A magnet is a substance which has the property of attracting the iron and its ores or the magnetic substances.
Different properties of a Magnet
It has the following properties:
- A magnet attracts iron, its alloys and other magnetic substances.
- It has the property of direction, if a magnet is freely suspended, it always indicates towards north and south.
- It has two poles, the pole which indicates towards geographical north is known as north pole and the one which indicates towards south is known as south pole.
- Like poles repel each other and unlike poles attract each other.
- The pole always exists in pairs, one isolated pole cannot exist.
- It induces the opposite polarity as soon as the magnetic material is brought near it, i.e., it induces the magnetic property into the other.
What are the different types of magnets?
These are the following types of the magnet:(a) Natural magnet (b) Artificial magnet
Natural magnet
A substance is found in the nature which exhibits the property of attracting the iron and its filings, simultaneously the property of directing north and south. It is in the shape of a stone as shown in Fig. I l. In the ancient era it was used by the navigators for direction purposes. It is known as leading stone or lodestone.
Artificial magnet
The magnets which are prepared by the human beings by artificial means like touch method etc. are known as artificial magnets. These are prepared by iron or steel.
The artificial magnets are of the following types:
(i) Temporary magnet
(ii) Permanent magnet.
Fig. 11.1. Natural magnet
(i) Temporary magnets. The magnets which loses their magnetism as soon as the magnetising means or force is removed. All the electromagnets are the temporary magnets as shown in Fig. 11 .2. The metal used in this case is the soft iron. The magnets are used in electric bell, buzzers, bell indicators, etc.
(ii) Permanent magnets. The permanent magnets are those which retain their magnetism for a long time. It is observed that if a piece of hard steel is magnetised, it acquire a substantial magnetism which it retains for an indefinite time. Such magnets are permanent magnets. The material used for permanent magnets are tungsten steel, high carbon steel and other hardened steel etc.
Fig. 11.2. Temporary magnet
Shapes. There are the different shapes like bar magnet, horse shoe magnet, U-shape magnet and magnetic needle, as shown in Fig. 11 .3.
Describe the molecular theory of a magnet.
If a permanent magnet is broken into pieces, every piece will behave like a separate magnet. There will be two poles north and south as shown in Fig. 1. Every substance is made Of a number of molecules. Each molecule is an independent magnet; having north and south. In an ordinary stage these molecules are not arranged in a proper sequence as shown Fig. I I .4(b) these are placed haphazardly. Thus the molecules neutralizes the effect of magnetism and as a result the piece do not behave like a magnet. If a magnetising force is applied, the molecules starts arranging in a proper sequence and the substance starts exhibiting the property of a magnet as shown in Fig. 11.4 (c). Otherwise also if the molecules are disarranged and if a bar magnet is brought as one end the molecules starts arranging in a straight line, thus two poles are formed. The end near the north of a magnet, will magnetise to be south of the piece and so on. The magnetic power of the piece to be magnetised will be proportional to the number of molecules arranged if less molecules are arranged the magnetism will be less and if more molecules then the magnetic power will be more. In case where all the molecules are arranged the maximum power will be exhibited, after this if the magnetising power is increased even then the magnetism will not increase, this point is known as saturation point.
Fig .4. Molecular structure of magnet.
Define the following: Magnetic pole, magnetic axis, magnetic flux, magnetic flux density, and magnetic field strength.
Magnetic pole
The two ends of the magnet where the magnetic power is maximum or concentrated, are known as the magnetic poles. There are two poles North and South.
Magnetic axis
It is an imaginary straight line joining the north and south poles of a magnet. Here the joining line AB is the magnetic axis. The other is the magnetic neutral axis, the line which right bisect the magnetic axis is called magnetic neutral axis, shown by CD in Fig. 1.10.
Magnetic field
The space or region occupied by the lines of force around a magnet is called magnetic field.
Magnetic flux
The total number of lines of force in a magnetic field is called magnetic flux. The unit is line and in MKS system weber.
The symbol is Φ.
Magnetic flux density
It is defined as the magnetic flux per unit area. It is represented by letter B and unit is Wb/m2 or max/cm2 or lines/cm2.
Magnetic field strength
The magnetic field strength is a vector quantity possessing the magnitude and direction. It is defined as the force experienced by a unit north pole of one Wb when it is placed there. The field strength at any point within a magnetic field is measured by the force experienced by a north pole of one weber placed at that point. It is denoted by letter H and the unit is N/Wb or ampere Turns/meter.
