paper
N
What are the forces between two magnets?
A bar magnet has two ends; one is its north pole, the other is its south pole.
If two magnets are placed so that their ends are close together as shown, they may
attract or repel.
A south pole attracts a north pole; a north pole attracts a south pole.
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N
N
S
S
N
N
S
S
A north pole repels another north pole; a south pole repels another south pole.
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S
S
N
N
N
N
S
S
We summarise this as ‘Like poles repel, unlike poles attract.’
‘Like poles’ means two of the same type. ‘Unlike poles’ means two of the opposite type.
The magnetic force between two poles is stronger if they are closer together.
Will two south poles attract or repel each other? Draw a
diagram to show this.
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Unit 6
What are the poles of a magnet?
If a bar magnet is hung so that it is free
to rotate, it will turn until one end points
towards the north – this end is called the
magnet’s north pole.
The other end points south, of course,
and is called the south pole. This is how
a compass works.
The magnet turns because it is in the Earth’s
magnetic field. There is a lot of iron and nickel
inside the Earth’s core and they make it behave
like a giant magnet, with one magnetic pole
close to each geographical pole.
A magnet’s north pole is
attracted towards the north.
What kind of magnetic pole
(north or south) must there
be in the north?
Something scientists don’t
know: does a north pole always
come with a south pole? Every
magnet we have discovered, no
matter how large or how small,
appears to have both poles
together. Some scientists
think that they may be
able to discover particles
that are just north poles
or south poles. But we
haven’t found any of these
‘magnetic monopoles’ yet.
If there was a magnetic
monopole that could move
freely near the Earth, what do
you think would happen to it?
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S
N
S
north
N
Magnets
What is a magnetic field?
A magnet can attract magnetic materials near to it, without touching them. We say that
there is a magnetic field around the magnet.
We can show up a magnet’s field using iron filings. We draw magnetic field lines to
represent the pattern.
S
N
The lines come out of the north pole and go into the south pole.
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The arrows on the lines show the direction of the field, from a north pole to a south
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pole. A compass will point in this direction.
Where the field lines are closer together, the field is stronger.
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The magnetic field of a bar magnet is strongest at its poles –
true or false? Explain how you can tell.
What is the field surrounding two magnets?
The magnetic field
surrounding two
magnets depends
on whether they
are attracting
or repelling.
S
N
S
N
S
N
N
S
Draw the magnetic field pattern for two south poles, repelling
each other.
PHYSICS 249
Unit 6
How are magnets and magnetic materials used?
Magnets and magnetic materials are used in:
compasses (for showing direction);
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fridge magnets;
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the stripe on a credit card that contains stored information;
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magnetic memories such as computer hard disk drives;
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loudspeakers, headphones and some microphones;
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electric motors and generators.
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The disc in a computer hard drive
is coated on both sides with a
magnetic material. This is the
drive’s memory. A pattern of tiny
magnetised areas is used to store
digital information.
List five places
where you might find
magnets in your home.
What is an electromagnet?
An electromagnet is another way of making a magnetic field.
A coil of wire is connected to a battery or a power supply.
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A current flows in the coil.
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A magnetic field appears all round the coil.
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One end of the coil is a north pole, the other
end is a south pole. The magnetic field of the
electromagnet is the same shape as the field of a
bar magnet.
coil
If the current is reversed, so that it passes
through the coil in the opposite direction, the
field will be reversed – the north pole will be at
the other end.
The coil of wire is sometimes called a solenoid.
It is usually made of copper wire – it doesn’t
have to be a magnetic material.
When the current is switched off, the coil stops
being magnetic. The field disappears.
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current
+
battery
–
Magnets
Electric current and magnetism are closely related. The connection between
electricity and magnetism was first discovered by Hans Christian Oersted in
1820. He noticed that, when he switched on an electric circuit, the needle of
his compass moved.
Oersted made his observation during a public lecture. The audience watched as
he repeated it several times to make sure that what he saw was really true.
Plan an investigation to test whether three different electrical
items are also magnetic when they are switched on and when
they are switched off.
How can an electromagnet be made stronger?
An electromagnet can be made stronger in three ways:
1. Use a coil with more turns of wire.
2. Put a bigger current into the coil.
3. Add an iron core to the coil.
Each of the electromagnets shown has the same current.
Put them in order, from strongest to weakest.
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Unit 6
How are electromagnets used?
Electromagnets have many uses because they can be
turned on and off easily, and because the direction of
the magnetic field can be reversed.
Electric bell – an electromagnet moves a striker
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backwards and forwards very quickly, producing the
ringing sound.
Electric motor – the electric current passing
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through the coil causes a rotor or axle to rotate and
generate turning forces or moments (see page 229).
Loudspeakers and headphones – the electric current
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passing through the coil causes a magnetic ‘piston’
to vibrate and produce sounds (see page 270).
Brain scanner – we can map the electric currents
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inside our brains using a person-sized electromagnet,
to show whether our brains are healthy.
Particle accelerators – we can ‘bend’ beams of
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electrically charged particles such as electrons and
protons, and speed them up or slow them down,
using huge electromagnets.
A scrapyard crane uses an
electromagnet to lift steel. The
steel falls off when the current is
switched off.
You need to remember that:
Some materials, such as iron, nickel and most types of steel, are magnetic.
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A magnet creates a magnetic field around itself.
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Magnetic field lines come out of the magnet’s north pole and go into its south pole.
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We can show the direction of magnetic field lines using a compass.
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Like poles repel; unlike poles attract.
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The Earth is a large magnet because its core contains iron and nickel.
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An electromagnet is a coil with an electric current flowing through it.
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The greater the current and the more turns on the coil, the stronger the electromagnet.
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An iron core makes an electromagnet stronger.
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Next time
Unit 7 In electricity, like charges repel; opposite charges attract (page 254).
Unit 8 An electric current in a circuit causes electromagnets to work or motors to turn on (page 261).
252 SMART SCIENCE