A history of magnetism
Magnetic materials
Which of the materials below are magnetic?
wood
wood
Magnetic or non-magnetic?
Can magnetism be blocked?
A fridge magnet may be coated in plastic but will stick
to a metal fridge.
Can magnetic forces act through any material?
1. Attach a paperclip to a piece of thread
and tape the other end of the thread
to a work bench.
2. Hold a magnet above the paperclip to
check that the paperclip is attracted.
3. Try inserting thin sheets of
different materials between
the magnet and the paperclip.
Note the effects of each material.
What is a magnetic field?
The region around a magnet where it has a magnetic
effect is called its magnetic field.
When a magnetic material
is placed in a magnetic field
it will experience a force.
The iron filings feel the
effect of the magnetic field
and line up along the
direction of the forces in
this region.
magnetic field
Shape of a magnetic field
Magnetic field lines
The shape of a magnetic field can be shown by drawing
magnetic field lines. These always point from the north
pole of a magnet to the south pole, along the direction of the
magnetic forces in each region. Where is the magnetic field
strongest?
strongest
field at
poles
The closer together
the magnetic field
lines, the stronger
the magnetic field.
weakest field further away
from poles
Forces between magnets experiment
Investigating magnetic fields
Viewing magnetic fields: N poles together
Bring the north poles of two bar magnets together.
What happens to the magnets?
Next, bring the two north poles as close to each other as
possible and place a piece of paper on top of the magnets.
1. Carefully scatter iron filings onto the paper.
2. Draw the pattern created by the iron filings.
Magnetic field pattern: N poles together
What do you notice about the pattern of the lines of force
in the region between the two north poles?
Viewing magnetic fields: N and S poles together
Bring the north and south poles of two bar magnets together.
What happens to the magnets?
Next, put the north and south poles close to each other,
without letting them touch, and place a piece of paper on top.
1. Carefully scatter iron filings onto the paper.
2. Draw the pattern created by the iron filings.
Magnetic field pattern: N and S poles together
What do you notice about the pattern of the lines of force
in the region between the north and south poles?
How does this pattern compare with the pattern between
the two north poles?
The Earth’s magnetic field
People have used magnetism to navigate for hundreds of
years. The Earth’s iron core creates a magnetic field.
The north poles of magnets, such as compass needles,
are attracted to the North Pole of the Earth.
But if like poles repel, why is the north
pole of a magnet attracted to the
North Pole of the Earth?
These poles were named before
magnetism was properly understood.
The ‘Magnetic North Pole’ of the
Earth is really a magnetic south pole.
Do you think that you would be able to use a compass
to navigate on other planets, or on the moon?
Magnetic fields: summary
Making a magnet
A magnet can be made by magnetizing a material that is
attracted to a magnet, e.g. a paper clip.
There are three methods that can be used to make a magnet:
1. Stroke a magnet along the paper
clip from one end to the other and
then, starting from the same place,
repeat the movement. The more
times this is done, the more
magnetic the paper clip becomes.
2. Hold a nail in a magnetic field and hit it with a hammer.
3. Put a magnetic material in a strong magnetic field.
Magnetic effects of current
Making an electromagnet
Investigating electromagnets
The strength of an electromagnet depends on whether it
has a core of iron.
Two experiments can be carried out to investigate the other
factors that can affect the strength of an electromagnet:
 Investigate how the number of coils affects the number
of drawing pins attracted to an electromagnet – keep the
current the same in this experiment.
 Investigate how the size of the current affects the
number of drawing pins attracted to an electromagnet –
keep the number of coils the same in this experiment.
Investigating electromagnets
How could you use the apparatus below to investigate the
effects of changing the current and the number of coils on
the strength of an electromagnet?
Investigating electromagnets – results
Here are some example results.
number
of coils
number of
drawing pins
attracted
current
(A)
number of
drawing pins
attracted
0
0
0
0
20
8
1
12
40
18
2
23
60
31
3
38
80
46
4
49
Investigating electromagnets – analysis
How did the number of coils affect the number of drawing
pins attracted to the electromagnet?
number of drawing pins attracted
50
40
30
20
10
0
0
20
40
60
number of coils
80
100
Investigating electromagnets – analysis
number of drawing pins attracted
How did the size of the current affect the number of
drawing pins attracted to the electromagnet?
60
50
40
30
20
10
0
0
1
2
current (A)
3
4
5
Using electromagnets – recycling
A large electromagnet is used on a recycling plant conveyor
belt to pick up and move metal cans.
Which metals would the electromagnet attract?
What advantages does an electromagnet
have over a permanent magnet?
Uses of electromagnets – electric bell
The circuit for a door bell includes an electromagnet.
When the circuit is closed, the
electromagnet pulls the
armature towards it, causing
the hammer to strike the bell.
The movement of the
armature breaks the circuit,
and the hammer returns to its
original position.
This sequence repeats,
causing the bell to sound
continuously.
Label the diagram – electric bell
Using electromagnets – door bells
Using electromagnets – the relay
Lifts, cars and other large electrical machines use
high currents.
Relays use an
electromagnet to allow a
small current in one
circuit to control a large
current in another circuit.
Can you predict what will
happen when the switch
is closed in this circuit?
Label the diagram – relay
Electromagnets: summary
Glossary
Anagrams
Magnets: true or false?
Multiple-choice quiz