Magnetism
Chapter 9
Table of contents
01
02
Magnetic field
03
Electromagnet
Earth’s
magnetic field
01
Magnetic field
Even the tiniest bolt must
be screwed on tightly in
order to perform its best.
—Jing Si Aphorism
Learning outcomes:
9.1.1 Describe what is meant by the term magnetic field.
9.1.2 Explain how to detect a magnetic field.
9.1.3 Draw magnetic field lines around a magnet and between two
magnets according to the directions and its strength.
Magnets
Most materials are not magnetic, but some are.
A magnetic material can be magnetised or will be
attracted to a magnet.
These metals are magnetic:
●
●
●
●
iron
cobalt
Nickel
Steel
Magnets
Bar magnet
A bar magnet is a
permanent magnet.
This means that its
magnetism is there all the
time and cannot be
turned on or off.
A bar magnet has two magnetic poles:
●
●
north pole (or north-seeking pole)
south pole (or south-seeking pole)
Attract and repel
If you bring two bar
magnets together, there
are two things that can
happen, attraction and
repulsion.
•
Opposite poles attract.
•
Like poles repel.
Opposite poles attract
Like poles repel
Paper clips are attracted to the
magnet if they are placed in the
magnetic field of the magnet.
Paper clips are not
attracted to the
magnet if they are
not in the magnetic
field of the magnet.
Magnetic field is the area around a magnet where the effects of
the magnet can be detected.
9.1.1 Describe what is meant by the term magnetic field?
Magnetic field is the area around a
magnet where the effects of the
magnet can be detected.
You cannot see a magnetic field, but
you can observe its effects.
Magnetic force is a non-contact force
because the magnet and the material
do not have to touch each other.
9.1.2 Explain how to detect a magnetic field.
You can use a compass or iron filings to detect a magnetic field.
Compass
Iron filings
9.1.3 Draw magnetic field lines around a magnet and between two
magnets according to the directions and its strength.
I see these in magnetic field.
1. Does the magnetic field lines cross with each other?
No.
North
2. The magnetic field lines point from the ___________
pole to
the _________
South pole.
3. Each line of magnetic field is continuous.
4. Are the magnetic field line closer or further away from each
other near the poles?
Closer. The strength of the magnetic force is the
strongest near the poles.
9.1.3 Draw magnetic field lines around a magnet and between two magnets
according to the directions and its strength.
In the diagram, note that:
●
●
●
each field line has an arrowhead
on it
the field lines come out of the
north pole and go into the
south pole
the field lines are more
concentrated at the poles
The magnetic field is strongest at the
poles, where the field lines are most
concentrated.
Field lines also show
what happens to the
magnetic fields of
two magnets during
attraction or repulsion.
Field lines lead from
one magnet to the
other when the
magnets attract each
other
Field lines also show
what happens to the
magnetic fields of two
magnets during
attraction or repulsion.
Field lines do not lead
from one magnet to the
other when the magnets
repel each other
Uses of magnets
Make a tight seal on the
doors to refrigerators and
freezers
A compass uses a
magnet to direct its
needle to the north pole
Children’s toys.
A compass comprises:
a magnetic needle mounted on a pivot
(so it can turn freely)
● a dial to show the direction
●
●
The north pole (north-seeking pole) of
the compass needle points towards the
Earth’s north pole.
●
If the needle points to the N on the dial,
you know that the compass is pointing
north.
●
This lets you navigate outdoors using a
map.
Compass
How to magnetise iron metal?
Iron, nickel and cobalt has magnetic
properties, but they do not show them
all the time.
Example: Steel paper clips do not
attract or repel each other.
To magnetise iron metal:
→ Rub the North pole from the
middle of the metal to the end. With
firm pressure, quickly run the magnet
across the piece of metal, repeatedly.
→ Test the magnetism.
Inside the magnet
Domains
🡪 The tiny particles in the magnet materials.
🡪 behave like tiny magnets.
A
A
Domains are arranged randomly.
🡪 The material does not attract other
magnetic materials.
🡪 does not point North-South.
domain
B
B
Domains are arranged in line.
→ all north poles in one direction, all
south poles in opposite direction.
→ the material is magnetised.
→ attract other magnetic materials.
Activity: Complete
workbook page 184 – 186
What did we learned?
9.1.1 Describe what is meant by the term magnetic field.
9.1.2 Explain how to detect a magnetic field.
9.1.3 Draw magnetic field lines around a magnet and between two
magnets according to the directions and its strength.
02
Earth’s
magnetic field
Learning outcomes:
9.2.1 Describe that Earth has a magnetic field.
9.2.2 Draw a diagram to show the earth's magnetic
field lines,
9.2.3 Describe the core of the Earth is the origin of the
Earth's magnetic field.
The Earth as a magnet
Outer core
- Liquid metal
- Contains iron and nickel.
Inner core
- Solid metal
- Contains iron and nickel.
As the Earth spins, the Outer
core and Inner core move at
different speeds
The Earth’s structure
→ generate magnetic fields
around the Earth
The Earth’s magnetism
It produces a magnetic
field in which the field lines
are most concentrated at
the poles.
This magnetic field can be
detected using magnetic
materials or magnets.
The Earth’s magnetic field affects the needles in compasses
The north pole of a
compass needle is a
magnetic north pole.
Geographic north pole
It is attracted to the
geographic North Pole,
which is a magnetic
south pole (opposite
magnetic poles attract).
Geographic south pole
Magnetic
North Pole
The magnetic north of the Earth is moving!
➢ The magnetic north moves slowly. It is currently in the
Arctic Ocean, north of Alaska.
➢ The movement in the liquid in outer core would
explain why the magnetic poles move slowly, and
have occasionally reversed.
➢ The magnetic north moves at a speed of about 60km
per year.
The Earth magnetic field causes the natural appearance of lights
visible in the night sky close to the North and South poles.
Aurora → caused by particles coming from the Sun arriving
into stronger parts of the Earth’s magnetic field.
Activity: Complete
workbook page 187 – 189
What did we learned?
9.2.1 Describe that Earth has a magnetic field.
9.2.2 Draw a diagram to show the earth's magnetic
field lines,
9.2.3 Describe the core of the Earth is the origin of the
Earth's magnetic field.
03
Electromagnet
Learning outcomes:
•
•
•
Understand the link between
magnetism and electricity
Construct an electromagnet.
Describe the uses of
electromagnet.
Electromagnets
• When an electric current flows in a wire, it creates a
magnetic field around the wire.
• This effect can be used to make an electromagnet.
What do you observe?
What is the aim of the
experiment?
What is the
C.V
I.V
D.V
Electromagnets
When an electric current flows in a
wire, it creates a magnetic field
around the wire.
This effect can be used to make an
electromagnet.
Components of a simple electromagnet:
●
●
●
●
Iron nail
Insulated wire
Battery/power supply
Switch
Electromagnet
You can make an electromagnet
stronger by doing these things:
●
adding more turns to the
coil
●
increasing the current
flowing through the coil
There is a limit to how much current can be
passed safely through the wire because the
resistance of the wire causes heating.
11 turns on coil
→ attracts more pins
6 turns on coil
→ attracts less pins
How does an electromagnet work?
Advantages of Electromagnet
Electromagnets have some
advantages over permanent
magnets. For example:
they can be turned on
and off
● the strength of the
magnetic field can be
varied.
● The polarity can be
reversed.
●
Bar magnet
(permanent magnet)
Electromagnet
(temporary magnet)
Applications of Electromagnet
1. Picking up scrap iron and steel in scrapyards.
https://mammothmemory.net/physics/magnets-and-electromagnetism/electromagnetism/uses-of-electromagnetism.html
2. Maglev trains is supported by electromagnet, to
reduce frictions with the tracks.
3. Electric bells like the ones used in most schools also
contain an electromagnet.
(open)
(closed)
The switch is
closed. An electric
current flows
through the
electromagnet,
producing a
magnetic field.
The switch is open and the metal
arm is away from the gong.
The metal arm is attracted to the
electromagnet. It hits the gong which
makes a sound.
cell
What did we learned?
•
•
•
Understand the link between
magnetism and electricity
Construct an electromagnet.
Describe the uses of electromagnet.
Activity: Complete
workbook page 193-195
End of chapter 9!
Do you have any questions?
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