Magnetism is a force of attraction or repulsion that acts at a distance. it is due to a magnetic field, which is caused by moving electrically charged particles. it is also inherent in magnetic objects such as a magnet.

A magnet is an object that exhibits a strong magnetic field and will attract materials like iron to it. magnets have two poles, called the north (n) and south (s) poles. Two magnets will be attracted by their opposite poles, and each will repel the like pole of the other magnet. magnetism has many uses in modern life.

The materials which can be made to behave like a magnet and which are easily magnetized by a magnetic field called as magnetic materials.

The materials which can be made to behave like a magnet and which are easily magnetized by a magnetic field called as magnetic materials.

Basic Definitions

1. Magnetic Induction (B) or Magnetic Flux Density
   It is the number of magnetic lines of force passing through unit area. Unit: weber/m² or tesla
2. Intensity of Magnetic Field or Magnetic Field Strength or Magnetising Field (H)
   It is the force experienced by an unit north pole placed at the given point in a magnetic field. H = F/m,  Unit: ampere/metre.
3. Magnetic Dipole
   The north pole and south pole are separated by a small distance constitute a magnetic dipole.
4. Magnetic Field
   The space around which the magnetic lines of forces are exist is called magnetic field.
5. Magnetic Lines of Force
   The continuous curve in a magnetic field that exist from north pole to south pole is called magnetic lines of force.
6. Magnetic Flux ( Φ )
   The group of all magnetic lines of force is called magnetic flux.
7. Magnetic Moment ( m )
   The product of pole strength and magnetic length is called magnetic moment. (m = p x 21)  Unit: weber metre.
8. Intensity of Magnetisation (I)
   Magnetisation is the process of converting a non-magnetic material in to a magnetic material.
   It is defined as the magnetic moment per unit volume. I = m/V  Unit: weber/m² .
9. Magnetic Permeability (μ)
   It is the ratio of the magnetic induction (B) to the applied magnetic field intensity (H). μ = B/H  Unit: henry/m
   Thus it is the measure of the ability of the material to permit magnetic lines of force.
10. Relative Permeability (μ_r )
    It is defined as the ratio of permeability of the medium to the permeability of the free space. μr = μ/μ_º  .
11. Magnetic Susceptibility (χ)
    It is defined as the ratio of intensity of magnetization and intensity of magnetic field. χ = I/H
    The sign and magnitude of χ  are used to determine the nature of the magnetic materials.
12. Bohr Magneton (μ_B )
    It is the magnetic moment produced by an unpaired electron in an atom. It is the fundamental unit of magnetic moment. One Bohr magneton, μ_B = 9.27 x 10^-24 Am² .

Relation between susceptibility [χ] and permeability [μ_r]

• We know that when a current is supplied through a coil, magnetic field is developed.
• When a magnetic material is placed inside a external magnetic field, the magnetic flux density [B] arises is due to applied magnetic field [H] and also due to the induced magnetization [I] i.e. The total flux density,

                                  B = μ_º ( I + H )                                              ………(1)

But we know that  μ = B/H                                                         ………(2)