Overview: Magnetism and Mater

• 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.

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.

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

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

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

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

Magnetisation M is the net magnetic moment per unit volume.

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

Unit: A m⁻¹

B_0​ = μ_0​nI

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.

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

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