# Magnetism - Physics Resources 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

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• 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

magnetized object.

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

the moon.

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• 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

south-seeking pole).

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

attract.

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

Ferromagnetic materials.

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

be isolated.

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.

Making magnets.

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-

South

orientation.

• 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

time.

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

are parallel.

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

Electro Magnetism

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

an electromagnet.

Advantages of an electromagnet over a natural magnet include strength and the ability to control the current

and direction.

Examples :

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

direction.

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• Form 5 Unit 2 Theme 6: Magnets and Motors Page 9

Electromagnetic Coil.

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

magnet.

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.

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• 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

and microphone.

Electric Bell

When the switch is pushed closed

the circuit is completed