Transcript Document

8J Magnets and electromagnets
8J Magnets and
electromagnets
Magnets
Magnetic fields
Electromagnetism
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8J Magnets and electromagnets
Magnets
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8J A few words to start you off
Match each word to its correct meaning.
1 magnetic force
2 repulsion
3 non-magnetic
4 north-seeking pole
5 magnetic
6 poles
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A the
C
attracts
two ends
a magnetic
of a magnet
material to a
magnet
B force
F
not attracted
that pushes
to a magnet
two objects
apart
D not
B
the attracted
end of a freely
to a magnet
suspended
magnet that faces north
F the
D
force
end
that
ofpushes
a freelytwo
suspended
objects
apart that faces north
magnet
E attracted to a magnet
C attracts
A
the two ends
a magnetic
of a magnet
material to a
magnet
8J And now a few experiments...
What happens?
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8J Magnets, magnetism and magnetic materials
A magnetic material is something that is attracted
by a magnet, for example, a steel paper clip.
Not all magnetic materials are magnets.
A magnet is a piece of magnetic material with a
north and south pole.
Magnetism is the force that pulls the opposite poles
of two magnets together or pushes the same poles
of two magnets apart.
Ancient Greeks identified a naturally magnetic rock
2500 years ago – do you know what it is called?
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8J What do you think?
True or false?
1. Aluminium is not magnetic.
TRUE
2. Some magnets also have an east and
west pole.
FALSE
3. Magnets attract materials made of iron.
TRUE
4. South poles attract each other.
FALSE
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8J What do you think?
True or false?
1. The magnetic field is the area around a
magnet where the magnetic force can be
felt.
TRUE
2. If you break a magnet, you get two smaller
magnets.
TRUE
3. The north pole of a magnet is always
stronger than the south pole.
FALSE
4. Some magnets only have a north pole.
FALSE
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8J Magnets and electromagnets
Magnetic fields
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8J What’s a field?
Field is a word we all understand – it’s a place with
lots of grass!
But scientists also use the word ‘field’ with another
meaning.
A field is an area where a force acts.
So, a magnetic field is the area where a magnet
could attract something.
Can you think of any other sort of field in science?
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8J What does a magnetic field look like?
Well, we can’t see it for a start, but...
Small particles of iron filings are pushed into patterns by
a magnetic field. If we sprinkle iron filings on a piece of
paper over a magnet we can see...
The lines of force are very
close together here – the field
is very strong.
The lines of force are further
apart here – the field is weaker.
A line of force links one pole of a magnet to the other pole.
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8J And the rule about directions...
The magnetic force acts in a direction.
The arrowheads on this diagram show you the way the
needle of a compass point would go if it was free to move.
You can see that the magnetic force runs from the north
pole to the south pole of the magnet.
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8J And now two magnets...
Can you explain what the lines of force show?
Where is the strongest part of the field?
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8J Turn one of the magnets around?
Can you explain what the lines of force show?
Where is the strongest part of the field?
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8J Magnetic fields – true or false?
True or false?
1. Iron filings can be used to show the shape
of the magnetic field.
TRUE
2. Magnetic field lines always go out of the
north pole.
TRUE
3. Magnetic field lines come out of both ends
of a magnet.
FALSE
4. Magnetic materials placed in the magnetic
field will be attracted towards the magnet.
TRUE
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8J Magnets and electromagnets
Electromagnetism
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8J Fields around wires
A wire carrying an electric current also shows magnetic
field lines around it.
They obey the same rules as
magnetic fields.
The closer the lines are
together the stronger the force.
But does this attract or repel a magnet?
That depends on which way the current is flowing.
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8J Fields in coils of wires
Electricity passing through a coil of wire produces a
magnetic field. Look at the diagram below to see the lines
of force.
What else creates field lines like
this?
What happens to the field if you put a bar of iron inside
the coil?
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8J Using electromagnets – alarm bells
Describe what is happening here.
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8J Test your magnetism
1. What is magnetism?
a) a kind of glue
b) an invisible force that pushes or pulls magnetic
materials
c) something that an attractive person has.
2. Which of the following metals can be magnetised?
a) copper
b) aluminium
c) iron.
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8J Test your magnetism
3. All magnets are surrounded by...
a) metal
b) a magnetic field
c) fluff.
4. Where is the Earth’s magnetic north pole?
a) close to the South Pole
b) close to the North Pole
c) at the North Pole in Summer and the South Pole in
winter.
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8J Test your magnetism
5. If magnetic field lines are close together it means...
a) the magnetic field is very strong
b) the magnetic field is very weak
c) there’s not much space for the field to spread into.
6. A wire carrying a current...
a) continues to act like a magnet when the current is
switched off
b) produces circular magnetic field lines
c) produces no magnetic field.
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