The word magnetism is derived from iron ore magnetite (Fe3O4), which was found in the island of magnesia in Greece. It is believed that the Chinese had known the property of the magnet even in 2000 B.C. and they used magnetic compass needle for navigation in 1100 AD. But it was Gilbert who laid the foundation for magnetism and had suggested that Earth itself behaves as a giant bar magnet. The field at the surface of the Earth is approximately 10-4 T and the field extends upto a height of nearly five times the radius of the Earth.
Earth?s magnetic field and magnetic elements
A freely suspended magnetic needle at a point on 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 near its geographical poles. Since the north pole of the magnetic needle approximately points towards geographic north (NG) it is appropriate to call the magnetic pole near NG as the magnetic south pole of Earth Sm. Also, the pole near SG is the magnetic north pole of the Earth (Nm). (Fig.)
The Earth?s magnetic field at any point on the Earth can be completely defined in terms of certain quantities called magnetic elements of the Earth, namely
(i) Declination or the magnetic variation θ.
(ii) Dip or inclination δ and
(iii)The horizontal component of the Earth?s magnetic field Bh
Causes of the Earth?s magnetism
The exact cause of the Earth?s magnetism is not known even today. However, some important factors which may be the cause of Earth?s magnetism are:
(i) Magnetic masses in the Earth.
(ii) Electric currents in the Earth.
(iii)Electric currents in the upper regions of the atmosphere.
(iv) Radiations from the Sun.
(v) Action of moon etc.
However, it is believed that the Earth?s magnetic field is due to the molten charged metallic fluid inside the Earth?s surface with a core of radius about 3500 km compared to the Earth?s radius of 6400 km.
The iron ore magnetite which attracts small pieces of iron, cobalt, nickel etc. is a natural magnet. The natural magnets have irregular shape and they are weak. A piece of iron or steel acquires magnetic properties when it is rubbed with a magnet. Such magnets made out of iron or steel are artificial magnets. Artificial magnets can have desired shape and desired strength. If the artificial magnet is in the form of a rectangular or cylindrical bar, it is called a bar magnet.
Basic properties of magnets
(i)When the magnet is dipped in iron filings, they cling to the ends of the magnet. The attraction is maximum at the two ends of the magnet. These ends are called poles of the magnet.
(ii) When a magnet is freely suspended, it always points along north-south direction. The pole pointing towards geographic north is called north pole N and the pole which points towards geographic south is called south pole S.
(iii) Magnetic poles always exist in pairs. (i.e) isolated magnetic pole does not exist.
The magnetic length of a magnet is always less than its geometric length, because the poles are situated a little inwards from the free ends of the magnet. (But for the purpose of calculation the geometric length is always taken as magnetic length.)
(v) Like poles repel each other and unlike poles attract each other. North pole of a magnet when brought near north pole of another magnet, we can observe repulsion, but when the north pole of one magnet is brought near south pole of another magnet, we observe attraction.
(vi)The force of attraction or repulsion between two magnetic poles is given by Coulomb?s inverse square law.
Note : In recent days, the concept of magnetic poles has been completely changed. The origin of magnetism is traced only due to the flow of current. But anyhow, we have retained the conventional idea of magnetic poles in this chapter. Pole strength is denoted by m and its unit is ampere metre.