Magnetism and Matter
Earth's magnetic field, magnetic properties of materials, and magnetic behaviour at the atomic level
Table of Contents
5.1 Introduction
Magnetic phenomena are universal in nature. Vast, distant galaxies, the tiny invisible atoms, humans and beasts all are permeated through and through with a host of magnetic fields from a variety of sources. The earth's magnetism predates human evolution.
5.2 Earth's Magnetic Field
The Earth behaves as if it has a giant bar magnet inside. The magnetic field lines emerge from the geographic south and enter at the geographic north (the internal magnetic poles are reversed).
- Magnetic Declination: Angle between geographic meridian and magnetic meridian at a place
- Magnetic Dip (Inclination): Angle that the total magnetic field vector makes with the horizontal at any point
- Horizontal Component (Bₕ): Component of Earth's field in the horizontal direction
5.3 Magnetic Dipole Moment
A current loop behaves like a magnetic dipole. The magnetic dipole moment is:
5.4 Magnetic Field Intensity
The magnetic field intensity at a point on the axis of a short bar magnet is:
5.5 Torque on a Magnetic Dipole
A magnetic dipole in a uniform magnetic field experiences a torque:
5.6 Diamagnetic Materials
Diamagnetic materials have all paired electrons and produce a weak opposing field when placed in an external magnetic field.
χ < 0 (small, negative). Weakly repelled by magnets. No permanent dipole moment. Examples: Cu, Ag, Au, Bi, water, NaCl.
5.7 Paramagnetic Materials
Paramagnetic materials have unpaired electrons with random thermal alignment, producing a weak net magnetization in an external field.
χ > 0 (small, positive). Weakly attracted by magnets. Permanent dipoles exist but randomly oriented. Examples: Al, Na, O₂, air, MnSO₄.
5.8 Ferromagnetic Materials
Ferromagnetic materials have unpaired electrons that spontaneously align in domains, producing strong magnetization.
χ >> 0 (large, positive). Strongly attracted by magnets. Domains aligned by external field. Retain magnetism (permanent magnets). Examples: Fe, Co, Ni, Fe₃O₄, CrO₂.