Samacheer Class 8 Science - Magnetism

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

Artificial magnets are magnets that are made by people in a laboratory or a factory through various methods such as stroking, electrical induction, or electromagnetic means. These magnets are created to have specific properties and strengths according to their intended use. Examples of artificial magnets include horseshoe magnets, bar magnets, U-shaped magnets, cylindrical magnets, disc magnets, ring magnets, and electromagnets. Horseshoe magnets are shaped like a horseshoe and are commonly used in laboratories and industries. Bar magnets are rectangular in shape and are widely used in compasses and educational demonstrations. Electromagnets are created by passing electric current through a coil of wire wound around an iron core and are used in various applications such as doorbells, electric motors, and lifting heavy iron objects.

Natural and artificial magnets differ in several important ways. Natural magnets are magnets that are found in nature and occur naturally in the earth, such as magnetite. They have irregular shapes and dimensions that cannot be controlled. The strength of a natural magnet is well determined and fixed, and it is difficult to change or modify their magnetic properties. Natural magnets are permanent magnets that retain their magnetic properties for a long time. They have limited usage in modern applications due to their irregular shapes and variable strength. Artificial magnets, on the other hand, are man-made magnets created in laboratories or factories. They can be manufactured in different shapes and dimensions according to specific requirements and applications. Artificial magnets can be made with required and specific strength by controlling the manufacturing process. Their magnetic properties are time-bound and can be modified or lost if not properly maintained. Artificial magnets have vast and widespread usage in day-to-day life in various devices such as motors, generators, loudspeakers, and medical equipment.

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 acts as a huge bar magnet, but it does not attract ferromagnetic materials in the way a typical bar magnet does for several important reasons. First, the magnetic field of Earth is relatively weak compared to the distances at which ferromagnetic materials are usually found. The strength of Earth's magnetic field decreases significantly with distance from the planet's surface, following the inverse square law, so objects on the surface experience only a very faint magnetic force. Second, ferromagnetic materials like iron and steel have their own internal magnetic domains that are randomly oriented at room temperature. For these materials to be attracted and aligned by an external magnetic field, the field must be strong enough to overcome the thermal energy that keeps the domains randomly oriented. Earth's magnetic field is simply not strong enough to align these domains effectively in ferromagnetic materials under normal conditions. Additionally, the magnetic character of ferromagnetic materials is affected by external temperature. When ferromagnetic materials are heated, they eventually reach the Curie temperature, at which point they lose their ferromagnetic properties and become paramagnetic, meaning they no longer respond strongly to external magnetic fields. Furthermore, Earth's magnetic field is not uniform everywhere on the surface, and its direction and strength vary with geographical location, which further reduces its ability to consistently attract ferromagnetic objects.

It is not advisable to slide a magnet on an iron bar back and forth during magnetization because this process works to cancel out the magnetization effect. When the magnet is moved in one direction along the iron bar, it aligns the magnetic domains of the iron in a particular direction, magnetizing the bar. However, when the magnet is moved back in the opposite direction, it tends to randomize and disalign these domains that were just aligned, effectively undoing the magnetization that was achieved in the first stroke. Each reverse motion counteracts the work done by the previous motion, resulting in a net reduction in the overall magnetization of the iron bar. To effectively magnetize an iron bar, the magnet should be moved in only one direction repeatedly, always from the same end to the other end. This ensures that the magnetic domains are progressively aligned in the same direction without being disrupted by reverse strokes. Moving the magnet in only one direction allows the domains to become increasingly organized and oriented, leading to stronger and more permanent magnetization of the iron bar.

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.