Earth's Magnetism & Magnetic Declination | Physics for JAMB PDF Download

Introduction to The Earth’s Magnetism

The reason why, a bar magnet, when suspended freely, points in north - south direction is due to earth’s giant magnetic field. It is believed that the electric currents circulating from earth’s core to the space give rise to the earth’s magnetic field. The earth’s magnetic field is supposed to save earth from the solar wind which might cause the ozone layer of the earth to strip away. The SI unit of earth’s magnetic field is given by tesla.

Fig: SI unit of earth’s magnetic field is given by tesla

Theory of Earth’s Magnetism

There is no valid reason for the cause of earth’s magnetism or why earth has giant magnetic field but there are some theory related to earth’s magnetic field which helps us to understand that why earth behaves as a giant magnet.

• It is believed that the magnetic field of earth is due to dynamo effect. Dynamo effect is caused by the motion of metallic fluids in the outer core of the earth which results in electric current. It is because of this electric current that the earth has its own magnetic field lines.

• Another theory suggests that the rotation of earth in its own axis produces strong electric current since the outer layers of earth is ionized. As a result of which when the earth rotates, there is a movement of charged ions, which in return produces electric current.
   

Distinctive Aspect of Earth’s Magnetism

• A hypothetical giant magnetic dipole is supposed to be located at the centre of the earth. It does-not coincides with the axis of earth. The dipole is tilted by 11.3° with respect to earth’s axis as shown in the diagram given below:

Fig: Pictorial representation of earth’s magnetic field

• As it can be seen from the above diagram that there are two north (magnetic north and geographic north) poles and two south (magnetic south and geographic south) poles  located on the poles of the earth. The magnetic north and magnetic South Pole is the result of the dipole. The magnetic north pole is located at 79.74° N (latitude) and 71.8° W (longitude). Similarly, the magnetic south pole is located at 79.74° S (latitude) and 108.22°E (longitude)

• If we observe carefully the magnetic field lines of the earth, we observe that the magnetic field lines enter the north pole and leaves the south pole unlike Bar magnet, where the magnetic field lines enter the south pole and leaves the north pole. This is because the magnetic north pole actually behaves like the south pole of a bar magnets and vice versa. It was named as magnetic north because the magnetic needle (North Pole) of the bar magnet pointed in this direction.
   

Component of Earth’s Magnetic Field

The components that are responsible for the magnitude as well as the direction of earth’s magnetic field at a particular place is given by:

• Magnetic Declination

• Horizontal Component of Earth’s Magnetic Field

• Angle of Dip or Magnetic Inclination
   

Magnetic Declination

When a magnetic needle is suspended freely in the air, it always points in the north-south direction. This direction in which a magnetic needle points freely in a particular direction free form all other attracting forces is known as Magnetic Meridian. 

Magnetic Declination is defined as the angle made by the Magnetic meridian with the geographic meridian. Here geographic meridian is defined as the plane passing through the north and south poles of the earth.

Fig: The angle θ is known as the magnetic declination

Fig: Pictorial representation of true geographic north and south-pole

Angle of Dip or Magnetic Inclination

Take a magnetic needle and suspend it freely so that it can rotate about a horizontal axis as shown in the diagram below:

Fig: Angle of dip or Magnetic inclination

The angle that the north pole of the needle makes with the horizontal axis is known as the Angle of Dip or Magnetic Inclination.