Magnetism, electromagnets, magnetic field

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A history of magnetism

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Magnetic materials
Which of the materials below are magnetic?
aluminium (Al)
wood
iron
(Fe)
gold (Au)
nickel (Ni)
cobalt (Co)
copper (Cu) polythene
magnesium
(Mg)
aluminium (Al)wood
gold (Au) copper (Cu)
polythene
magnesium (Mg)
nickel(Ni)
cobalt(Co)
iron
(Fe)

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Magnetic or non-magnetic?

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A fridge magnet may be coated in plastic but will stick
to a metal fridge.
Can magnetic forces act through any material?
Can magnetism be blocked?
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.

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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

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Shape of a magnetic field

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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?
weakest field further away
from poles
strongest
field at
poles
The closer together
the magnetic field
lines, the stronger
the magnetic field.

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Forces between magnets experiment

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Investigating magnetic fields

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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.

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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?

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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.

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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?

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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.
Do you think that you would be able to use a compass
to navigate on other planets, or on the moon?
The Earth’s magnetic field
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.

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Magnetic fields: summary

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There are three methods that can be used to make a magnet:
Making a magnet
A magnet can be made by magnetizing a material that is
attracted to a magnet, e.g. a paper clip.
3. Put a magnetic material in a strong magnetic field.
2. Hold a nail in a magnetic field and hit it with a hammer.
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.

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Magnetic effects of current

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Making an electromagnet

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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.

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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

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Investigating electromagnets – results
Here are some example results.
0
8
18
31
46
0
20
40
60
80
number
of coils
number of
drawing pins
attracted
0
12
23
38
49
0
1
2
3
4
current
(A)
number of
drawing pins
attracted

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50
40
30
20
10
0
20 40 60 80 1000
number of coils
numberofdrawingpinsattracted
Investigating electromagnets – analysis
How did the number of coils affect the number of drawing
pins attracted to the electromagnet?

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Investigating electromagnets – analysis
How did the size of the current affect the number of
drawing pins attracted to the electromagnet?
60
40
30
20
10
0
1 2 3 4 50
current (A)
numberofdrawingpinsattracted
50

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Using electromagnets – recycling
A large electromagnet is used on a recycling plant conveyor
belt to pick up and move metal cans.
What advantages does an electromagnet
have over a permanent magnet?
Which metals would the electromagnet attract?

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Uses of electromagnets – electric bell
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.
The circuit for a door bell includes an electromagnet.

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Label the diagram – electric bell

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Using electromagnets – door bells

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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?
Using electromagnets – the relay

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Electromagnets: summary

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Glossary

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Anagrams

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Multiple-choice quiz

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Magnets: true or false?