### Revision of Magnetism

Magnetism is a property by virtue of which a piece of solid body attracts iron pieces and pieces of some other metals. Such a piece of solid body is called a natural magnet. The two ends of a magnet are called its poles. When such a magnet is suspended freely by a piece of a silk fiber, it turns and adjusts itself in the direction of North and South of the earth. The end adjusting itself in the direction of North is called N pole while other is called S pole.When such two magnets are brought near each other, their behaviour is governed by some laws called laws of magnetism.
1. Laws of Magnetism
Law 1 : It states that 'like' magnetic poles repel and 'unlike' poles attract each other.
When the two magnets are brought near each other, such that two like poles i.e. N and N or S and S are facing each other, then the two magnets experience a force of repulsion. As against this, if two unlike poles i.e. N and S or S and N are facing towards each other, then they experience a force of attraction and try to attract each other.
Law 2 : This law is experimentally proved by Scientist Coulomb and hence also known as coulomb's law.
The force (F) exerted by one pole on the other pole is,
1. Directly proportional to the product of the pole strengths.
2. Inversely proportional to the square of the distance between them and
3. Dependent on the nature of medium surrounding the poles.
Mathematically this law can be expressed as,
Where M1 and M2 are the pole strengths of the poles while 'd' is the distance between the two poles.
Where K is constant which depends on the nature of the surrounding.
2. Magnetic Field and Flux
The region around a magnet within which the influence of the magnet can be experienced is called its magnetic field. The presence of magnetic field is represented by imaginary lines around a magnet. These are called magnetic lines of force.
The total number of lines force existing in a particular magnetic field is called magnetic flux, denoted by a symbol ''. It is measured in a unit weber.
1 weber = 108  lines of force
Key point : The lines of force never intersect each other and are like stretched rubber bands and always try to contract in length.
These properties of lines of force play an important role in the understanding of the working principle of the d.c. machines.
The lines of flux have a fixed direction. These flux lines start at N-pole and terminate at S-pole, external to the magnet. While the direction of flux lines is from S-pole to N-pole, internal to the magnet. The distribution and direction of such flux lines for a bar magnet is shown in the Fig. 1.
 Fig.  1

These lines always from a closed loop and never intersect each other.