Magnetisation and Magnetic Intensity

A useful way to understand magnetisation is to imagine that the tiny magnetic dipoles inside a material begin to align when the material is placed in a magnetic field. Greater alignment means greater magnetisation.

Analogy: Just as students standing randomly in a playground create no overall pattern, magnetic dipoles in random orientation create little or no net magnetisation. When they begin facing nearly the same direction, the material shows a net magnetic effect.

The ratio of magnetic moment to the volume of the material is called magnetisation.

The ratio of the magnitude of the total field inside the material to that of the intensity of the magnetising field is called magnetic permeability.

The ratio of the magnitude of intensity of magnetisation to that of magnetic intensity is called magnetic susceptibility.

The ratio of the strength of the magnetising field to the permeability of free space is called magnetic intensity.

The ratio of magnetic permeability of the material (μ) and magnetic permeability of free space (μ₀) is called relative permeability.

The magnetic field that exists in a vacuum and induces magnetism is called a magnetising field.

The total magnetic field inside a magnetic material, which is the sum of the external magnetising field and the additional magnetic field produced due to magnetisation of the material, is called magnetic induction.

The ability of a magnetising field to magnetise a material medium is called magnetising field intensity.

The magnetic moment developed per unit volume of a material when placed in a magnetising field is called the 'intensity of magnetisation'.

M = \[\frac {\text {magnetic moment​}}{\text {volume}}\]

• SI unit: A/m

H = nI

where n is the number of turns per unit length and I is current.

• SI unit: A/m

B = μ₀​(H + M)

• SI unit: tesla (T)

M = χH

or

χ = HM​

B = μH

• SI unit: H/m

μr ​= \[\frac {μ}{​μ_0}\]

A solenoid has 1000 turns per metre and carries a current of 2 A. The core material has a relative permeability of 400. Find H, B, and the nature of magnetisation.

Step 1: Magnetic intensity

Step 2: Magnetic induction

Substituting values gives approximately 1.0 T.

Step 3: Interpretation

A large value of μ_r means the material greatly enhances the magnetic field inside the solenoid. This is why magnetic cores are used in electromagnetic devices.

• Transformer cores: Soft ferromagnetic materials are used because they magnetise strongly and improve the magnetic effect.
• Permanent magnets: Hard magnetic materials are chosen because they retain magnetisation for longer durations.
• Medical imaging: Magnetic properties of materials are important in technologies such as MRI systems and magnetic shielding applications.