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Magnetism
Magnet
Some materials like the mineral magnetite (Fe3O4) show magnetic properties, i.e.,
they attract pieces of iron and tend to orient themselves in the north-south direction.
Electromagnet
Electric current passing through an insulated copper wire wound around a piece of
iron makes it a temporary magnet. This magnet works as long as the current passes
through it.
Magnetic field
The space around a permanent magnet or an electromagnet in which, at any point
where the influence of a magnet can be felt is called the magnetic field.
Tesla (T) is the S.I. unit of magnetic field.
Magnetic effect of current
When current flows through a conductor, magnetic field is produced.
Electric Motor
Motor is a device, which changes electrical energy into mechanical energy.
E.g.: AC Motor, DC Motor
Properties of a Bar Magnet
1. A freely suspended bar magnet always points in a north-south direction.
2. The ends of a bar magnet show maximum effect and these are called the poles
of a bar magnet.
3. lm = 7 lg
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4. Like poles repel and unlike poles attract each other.
5. A magnet attracts other magnetic materials.
6. Repulsion is the surest test for a magnet.
7. A magnet shows the property of magnetic induction (this property will last as
long as the magnet is present. The moment it is removed, there will be no
magnetism left).
Magnetic Induction
It is the phenomenon in which an iron piece behaves as a magnet in the presence of a
permanent magnet.
Tests for a magnet
1. We can suspend a magnet to be tested freely. If it points in a north-south
direction, it is a magnet.
2. We can dip the piece in iron filings. If the iron filings stick, it is a magnet.
3. We bring another bar magnet and bring the North Pole of the Bar magnet
towards the piece to be tested and we do the same with the piece’s other pole.
One should attract and the other should repel. If both attract, then it is not a bar
magnet. Repulsion is the surest test for a magnet.
Temporary Magnet
Temporary magnets are usually made of iron and are able to keep their magnetism for
a short time. When the core is of soft iron, it loses all its magnetism when current
stops flowing.
In order to convert a magnetic substance into a magnet, it is required to rearrange the
tiny atomic magnets (domains) parallel to each other in the same direction.
Sometimes this reorientation last for a short duration or as long as the cause is present.
Such magnets are called temporary magnets. An electromagnet behaves as a magnet
as long as the electric current flows through it.
Permanent Magnet
In some magnets, the atomic magnets remain parallel to each other even when the
source is removed and the effect lasts longer.
Temporary Magnet
They are made up of soft iron.
Permanent Magnet
They are made up of steel, cobalt, nickel,
alnico.
It remains a magnet as long as the source It remains a magnet even if the source of
of magnetisation is present
magnetisation is removed
The alignment of microscopic domains is The alignment of domains is permanent
temporary
Domain theory
All magnetic materials are made up of tiny regions called domains. These domains
have north and south poles. Each domain is a magnet in itself. In an ordinary piece of
iron, domains are oriented randomly. The magnetic effect is absent. But in a magnet,
those domains are properly oriented. Therefore there is a net magnetic effect on one
end i.e., north pole and the other, south pole.
The Earth as a magnet (Terrestrial Magnetism)
A freely suspended magnet always comes to lie in the geographic north-south
direction because the earth behaves as a magnet. Near the earth’s geographic north
pole is the magnetic south and near the earth’s geographic south pole is the magnetic
north. So, a freely suspended magnet’s poles get attracted to the magnetic poles of the
earth and the north pole of the magnet points to the geographic north of the earth and
the south pole points to the geographic south of the earth.
Making magnets
There are two methods by which a piece of iron can be magnetized:
Single-touch method
Place an iron piece on the table. Hold a strong bar magnet on it, almost vertically with
its north pole touching the end of the iron bar. Move the magnet along the piece of
iron and then lift it and bring it back to the starting point. Repeat this action for about
50 times. The piece of iron becomes a magnet.
Double-touch method
Place a piece of iron to be magnetized on the table. Take two bar magnets and place
them at the centre of the iron bar. Move the magnet in a circular direction away from
the centre for about 30 times. The iron piece gets magnetized. This method is better
than single-touch method.
Solenoid
If a rod of soft iron is introduced within a solenoid carrying current, the magnetic field
becomes very strong. The specimen introduced within is called the core. Strength of
an electromagnet can be increased by increasing the number of turns in the coil or by
increasing the amount of current flowing through the solenoid.
Uses of Electromagnets
1. Large electromagnets are used in industry for lifting heavy pieces of iron.
2. For removing iron bits from wounds by surgeons.
3. In electric bells.
4. In telegraph and telephones.
5. In electric motors.
6. They are used for separating iron from non-magnetic scraps.
7. Horse-shoe type magnet is used in an electric bell.
8. They are used in dynamo, electric motor, microphone, fans, loudspeakers,
telegraph and telephone.
Demagnetisation of magnet
1. Heating demagnetises magnets as it disturbs the alignment of the domains.
2. A magnet loses its magnetism if dropped or hammered or handled roughly.
This also disturbs the alignment of the molecular magnets and domains.
3. If two magnets are kept side by side with like poles next to each other, they
get demagnetised.
Uses of Magnet
1. In a magnetic compass for finding direction. A mariner’s compass is used by
sailors for navigation.
2. In dynamos of cars and bicycles, electricity meter, refrigerator door, etc.
3. Magnetic tapes (plastic tapes coated with magnetic material) are used in audio
and video tapes and in computers to store data.
4. In medicine, magnet therapy is gaining popularity today.
5. Nuclear magnetic resonance (NMR) imaging is used to study the brain.
6. Magnets are used in bulletin boards and toys.