Samacheer Class 8 Science - Magnetism

Magnetite – Natural magnet
A tiny pivoted magnet – Compass box
Cobalt – Ferromagnetic material
Closed curves – Magnetic lines
Bismuth – Diamagnetic material

Magnets that are made by people in a laboratory or a factory are called artificial magnets.
Example:
Horse shoe magnet, bar magnet, U-shaped magnet, cylindrical magnets, disc magnets, ring magnets and electromagnets.

1. Natural Magnets:
These are found in nature.
Have irregular shapes and dimensions.
The strength of a natural magnet is well determined and difficult to change.
These are permanent magnets.
They have a less usage.
2. Artificial Magnets:
These are man-made magnets.
They can be made in different shapes and dimensions.
Artificial magnets can be made with required and specific strength.
Their properties are time bound.
They have a vast usage in day-to-day life.

A freely suspended magnetic needle at a point on the Earth comes to rest approximately along the geographical north – south direction.
This shows that the Earth behaves like a huge magnetic dipole with its magnetic poles located near its geographical poles.
The north pole of a magnetic needle approximately points towards the geographic north (NG).
The magnetic north pole of the needle is attracted by the magnetic south pole of the Earth (Sm), which is located at the geographic north NG.
Also, the magnetic south pole of the needle is attracted by the magnetic north pole of the Earth (Nm), which is located at the geographic south SG.

In ancient times, the magnet in the form of ‘direction stone’ helped seamen to find the directions during a voyage.
Nowadays, magnets are used to generate electricity in dynamos.
Electromagnets are used in our day-to-day life.
They are used in electric bells and electric motors.
They are used in loudspeakers and microphones.
An extremely powerful electromagnet is used in the fast moving Maglev train to remain floating above the tracks.
In industries, magnetic conveyor belts are used to sort out magnetic substances from scraps mixed with non-magnetic substances.
Magnets are used in computer in its storing devices such as hard disks.
In banks, the magnets enable the computers to read the MICR numbers printed on a cheque.
The tip of the screw drivers are made slightly magnetic so that the screws remain attached to the tip.
At hospitals, extremely strong electro magnets are used in the MRI (Magnetic Resonance Imaging) to scan the specified internal organ.

Spread some steel pins on a wooden board and bring an iron nail near them.
Now, make one of the magnetic poles of the bar magnet touch one end of the iron nail.
Slide it along its length in one direction slowly till the other end is reached.
Repeat the process, as shown in the diagram, 20 to 30 times.
The magnet has to be moved in one direction only.
Avoid the swiping of the magnet back and forth.
Now, bring the iron nail near the steel pins.
The steel pins stick to the iron nail because nail has become a temporary magnet.

Earth has been assumed or imagined by the scientists as a huge magnetic dipole.
The south pole of the imaginary magnet inside the Earth is located near the geographic north pole and the north pole of the Earth’s magnet is located near the geographic south pole.
The line joining these magnetic poles is called the magnetic axis.
The magnetic axis intersects the geographic north pole at a point called the north geomagnetic pole or northern magnetic pole.
It intersects the geographic south pole at a point called the south geomagnetic pole or southern magnetic pole.
The magnetic axis and the geographical axis (axis of rotation) do not coincide with each other.
The cause of the Earth’s magnetism, are as follows.
Masses of magnetic substances in the Earth
Radiations from the Sun
Action of the Moon

Earth is not attracting other ferromagnetic materials because the magnetic character of ferromagnetic materials is affected by the external temperature. When they are heated, they become paramagnetic at curie temperature.

It is not advisable to slide a magnet on an iron bar back and forth because moving it in opposite directions will work to cancel each other out.

Each broken piece behaves like a separate magnet. Therefore, four pieces will have eight poles.
Conclusion:
Thus, we can conclude that unlike poles of a magnet attract each other, i.e., the north pole and the south pole of a magnet attract each other.