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Fun with Magnets
Nearly 5,000 years ago, people of Magnesia, a province in Asia Minor, discovered that a particular type of rock could attract small pieces of iron towards it. This wonderful rock was named Magnetite after Magnesia, the place where magnetic iron ore was first.
WHAT ARE MAGNETS?
Any substance which possesses the following two properties is called a magnet.
1. It attracts small pieces of iron towards itself.
2. It always comes to rest in the north-south direction when suspended freely.
NATURAL AND ARTIFICIAL MAGNETS
Lodestone and magnetites are natural magnets because they are found in nature.
Artificial magnets are prepared by man to be used at any time and at any place. These magnets are much stronger than lodestone.
Types of magnets
There are two types of magnets: temporary and permanent.
MAGNETIC AND NON-MAGNETIC SUBSTANCES
Substances which are attracted by a magnet are called magnetic substances. Iron, cobalt, nickel and their alloys are magnetic substances.
Substances which are not attracted by a magnet are called non-magnetic substances. Aluminium, copper, brass, stainless steel, wood, plastic, etc. are non-magnetic substances.
PROPERTIES OF MAGNETS
Let us now study some important properties of magnets.
1. Poles of a Magnet: The two ends of a magnet where the magnetic force is maximum are called the poles of the magnet. Each magnet has two poles.
2. Directional Property: A magnet always points in the north-south direction when suspended freely. The end of the magnet that points towards the north is called the North Pole (N-Pole) and the other end of the magnet pointing towards the south is called the South Pole (S-Pole).
3. Like Poles Repel, Unlike Poles Attract: If we bring the north pole of a magnet close to the north pole of another magnet, we will see that they repel each other. However, when the north pole of a magnet is brought close to the south pole of another, they attract each other.
4. Magnetic Poles Always Exist in Pairs: If a bar magnet is broken into two pieces you will see that each piece behaves as a whole magnet. This shows that new poles are formed at the broken ends as shown in figure. If these pieces are broken again, each smaller piece still remains a whole magnet with two opposite poles. Even a very small piece of a magnet is a whole magnet.
A magnetic compass is a simple device which has a magnetic needle (free to rotate) on a pivot at the centre of a round box as shown in figure. It is used by sailors and navigators to know the northern, souther, eastern and western directions. In the compass, the needle which is free to rotate, always rests in the north-south direction.
Magnetic field: Around every magnet there is a certain zone within which the magnetic force can be experienced. This certain zone is called as magnetic field.
Magnetic field lines: Within the field the magnetic force acts along some lines. These lines are known as magnetic field lines.
A current carrying straight conductor or a circular conductor produces a magnetic field around them. Magnetic Field due to a Current through a Straight Conductor: When a straight conductor carries electric current, a magnetic field is set up around the conductor. The field lines around the conductor are concentric circles whose centres lie on the conductor as shown in the figure.
The magnetic field produced causes the deflection in the compass needle. It is found that the deflection in the needle increases when current increases. Again if the needle is moved away from the conductor, the deflection in the needle decreases i.e., the magnetic field produced by given current in the conductor decreases as the distance from it increases.
Right Hand Thumb Rule: The direction of the magnetic field associated with a current carrying straight conductor can be obtained by applying the “right hand thumb rule”. According to this rule:
Hold the current carrying conductor in your right hand such that the thumb Field mpoints in the direction of current, then the direction in which your fingers
encircle the conductor will give the direction of the field lines of the magnetic
Magnetic Field due to a Current through a Circular Loop: When a current is passed through the circular coil, a magnetic field is produced around it. The pattern of magnetic field lines is shown in the figure. The field lines are circular near the wire but they become straight at the centre of the coil.
Earth Magnetism: The earth is also a huge magnet whose poles and the nature of the magnetic field is
We shall learn more about magnetism in higher classes.