CBSE Class 12th Physics Notes: Magnetism and Matter (Part ‒ III)

Mayank Uttam Created On: Dec 14, 2016 16:30 IST

Atom as Magnetic Dipole

In an atom, an electron revolves around the nucleus. The orbit of electron behaves like a current loop of current which has a definite magnetic dipole moment associated with it.

Magnetic dipole moment of an orbit of radius r having electron moving with velocity ω, is given by,

CBSE Class 12 Physics Syllabus 2017

Important terms used in Magnetism

Relative Magnetic Permeability

It is the ability of a material to permit magnetic lines of force to pass through it. It is given by,

μ_R = (B/B_o) = (μ/μ_o)

Where B_o is number of magnetic lines per unit area in vacuum, B is number of magnetic lines of force per unit area in the given material medium. μ is magnetic permeability of a material and μ_o is magnetic permeability of free space.

μ_R ha no dimension and its value for vacuum is one.

Magnetic Intensity

Degree to which a magnetic field can magnetise a material is represented in terms of magnetic intensity.

It is given by, H = n i or H = B/ μ, where, n is number turns per unit length of a toroidal solenoid carrying a current i. It is a vector quantity and its SI unit is Am^‒1.

Intensity of Magnetisation

Intensity of Magnetisation of a magnetic material is defined as magnetic moment per unit volume of the material.

Magnetic Susceptibility

Susceptibility of a magnetic material is defined as the ratio of the intensity of magnetisation (I) induced in the material to the magnetizing force (H) applied to it.

It is given by,

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Relation between Magnetic Susceptibility and Magnetic Permeability

A magnetic material placed in a magnetising field of magnetising intensity H, the material gets magnetised. Total magnetic induction B in the material is the sum of the magnetic induction B_o in vacuum produced by the magnetic intensity and magnetic induction B_m, due to magnetisation of the material, therefore, B = B_o + B_m

Now, B_o = μ_o H and B_m = μ_o I, where I is the intensity of magnetistion induced in the magnetic material. So, B = μ_o (H + I)

We know that, B = μ H and I = χ_m H

Classification of Magnetic Material

Different types of material behave differently in presence of external magnetic field and are categorised into following types:

(i) Diamagnetic Material (ii) Paramagnetic Material and (iii) Ferromagnetic Material

Diamagnetic Material

Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field. Materials in whose atoms (molecules) or ions do not possess any net magnetic moment of their own.

Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride.

Paramagnetic Material

Paramagnetic materials are those which get weakly magnetised when placed in an external magnetic field. They have tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get weakly attracted to a magnet.

The individual atoms (or ions or molecules) of a paramagnetic material possess a permanent magnetic dipole moment of their own.

Ferromagnetic Material

Ferromagnetic substances are those which gets strongly magnetised when placed in an external magnetic field. They have strong tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get strongly attracted to a magnet.

The individual atoms (or ions or molecules) in a ferromagnetic material possess a dipole moment as in a paramagnetic material but in a more cooperative way.

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Curie’s Law in Magnetism

According to this law, the intensity of magnetization (I) of magnetic material is directly proportional to the magnetic field (H), and inversely proportional to the absolute temperature (T) of the material.

Where, C is a constant known as Curie’s constant.

Hysteresis Curve:

Hysteresis curve is the relation between magnetic induction or intensity of magnetization of a ferromagnetic material with magnetizing force or magnetic intensity.

The graph given above shows hysteresis loop for ferromagnetic materials taken through one cycle of magnetisation.

Chapter Wise Revision Notes on Chapter 5: Magnetism and Matter (Part I)

Chapter Wise Revision Notes on Chapter 5: Magnetism and Matter (Part II)