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

Magnetic field lines are imaginary lines representing the direction of the magnetic field

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

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

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

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

Substances which when placed in a magnetic field are feebly magnetised in a direction opposite to that of the magnetising field are called diamagnetic substances.

Substances which when placed in a magnetic field are feebly magnetised in the direction of the magnetising field are called paramagnetic substances.

Substances which when placed in a magnetising field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.

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

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.

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

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

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

The magnetic susceptibility of a paramagnetic material varies inversely with its absolute temperature. Mathematically,

On cooling, paramagnetic substances get converted to ferromagnetic materials at the Curie temperature.

For ferromagnetic substances above the Curie temperature, the magnetic susceptibility is inversely proportional to (T − T_C), where T_C is the Curie temperature. Mathematically,

On heating beyond the Curie temperature (T_C(iron) = 770 °C), ferromagnetic substances get converted into paramagnetic materials.

Direction:

• Outside magnet: North → South
• Inside magnet: South → North

Key Properties:

• Closed curves (no start or end)
• Never intersect each other
• The tangent at any point gives the direction of the magnetic field
• Closer lines → stronger field
• The field is strongest at the poles

• Relative permeability ranges:   μ_r ≫ 1, of the order of 10²; μ ≫ μ₀
• Diamagnetic: B ≫ B₀​; B_m ≫ B₀
• Magnetic susceptibility (χ): positive and high, χ ≈ 10²; very large, positive, temperature dependent, χ_m ∝ \[\frac {1}{T−T_C}\]​ (Curie–Weiss law)
• Magnetic moment: very high
• Intensity of magnetisation (I) vs H: I is very large, positive, varies non-linearly with H (I is in the direction of H, value of I is very high)

• Relative permeability ranges: μ_r < 1 (as B is less than μ₀H); also 1 > μ_r > 0, μ < μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and negative, ∣χ∣ ≈ 1; small, negative and temperature-independent, χ_m ∝ T₀
• Magnetic moment: very low (≈ 0)
• Intensity of magnetisation (I) vs H: I is small, negative, varies linearly with H (I and H in opposite direction, I is negative with respect to H)

• Relative permeability ranges:  μ_r > 1 (as B is slightly greater than μ₀H); (1 + ε) ≥ μ_r > 1, μ > μ₀
• Diamagnetic: B < B₀​; B_m < B₀
• Magnetic susceptibility (χ): low and positive, χ ≈ 1; small, positive, varies inversely with temperature, χ_m ∝ \[\frac {1}{T}\]​ (Curie law)
• Magnetic moment: very low but not zero
• Intensity of magnetisation (I) vs H: I is small, positive, varies linearly with H (I and H in same direction, value of I is low)

• 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.