Transcript 8J Magnets and Electromagnets

KS3 Physics
8J Magnets and
Electromagnets
1 of 20
29
© Boardworks Ltd 2005
2004
Contents
8J Magnets and Electromagnets
Magnetic materials
Magnetic fields
Electromagnets
Summary activities
1
2 of 20
29
2004
© Boardworks Ltd 2005
Magnetic materials
Which of the metals below are magnetic metals?
silver (Ag)
1
3 of 20
29
2004
© Boardworks Ltd 2005
Magnetic materials
A magnetic material is attracted to a magnet.
Only iron (Fe), nickel (Ni) and cobalt (Co) are magnetic.
Ag
1
4 of 20
29
2004
© Boardworks Ltd 2005
Contents
8J Magnets and Electromagnets
Magnetic materials
Magnetic fields
Electromagnets
Summary activities
1
5 of 20
29
2004
© Boardworks Ltd 2005
Forces between magnets – experiment
1
6 of 20
29
2004
© Boardworks Ltd 2005
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.
S
N
The iron filings feel the effect of the magnetic field and
show the direction of the forces in this region.
1
7 of 20
29
2004
© Boardworks Ltd 2005
Shape of a magnetic field
What is the shape and direction of the lines of force in the
magnetic field around a bar magnet?
strongest
field
at poles
N
S
strongest
field
at poles
weakest field further
away from poles
Where is the magnetic field strongest?
1
8 of 20
29
2004
© Boardworks Ltd 2005
Viewing magnetic fields: N poles together
Bring the north poles of two bar magnets together.
S
N
N
S
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.
Carefully scatter iron filings onto the paper.
Draw the pattern created by the iron filings.
1
9 of 20
29
2004
© Boardworks Ltd 2005
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?
S
1
10ofof20
29
N
2004
© Boardworks Ltd 2005
Viewing magnetic fields: N and S poles together
Bring the north and south poles of two bar magnets together.
S
N
S
N
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.
Carefully scatter iron filings onto the paper.
Draw the pattern created by the iron filings.
1
11ofof20
29
2004
© Boardworks Ltd 2005
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?
S
N
S
N
How does this pattern compare with the pattern between
the two north poles?
1
12ofof20
29
2004
© Boardworks Ltd 2005
Magnetic fields – summary
1. When two like poles (e.g. two north poles or two south
poles) are put together, they repel each other.
2. When two unlike poles (e.g. a north and a south pole)
are put together, they attract each other.
3. Scattering iron filings around a bar magnets makes it
possible to see the lines of force of the magnetic field.
1
13ofof20
29
2004
© Boardworks Ltd 2005
Making a magnet
A magnet can be made by magnetizing a material which is
attracted to a magnet, e.g. a paper clip.
There are three methods that can be used to make a magnet:
 Stroke a magnet along the paperclip 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 clip becomes.
 Hold a nail in a magnetic field and hit it with a hammer.
 Put a magnetic material in a strong magnetic field.
1
14ofof20
29
2004
© Boardworks Ltd 2005
Contents
8J Magnets and Electromagnets
Magnetic materials
Magnetic fields
Electromagnets
Summary activities
1
15ofof20
29
2004
© Boardworks Ltd 2005
Making an electromagnet
When electricity is passed through a coil of wire, the coil has
a magnetic field around it. This is called an electromagnet.
If the coil of wire is wrapped around a piece of iron, such as
an iron nail, the magnetic field gets stronger.
1
16ofof20
29
2004
© Boardworks Ltd 2005
Investigating an electromagnet
An iron core at the centre of a coil of wire, increases the
strength of an electromagnet.
Two experiments can be carried out to investigate the other
factors that can affect the strength of an electromagnet:
1. Investigate how the number of coils affects the
number of paper clips attracted to an electromagnet
– keep the current the same in this experiment.
2. Investigate how the size of the current affects the
number of paper clips attracted to an electromagnet
– keep the number of coils the same in this experiment.
1
17ofof20
29
2004
© Boardworks Ltd 2005
Investigating an electromagnet – results 1
Number of coils
Number of paper
clips attracted
0
0
20
8
40
18
60
31
80
46
Remember – keep the current the same
throughout this experiment!
1
18ofof20
29
2004
© Boardworks Ltd 2005
Investigating an electromagnet – results 2
Current (A)
0
Number of paper
clips attracted
0
1
12
2
23
3
38
4
49
5
60
Remember – keep the number of coils the same
throughout this experiment!
1
19ofof20
29
2004
© Boardworks Ltd 2005
Investigating an electromagnet – graph 1
Graph to show how the number of coils affects
the strength of an electromagnet
number of clips attracted
50
40
30
20
10
0
0
1
20ofof20
29
20
40
60
number of coils
80
100
2004
© Boardworks Ltd 2005
Investigating an electromagnet – graph 2
number of clips attracted
70
Graph to show how the current affects the
strength of an electromagnet
60
50
40
30
20
10
0
0
1
2
3
4
5
current (A)
1
21ofof20
29
2004
© Boardworks Ltd 2005
Using electromagnets – scrap yards
A large electromagnet is used in a scrap yard to pick up
and move heavy pieces of scrap metal.
What metals would the
electromagnet attract?
What advantages does
an electromagnet have
over a permanent
magnet?
1
22ofof20
29
2004
© Boardworks Ltd 2005
Using electromagnets – door bells
The circuit for a door bells includes an electromagnet.
1
23ofof20
29
2004
© Boardworks Ltd 2005
Using electromagnets – relay
Lifts, cars and other large electrical machines use high
currents.
A relay, which includes an electromagnet, is used to allow
a small current in one circuit to control a large current in
another circuit.
1
24ofof20
29
2004
© Boardworks Ltd 2005
Electromagnets – summary
1. When electricity is passed through a coil of wire, the coil
behaves like a magnet and has a magnetic field around it
– this is an electromagnet.
2. There are three ways to make an electromagnet stronger:
 wrap the coil of wire around an iron core;
 increase the number of coils;
 increase the size of the current.
3. An electromagnet can be easily turned on and off.
This is why electromagnets can be used in
scrapyards and as switches in electrical devices.
1
25ofof20
29
2004
© Boardworks Ltd 2005
Contents
8J Magnets and Electromagnets
Magnetic materials
Magnetic fields
Electromagnets
Summary activities
1
26ofof20
29
2004
© Boardworks Ltd 2005
Glossary
 attraction – The force that pulls things together,







1
27ofof20
29
e.g. opposite poles of two magnets.
electromagnet – A magnet made by passing electricity
through a coil of wire, which often has a core inside.
magnet – An object that has a magnetic field and can
attract magnetic materials.
magnetic field – The area around a magnet where its
magnetic force can be felt.
magnetic materials – Materials that are attracted to a
magnet, e.g. iron, cobalt and nickel.
magnetism – The non-contact force of a magnetic field.
poles – The parts of a magnet where its magnetic field is
strongest.
repulsion – The force that pushes things away from each
other, e.g. similar poles of two magnets.
2004
© Boardworks Ltd 2005
Anagrams
1
28ofof20
29
2004
© Boardworks Ltd 2005
Multiple-choice quiz
1
29ofof20
29
2004
© Boardworks Ltd 2005