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 the 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). (Figure)
Causes of Earth’s Magnetism
Earth’s Magnetism is generated by convection currents of molten iron and nickel in the earth’s core. The earth’s magnetic field extends millions of kilometers into outer space and looks very much like a bar magnet. These currents carry streams of charged particles and generate magnetic fields. The earth’s south magnetic pole is actually near the North Pole and the magnetic north pole is in Antarctica! This is why a compass magnet’s the North Pole actually points north (north and south poles attract). The Earth’s magnetic field extends far and wide but is very weak in terms of field strength. The Earth’s magnetic field extends far and wide but is very weak in terms of field strength.
Components of Earth’s Magnetic Field
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
It is defined as the angle between the true north and the magnetic north. On the horizontal plane, the true north is never at a constant position and keeps varying depending upon the position on the earth’s surface and time.
(ii) Dip or inclination δ
It is the angle made the horizontal plane on the earth’s surface. At the magnetic equator, the angle of dip is 0° and at the magnetic poles, the angle of dip is 90°.
(iii) The horizontal component of the Earth’s magnetic field Bh.
The magnetic intensity of the earth’s magnetic field makes an angle known as the Angle of Dip (δ) with the horizontal axis.
The earth’s magnetic poles are not aligned to the actual geographic north and south poles. Instead, the magnetic south pole is in Canada while the magnetic north pole lies in Antarctica. The magnetic poles are inclined by about 10 degrees to the earth’s rotational axis. So, all this time your compass was really pointing to Canada, not the true North!
