Magnetism is the set of physical phenomena associated with magnetic fields magnetic moments and the interactions produced by moving electric charges and magnetic materials. At a fundamental level magnetism is connected with electric charge in motion and with intrinsic magnetic moments associated with elementary particles such as electrons. In classical electromagnetism electric currents generate magnetic fields while magnetic fields exert forces on moving charges. In quantum physics electron spin and orbital angular momentum contribute to magnetic moments and interactions among microscopic magnetic moments help produce the magnetic properties of matter. Magnetism therefore spans classical electromagnetic theory quantum mechanics materials science chemistry geology biology and engineering.
Magnetic behaviour in materials is commonly classified according to responses such as diamagnetism paramagnetism ferromagnetism antiferromagnetism and ferrimagnetism. Diamagnetic materials develop a weak induced magnetic response opposing an applied field. Paramagnetic materials have magnetic moments that tend to align partially with an applied field but normally lose strong alignment when the field is removed. Ferromagnetic materials can develop strong persistent magnetisation because microscopic interactions favour aligned magnetic moments. Antiferromagnetic materials exhibit opposing ordered moments that can largely cancel macroscopically, while ferrimagnetic materials contain oppositely oriented moments with unequal magnitudes and therefore retain a net magnetic moment.
Ferromagnetism depends strongly on magnetic domains. Within a domain many microscopic magnetic moments can become aligned, while different domains can initially have different orientations. An external magnetic field can cause favourable domains to grow and rotate, producing magnetisation. After the field is removed some materials retain substantial magnetisation because domain structures do not immediately return to their original state. This produces hysteresis, in which magnetisation depends partly on the previous magnetic history of the material. Important properties include coercivity, remanence and saturation. Hard magnetic materials are useful for permanent magnets, while soft magnetic materials are often preferred for transformer cores and electromagnetic devices because they can magnetise and demagnetise efficiently.
Magnetism has a long history beginning with naturally magnetic materials such as lodestone and practical uses of magnetic compasses. Ancient observations of magnetic attraction were eventually incorporated into systematic physical investigation. William Gilbert studied magnetism extensively in the early modern period and recognised important properties of Earth as a magnet. Nineteenth century discoveries connected electricity and magnetism through experiments involving currents magnetic fields and induction. Maxwell subsequently formulated a mathematical electromagnetic framework showing that electric and magnetic phenomena belong to a unified theory. Twentieth century quantum mechanics then provided a deeper explanation of microscopic magnetic moments and magnetic ordering in matter.
Magnetism has extensive technological applications. Electric motors convert electrical energy into mechanical motion using magnetic forces. Generators use electromagnetic induction to convert mechanical energy into electrical energy. Transformers transfer electrical energy between circuits through changing magnetic flux. Magnetic recording stores information through controlled magnetic states. Loudspeakers and microphones convert electrical and mechanical energy through magnetic interactions. Magnetic sensors detect position speed current field strength and material properties. Magnetic resonance imaging uses strong magnetic fields and radiofrequency electromagnetic signals to obtain information about the internal structure of the body.
Magnetism is also important in geology and planetary science. Rocks can preserve information about past magnetic fields through remanent magnetisation. Paleomagnetic evidence has contributed to understanding continental movement and seafloor spreading. The magnetic field of Earth changes over geological time and has undergone polarity reversals. The record of these reversals in oceanic crust provided important evidence for plate tectonic processes. Planetary magnetic fields vary according to internal composition rotation and electrically conducting fluids. Observations of magnetic fields around planets therefore provide information about planetary interiors and interactions with space environments.
At the microscopic level magnetism cannot be fully explained using only the classical picture of tiny charged particles orbiting like miniature planets. Quantum mechanics shows that intrinsic electron spin is associated with magnetic moment even though spin is not simply classical rotation of an extended charged sphere. Orbital motion and spin interactions contribute to magnetic behaviour, while the Pauli exclusion principle and exchange interactions are important in explaining magnetic ordering in solids. Quantum electrodynamics provides an even deeper theoretical framework for electromagnetic interactions. These microscopic mechanisms explain why different materials display dramatically different magnetic properties.
Common misconceptions include assuming that magnetism and electricity are completely separate forces or that every magnetic material behaves like a permanent magnet. Magnetism is part of electromagnetism and can arise from currents and changing electric fields as well as permanent magnetic ordering. Most materials are not strongly ferromagnetic. Another misconception is that magnetic fields can perform work on a free charged particle by simply increasing its speed.
The magnetic force is perpendicular to the velocity component affected in the ideal Lorentz force description and therefore primarily changes direction rather than kinetic energy. Magnetism is consequently best understood as a broad physical phenomenon linking charge motion fields microscopic quantum properties and material structure.
