Magnetic Field

Magnetic Field

• We have seen that magnet has its influence on the surrounding medium. 'The region around a magnet within which the influence of the magnet can be experienced is called magnetic field'. Existence of such field can be experienced with the help of compass needle, iron or pieces of metals or by bringing another magnet in vicinity of a magnet.

1. Magnetic Lines of Force

• The magnetic field of magnet is represented by imaginary lines around it which are called magnetic lines of force. Note that these lines have no physical existence, these are purely imaginary and were introduced by Michael Faraday to get the visualization of distribution of such lines of force.

2. Direction of Magnetic Field

• Consider the isolated N pole (we cannot separate the pole but imagine to explain line of force) and it is allowed to move freely, in a magnetic field. Then path along which it moves is called line of force. Its shape is as shown in the Fig. 1.3.1 and direction always from N-pole towards S-pole.

• The lines of force for a bar magnet and U-shaped magnet are shown in the Fig. 1.3.2.

• Attraction between the unlike poles and repulsion between the like poles of two magnets can be easily understood from the direction of magnetic lines of force. This is shown in the Fig. 1.3.3 (a) and (b).

3. Properties of Lines of Force

• Though the lines of force are imaginary, with the help of them various magnetic effects can be explained very conveniently. Let us see the various properties of these lines of force.

 1) Lines of force are always originating on a N-pole and terminating on a S-pole, external to the magnet.

2) Each line forms a closed loop as shown in the Fig. 1.3.4.  

Key Point : This means that a line emerging from N-pole, continues upto S-pole external to the magnet while it is assumed to continue from S-pole to N-pole internal to the magnet completing a closed loop. Such lines internal to the magnet are called as lines of induction.

3) Lines of force never intersect each other.

4) The lines of force, are like stretched rubberbands and always try to contract in length.

5) The lines of force, which are parallel and travelling in the same direction repel each other.

6) Magnetic lines of force always prefer a path offering least opposition.

Key Point : The opposition by the material to the flow of lines of force is called reluctance. Air has more reluctance while magnetic materials like iron, steel etc. have low reluctance. Thus magnetic lines of force can easily pass through iron or steel but cannot pass easily through air.

Review Question

1. What is magnetic field and magnetic lines of force? State the properties of lines of force.