Form 5 Unit 2 Theme 6: Magnets and Motors Page 1
Magnetism Magnetism is an effect that we cannot see, hear or touch.
It is caused by magnets.
A magnet is a body which attracts iron, combinations of
metals (known as alloys) or other materials which are
composed of iron or iron-like substances. Additionally, the magnet can "magnetize" other objects which in turn act like
magnets. Other iron objects will be attracted to the
Types of magnets .
There are many different types of magnets, the most
common are Bar magnet, Flat magnet, horseshoe magnet
and ring magnets.
The Earth itself is a magnet and
contains a great deal of iron. On the other hand the Earth's moon is largely
made of aluminum, not iron. The
moon does not act like a magnet and
magnetic compasses do not work on
Form 5 Unit 2 Theme 6: Magnets and Motors Page 2
A magnetic compass has a non-magnetic
casing (container). The compass needle is
magnetic and is free to pivot around in a
circle. The compass needle points to the
earth's magnetic north. A magnetic compass
works through air, water, glass, and walls.
The Magnetic Compass Points Towards Magnetic North
The two ends of the magnet are different and are referred to
as the north and south pole (or north-seeking pole and
Like poles repel. The south poles of two magnets will not
attract one another. Neither will the north poles of two
magnets. The north pole of one magnet and the south pole
of another magnet will attract one another. Unlike poles
Like Poles Repel -- Unlike Poles Attract
Form 5 Unit 2 Theme 6: Magnets and Motors Page 3
So, what is the difference between a magnet and a
piece of magnetic material?
The easiest way to tell them apart is that a magnet can
repel and attract another magnet. Whereas, a piece of
magnetic material can only attract a magnet!
There are only three magnetic elements, iron, nickel and
cobalt that can form a magnet. These are called
In practice you will only use iron,or steel which is an alloy of
iron. Even some plastics are magnetic because they have
magnetic substance mixed in them.
If you break up a magnet, you will see, new north and
south poles form immediately. A magnetic pole cannot
Cutting a magnet in half will not isolate a single north or south. One magnet becomes two, then four, and so on.
This process will never end; even when the last electron spin dipole is reached, it cannot be cut to reveal a single north or single south pole.
Form 5 Unit 2 Theme 6: Magnets and Motors Page 4
Magnetic field and magnetic lines of force
The space around a magnet is
called its "magnetic field".
The pattern is called the magnetic
lines of force. The effect the magnet
has around its surrounding is called
the magnetic field. The strength of
a magnetic force,is inversely
proportional to the square of the
distance from the magnet.
The properties of lines of force :
1. They are said to originate from the north pole and end at
the south pole.
2. The lines of forces of a particular magnet do not intersect
with each other.
3. A line of force is continuous : starts from the north pole
and ends at the south pole.
4. There is no line of force within the magnet.
Form 5 Unit 2 Theme 6: Magnets and Motors Page 5
Magnetic effects can be forced on substances that can then
become magnetic themselves. The substances have to be
made of magnetically favourable materials like iron, nickel,
steel, etc. This is called Magnetic Induction.
Take an iron nail. It is not magnetic initially. Bring
its head close to a bar magnet. Let it get attached
to the north pole of the magnet. Slowly the nail
itself will start attracting other nails. Thus the first nails head gets induced as a south pole and
it itself becomes a magnet. Its free end acquires a
north pole. Thus the iron nails become induced as
magnets. After the nails are removed, they may
continue to behave as tiny magnets, but will loose
their magnetic effect after some time.
1. Hammerring steel
2. Stroking method - This involves inducing magnetism
in bars of iron by rubbing them in a particular manner over
other permanent magnets.
Stroking a piece of
unmagnetised iron or
steel with a known
magnet can make it
into a magnet.
This causes the
molecules to line
up in the North-
Form 5 Unit 2 Theme 6: Magnets and Motors Page 6
3. Solenoid method This involves introducing a magnet
into a solenoid and switching the direct current for a short
Handling or breaking, heating, etc may destroy magnetism.
Such effects are called demagnetizing effects. In order to
avoid demagnetizing effects, a permanent magnet is always
kept stuck to soft iron materials called keepers. These
keepers help the poles to stay apart and not get stuck to
each other or bang against each other.
Iron Soft magnetic material .
Iron is very easy to become a magnet but it also loses its
magnetism easily as well. Very useful for making
temporary magnets. E.g. electromagnets, transformers,
reed switches, magnetic switches
Steel Hard magnetic material .
Steel is much more difficult to become a magnet but it
retains its magnetism. Very useful for making permanent
magnets. E.g. compass, fridge door rubber, oil sump tap, magnetic disks, tapes
Form 5 Unit 2 Theme 6: Magnets and Motors Page 7
Magnetic Field patterns
Notice that the lines of magnetic force between the poles
The region in the centre of the poles shown by the red star
has no magnetic field, as the two opposing fields cancel
each other out.
Form 5 Unit 2 Theme 6: Magnets and Motors Page 8
An electric current produces a magnetic field. The Danish
physicist H. C. Oersted first discovered this relationship.
This magnetic field strength could be increased when the
electrical wire was wound into a coil. Winding the wire
around a soft-iron core can increase magnetic strength hundreds or thousands of times. Such a device is known as
Advantages of an electromagnet over a natural magnet include strength and the ability to control the current
1. Salvage yard cranes.
2. Lifting magnets are also used to load machine parts,
steel rails and scrap iron or steel.
3. Burglar alarms use magnets.
The magnetic field around
a straight wire is circular, at
right angles to the wire.
You can work out the direction
of the field using your right
clenched fist. Point your
thumb upwards in the same
direction as the current. The
direction of the field is the
same direction in which your
fingers curl. Reversing the
direction of the current will reverse the magnetic field
Form 5 Unit 2 Theme 6: Magnets and Motors Page 9
The magnetic field can be made stronger by coiling the wire
around a piece of soft iron. This electromagnet is
sometimes called a solenoid.
The shape of the magnetic field is the same as a bar
The soft iron inside the coil makes the magnetic field
stronger because it becomes a magnet itself when the
current is flowing.
Soft iron is used because it loses its magnetism
as soon as the current stops flowing. Soft iron is said to form
a temporary magnet. In this way, the electromagnet can
be switched on and off by turning the electricity on and off.
Steel forms a permanent magnet. If steel was used inside
the coil, it would continue as a magnet after the electricity
was switched off.
It would not be useful as an electromagnet. Permanent
magnets are needed for electric motors, generators,
loudspeakers and microphones.
Form 5 Unit 2 Theme 6: Magnets and Motors Page 10
The strength of the magnetic field around the coil can
be increased by
1. Using a soft iron core (core means middle bit).
2. Using more turns of wire on the coil.
3. Using a bigger current.
Reversing the direction of the current will reverse the
magnetic field direction.
An electromagnet is used in the electric bell, relay,
circuit breaker, loudspeaker
When the switch is pushed closed
the circuit is completed