Magnetism and Matter

The One Thing

Magnetism is described through magnetic dipoles, their fields, and the response of atomic magnetic moments to an applied field. Magnetic field lines, dipole equations, and quantities such as magnetisation and susceptibility provide a quantitative account of magnetic behaviour in matter.

Reactions, Processes and Experiments

Key Terms

• Bar magnet: A magnet with two poles, north and south, whose magnetic effect is strongest near its ends.
• Magnetic poles: The two ends of a magnet where its magnetic strength is greatest; isolated magnetic poles have not been observed.
• Magnetic dipole: A system having equal and opposite magnetic poles separated by a small distance; a bar magnet behaves approximately as a magnetic dipole.
• Magnetic dipole moment: A vector quantity that measures the strength and orientation of a magnetic dipole. For a bar magnet, its magnitude is pole strength multiplied by magnetic length.
• Magnetic field: The region around a magnet or current-carrying conductor in which another magnet or moving charge experiences a magnetic force.
• Magnetic field lines: Imaginary curves whose tangent at any point gives the direction of the magnetic field.
• Magnetisation: The net magnetic dipole moment developed per unit volume of a material, represented by M.
• Magnetic intensity: The magnetising field applied to a material, represented by H.
• Magnetic susceptibility: A dimensionless measure of how easily a material becomes magnetised, defined as chi_m = M/H.
• Relative permeability: The ratio of a material's permeability to the permeability of free space, given by mu_r = 1 + chi_m for a linear material.
• Diamagnetic material: A material weakly repelled by a magnetic field, with negative susceptibility and relative permeability slightly less than one.
• Paramagnetic material: A material weakly attracted by a magnetic field, with small positive susceptibility and relative permeability slightly greater than one.
• Ferromagnetic material: A strongly attracted material in which magnetic domains can align, producing large magnetisation and possible permanent magnetism.
• Magnetic domains: Small regions in a ferromagnetic substance within which atomic magnetic moments are aligned in the same direction.
• Curie temperature: The temperature above which a ferromagnetic material loses its ferromagnetism and behaves approximately as a paramagnetic material.
• Earth's magnetism: Earth behaves approximately like a giant magnetic dipole, producing a magnetic field with horizontal and vertical components.
• Magnetic declination: The angle between the geographic meridian and the magnetic meridian at a place.
• Magnetic inclination or dip: The angle made by Earth's total magnetic field with the horizontal plane.
• Magnetic elements of Earth: Declination, dip, and horizontal component of Earth's magnetic field together describe the magnetic field at a location.

Easily Confused

• Axial field and equatorial field: At the same distance from a short dipole, the axial field is twice the magnitude of the equatorial field: $B_{\text{axial}}=({\mu }_0/4\pi )(2m/r^3)$, whereas $B_{\text{equatorial}}=({\mu }_0/4\pi )(m/r^3)$.
• Uniform and non-uniform magnetic fields: A dipole in a uniform field experiences torque but no net force; in a non-uniform field it can experience both force and torque.
• Stable and unstable equilibrium: The dipole is in stable equilibrium when its magnetic moment is parallel to the external field; the antiparallel position is unstable equilibrium.
• Diamagnetic and paramagnetic materials: Diamagnetic materials have ${\chi }_m<0$ and are weakly repelled, whereas paramagnetic materials have small positive ${\chi }_m$ and are weakly attracted.
• Paramagnetic and ferromagnetic materials: Both have positive susceptibility, but paramagnetic attraction is weak, while ferromagnetic attraction is strong because magnetic domains can align.
• Magnetic declination and magnetic dip: Declination is the angle between the geographic and magnetic meridians; dip is the angle between Earth’s total field and the horizontal plane.
• Magnetic field direction inside and outside a bar magnet: Outside the magnet, field lines run from north to south; inside it, they run from south to north.
• Magnetisation and magnetic intensity: Magnetisation $M$ is the net magnetic dipole moment per unit volume, whereas magnetic intensity $H$ is the magnetising field applied to the material.

What Gets Asked

• Calculations involving a short magnetic dipole: Questions may require the axial or equatorial field equations. The common mark-losing slip is forgetting the factor of 2 in $B_{\text{axial}}$ or failing to note that the axial field is twice the equatorial field at the same distance.
• Torque and potential-energy problems: Questions may use $\tau =mB\sin \theta$ or $U=-mB\cos \theta$. The key distinction is that torque tends to align the dipole, while the minimum potential energy occurs in the parallel orientation.
• Field-line interpretation: Questions may ask for field direction, relative field strength, or the reason field lines are closed. Marks are lost by drawing intersecting lines, reversing the internal direction, or treating magnetic poles as isolated monopoles.
• Classification of magnetic materials: Questions may require comparison of diamagnetic, paramagnetic, and ferromagnetic substances using susceptibility, attraction or repulsion, and examples. The specific examples must be matched correctly: bismuth, copper, water, and quartz are diamagnetic; aluminium, platinum, and oxygen are paramagnetic; iron, cobalt, nickel, and some alloys are ferromagnetic.
• Material equations: Questions may use $B={\mu }_0(H+M)$, ${\chi }_m=M/H$, or ${\mu }_r=1+{\chi }_m$. A common error is confusing magnetisation $M$ with magnetic intensity $H$.
• Earth’s magnetism: Questions may involve declination, dip, or the component relations $B_H=B\cos \delta$ and $B_V=B\sin \delta$. The mark-losing slip is confusing declination with dip or reversing the horizontal and vertical component relations.