Magnetic Field

Keep a small magnetic needle near a bar magnet. The magnetic needle rotates and stops in a particular direction only. This shows that a force acts on the magnetic needle that makes it rotate and rest in a particular direction only. This force is called torque.

The region around the magnet where the force on the magnetic needle occurs and the needle stops at a specific direction, is called a magnetic field. The direction of magnetic field is represented by magnetic line of forces.

The direction of magnetic needle changes continuously and it takes the curved path while moving from north to south. This curved path is known as magnetic line of forces. Tangent line draw at any point on magnetic line of force, represent the direction of magnetic field at that point.

Properties of Magnetic Lines of Force

These magnetic line of forces have following properties:

  1. Magnetic line of forces always start from north pole and end at south pole of the magnet.
  2. These line of forces never intersect each other.
  3. Near the poles magnetic lines are very near to each other which shows that magnetic field at the poles is stronger as compare to other parts.

Earth as Magnet

The Earth acts as a giant magnet with south magnetic pole somewhere in the Arctic and north magnetic pole in Antaractic. The Earth also behaves like a bar magnet. Its hot liquid centre core contains iron and as it moves, it creates an electric current that cause a magnetic field around the Earth.

The Earth has a north and south magnetic pole. These poles are not same with the geographic north and south poles on a map and tilted at an angle of 11.3 degree with respect to it. Due to this, if a magnetic needle is suspended freely, it rests in the north-south direction and is useful for navigation.

Magnetic Field around Current Carrying Wire

If an electric current is made to flow in a wire, magnetic field produce around it.

A magnetic needle placed parallel to the conducting copper wire (attached to the two ends of a battery) shows deflection. This shows that when electric current flows through a conductor, magnetic field is produced around the conductor.

If the current is increased, there is greater amount of deflection. If the direction of flow of electric current is changed (by reversing the end of the battery) the direction of deflection in the magnetic needle is also reversed. If the current flow is stopped the deflection in the magnetic needle also ceases. Thus, magnetic field is an effect of flow of electric current through conducting wire. In the year 1820 a scientist from Denmark named H.C. Oersted observed this effect for the first time.