Revision: Magnetic Effects of Current and Magnetism >> Magnetism and Matter Physics Science (English Medium) Class 12 CBSE

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

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

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

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

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

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

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

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

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

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

Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.

The value of the reverse magnetising field (H₀) required to reduce the residual magnetic induction of a ferromagnetic material to zero (represented by point F on the hysteresis loop) is called coercivity.

The lagging of intensity of magnetisation (I) or magnetic induction (B) behind the magnetising field (H) during the process of magnetisation and demagnetisation of a ferromagnetic material is called hysteresis.

The residual value of magnetic induction (B) retained by a ferromagnetic material when the magnetising field (H) is reduced to zero (represented by point B/C on the hysteresis loop) is called retentivity.

The temperature above which a ferromagnetic substance becomes paramagnetic is called curie temperature. 

A macroscopic region in a ferromagnetic material where magnetic dipoles are aligned in a common direction.

Magnetisation M is the net magnetic moment per unit volume.

\[\mathbf{M}=\frac{m_{\mathrm{net}}}{V}\]

Unit: A m⁻¹

The magnetic moment of a bar magnet is equal to the magnetic moment of an equivalent solenoid producing the same magnetic field.

A bar magnet is equivalent to a solenoid carrying current, as both produce similar magnetic field patterns, especially at large distances.

Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field.

A bar magnet behaves like a magnetic dipole, having two equal and opposite poles separated by a small distance.

A bar magnet is a magnet in the shape of a rectangular bar having two poles — a north pole and a south pole — at its ends.

Ferromagnetic substances are those which get strongly magnetised when placed in an external magnetic field.

Diamagnetic substances are those which have a tendency to move from stronger to weaker parts of an external magnetic field.

Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field.

\[\vec τ\] = \[\vec p\] × \[\vec E\]

Magnitude: τ = pE sin θ

\[B=\frac{\mu_0}{4\pi}\frac{2m}{r^3}\]

Where:

• m = magnetic dipole moment
• r = distance from centre
• μ₀ = permeability of free space

τ = m × B

Magnitude:

τ = mB sin θ

U = −m ⋅ B

B_0​ = μ_0​nI

When a ferromagnetic material is subjected to a cycle of magnetisation and demagnetisation, the intensity of magnetisation (I) or magnetic induction (B) lags behind the magnetising field (H). This lagging behaviour is called hysteresis. When plotted on a B–H graph, the curve forms a closed loop (hysteresis loop) in which:

• B₀​ (point A) denotes the saturation magnetic induction,
• The intercept OB (or OC) on the B-axis when H is reduced to zero represents retentivity (residual magnetism),
• The intercept OF on the H-axis (reverse field H_0​) needed to reduce B to zero represents coercivity,
• Points D and E represent reverse saturation and reverse retentivity respectively.

The area enclosed by the hysteresis loop is equal to the energy loss per cycle per unit volume of the material, and this area is different for different materials. Therefore, materials used as electromagnets require a narrow loop (low hysteresis loss), while permanent magnets require a wide loop (high retentivity and coercivity).

Tangent law states that, if a magnetic field ‘B’ is applied at right angles to the horizontal component of the earth's field B_H, the needle comes to equilibrium at an angle ‘$\theta ’$ to the magnetic meridian such that, tan θ = `B/B_H`.

• Magnetic phenomena are universal and exist at all scales, from atoms to galaxies, including the Earth.
• The Earth behaves like a giant bar magnet, with its magnetic field directed approximately from geographic south to geographic north.
• A freely suspended bar magnet aligns itself along the north–south direction; like poles repel and unlike poles attract.
• Magnetic monopoles do not exist; breaking a magnet always produces smaller magnets, each having both north and south poles.
• Certain materials, such as iron and its alloys, can be magnetised, and materials are classified as diamagnetic, paramagnetic, or ferromagnetic based on their magnetic properties.