7.1 The properties of magnets
-Lodestone is an iron- containing mineral that is naturally
magnetic.
-The Ancient Greeks had discovered magnetism.
- A magnetic force is a non-contact force .
• Simple Phenomena of magnetism
• What is magnet?
• Magnets are able to attract objects made
from magnetic materials such as iron, steel,
nickel and cobalt.
• Magnets cannot attract objects made from
materials such as plastic, wood, paper or
rubber. These are non-magnetic materials.
Attraction and repulsion
-Every magnet has a north pole and a south pole. The poles of
magnets
attract or repel each other.
The force between magnetic poles decreases as their separation
increases.
• Some materials are attracted to magnets. These are called
magnetic materials.
• Iron, cobalt, and nickel are all magnetic materials that are
• elements.
• Some of the alloys or oxides of these elements are also magnetic.
• Steel is magnetic because it contains mainly iron. Lodestone, or
magnetite , contains iron oxide which is also magnetic.
• Iron, cobalt, and nickel are all magnetic materials that are
elements.
• Some of the alloys or oxides of these elements are also
magnetic.
• Steel is magnetic because it contains mainly iron. Lodestone
contains iron oxide which is also magnetic.
• PG 137
1 Sort the list below into materials that are magnetic and materials
that are not magnetic.
brass steel iron copper wool wood nickel cotton
Magnetic : Steel , iron , nickel
Non-Magnetic : brass, copper , wool, wood, cotton.
2 You pick up some nails with your fingers from a pile of nails
and find that they appear to stick together. Are they made out of
iron or steel? Explain your answer.
Steel .
Steel can be magnetized.
• PG 137
4 Extension: Use the idea of domains to explain:
a why if you break a magnet in two you get two magnets each with a north and
south pole.
In a magnet there are lots of small regions call domains, each one behaves
like a small magnet so when a magnet is broken both parts still have a
north and south pole.
b why you can demagnetize a permanent magnet by heating it.
Heating a magnet disrupts the domains and they no longer line up.
c why iron does not stay magnetized when you remove the magnet, but steel
does.
Iron does not stay magnetized because the domains are easy to line up, this
means they are also easy to disrupt again and they will not stay lined up.
7.2 Magnetic field
What is a magnetic field?
If you put a steel pin near a magnet, the pin will move. The magnet attracts
the pin because it is exerting a force on it.
The region around the magnet where magnetic materials experience a
force is called the magnetic field.
Notes
A region where magnetic materials experience a force
is called a magnetic field.
● You can find the shape of a magnetic field using iron
filings or plotting compasses.
● The Earth behaves as if it has a bar magnet inside it.
PG 139
1 What is a magnetic field?
A magnetic field is the region around a magnet where magnetic
materials will experience a force.
2 Where is the magnetic field of a bar magnet strongest? How do you
know?
The magnetic field of a bar magnet is strongest at the two poles ,
because if you plot field lines around a magnet to show the field they
are closest together at the poles , indicating the field is strongest here.
3 Look at the pictures of iron filings around the two magnets repelling
and two magnets attracting.
a Draw simplified diagrams of the magnetic field pattern in each
picture.
3.b What would happen if you put a small steel ball at the neutral point?
If you place a steel ball at the neutral point nothing would happen.
C Explain your answer.
Nothing would happen because the two magnetic fields have
cancelled each other out.
4 A student says that he has made a magnetic field with some iron
filings. What would you say to him?
He has not made a magnetic field with the iron filings. He has
used the iron filings to show to location of the magnetic field of
a magnet.
7.3 Electromagnets
Electromagnets work when an electric current flows through a wire, a
magnetic field is produced.
Making an electromagnet
You can make a circular loop of wire and pass a current through it.
The magnetic field lines at the centre are straight.
A coil of wire is called a solenoid.
The strength of an electromagnet
The strength of an electromagnet depends on:
- the number of turns, or loops, on the coil – more turns will
make a stronger electromagnet
- the current flowing in the wire – more current will make a
stronger electromagnet
- the type of core – using a magnetic material in the core will
make a stronger electromagnet.
Permanent magnets and electromagnets
There are two main differences between a permanent magnet
and an electromagnet.
- can turn an electromagnet on and off.
- can make an electromagnet that is much stronger than
a permanent magnet.
Notes
-A current flowing through a wire produces a magnetic field.
-An electromagnet is a coil of wire usually wrapped around
an iron core.
-You can switch an electromagnet on and off by turning the
current on and off.
- The magnetic field around an electromagnet is similar to
that around a bar magnet.
• Using electromagnets
A relay is a type of switch that can be used in a low-current
circuit to turn on a high- voltage or high-current circuit.
Fire doors use electromagnets to close automatically.
An electric bell works by continuously making and
breaking a circuit.
Scanners use magnets to take pictures of organs in the body.
• Using electromagnets
The relay
A relay uses a small current in one circuit to operate a switch in a
separate high-voltage or high-current circuit.
A relay is a type of switch that can be used in a low-current
circuit to turn on a high- voltage or high-current circuit.
Fire doors use electromagnets to close automatically.
Scanners use magnets to take pictures of organs in the body.
• Magnets can be used to produce an image of the inside of your body.
• The device is called a magnetic resonance imaging (MRI) scanner.
• An MRI scanner contains very strong magnets. These produce a very
• intense magnetic field that interacts with the hydrogen in the water
• molecules in your body. The MRI scanner enables doctors to take
• pictures of people’s organs.
Scanners use magnets to take pictures of organs in the body.
Magnets can be used to produce an image of the inside of your body.
The device is called a magnetic resonance imaging (MRI) scanner.
An MRI scanner contains very strong magnets. These produce a very intense
magnetic field that interacts with the hydrogen in the water molecules in your
body. The MRI scanner enables doctors to take pictures of people’s organs.
An electric bell works by continuously making and
breaking a circuit.
● A current passes through
the coil and the iron core is
magnetised.
● The iron armature is
attracted to the core and the
hammer hits the
gong, making a sound.
● This breaks the circuit at X
so the iron core is no longer
magnetised.
● The armature springs back
and completes the circuit.
• Magnetic field lines are a visual representation of a magnetic
field
• Direction
• The direction of the magnetic field is shown by the alignment of
the field lines, which always point from north to south. An arrow
on the line indicates the direction of the force on a magnetic
north pole
• that show the direction and strength of the field:
• Strength
• The strength of the magnetic field is shown by the spacing of
the field lines. The closer the lines are together, the stronger the
field.