Magnetism and MatterMind Map

A bar magnet is a magnetic dipole with two inseparable poles: north and south. Its magnetic dipole moment is directed from south pole to north pole inside the magnet and measures the strength of the magnet. A bar magnet can be treated as an equivalent solenoid because both produce similar magnetic field patterns. Magnetic field lines emerge from north pole and enter south pole outside the magnet, while inside they run from south to north, forming closed loops. Earth behaves approximately like a giant magnet, but its geographic north lies near Earth’s magnetic south pole. Earth’s magnetism is described using declination, inclination or dip, and horizontal component of Earth’s magnetic field.

Magnetic field lines represent the direction and strength of magnetic field. Outside a magnet, they go from north to south; inside the magnet, they return from south to north, forming closed loops. The tangent to a field line gives the magnetic field direction, and closer lines indicate stronger field. Magnetic flux through a surface is ΦB = B·A = BA cosθ. Gauss’s law in magnetism states that net magnetic flux through any closed surface is zero: ∮B·dA = 0. This means magnetic field lines entering a closed surface always equal field lines leaving it. The law expresses the absence of magnetic monopoles, meaning isolated north or south poles do not exist.

Magnetisation describes how strongly a material becomes magnetised in an external magnetic field. It is defined as magnetic dipole moment per unit volume: M = mnet/V. Magnetic intensity H represents the magnetising field produced by free currents. Magnetic susceptibility χm measures how easily a material gets magnetised and is defined by M = χmH for linear materials. Magnetic permeability μ measures how much magnetic field is supported in a material, and relative permeability μr compares it with free space. The important relation inside a magnetised material is B = μ0(H + M). For linear isotropic materials, B = μH and μ = μ0(1 + χm). These quantities help classify diamagnetic, paramagnetic and ferromagnetic materials.

Materials respond differently to an applied magnetic field depending on their atomic magnetic moments. Diamagnetic materials develop induced magnetisation opposite to the applied field and are weakly repelled. Paramagnetic materials have permanent atomic dipoles that partially align with the field and are weakly attracted. Ferromagnetic materials have strong interactions between neighbouring dipoles, forming domains that align strongly in a field; they show large magnetisation, hysteresis and retentivity. Hysteresis is the lag of magnetisation behind the magnetising field, and the area of the hysteresis loop represents energy loss per cycle. Above Curie temperature, ferromagnetic materials become paramagnetic. These properties decide applications such as electromagnets, transformer cores and permanent magnets.