Magnetism

Magnetism is a fundamental physical phenomenon in which materials and moving electric charges exert attractive or repulsive forces on one another through a magnetic field. Recognized since antiquity through the behavior of naturally occurring lodestone, magnetism was mathematically unified with electricity by James Clerk Maxwell in the 1860s into the theory of electromagnetism, one of the cornerstones of modern physics.

History of magnetism

The earliest recorded observations of magnetic phenomena date to ancient Greece and China, where lodestone — a naturally magnetized form of the mineral magnetite — was found to attract iron. The word magnet is generally traced to the Greek region of Magnesia, where magnetic stones were reportedly abundant.

Pre-modern understanding

In 1269, Petrus Peregrinus de Maricourt wrote Epistola de Magnete, one of the first systematic experimental studies of magnetic poles. William Gilbert's De Magnete (1600) established that Earth itself behaves as a giant magnet, explaining the behavior of the compass needle.

Classical electromagnetism

Hans Christian Ørsted demonstrated in 1820 that electric current deflects a compass needle, revealing the link between electricity and magnetism. André-Marie Ampère subsequently formulated mathematical laws describing the magnetic forces between current-carrying conductors. Michael Faraday's discovery of electromagnetic induction in 1831 showed that a changing magnetic field generates an electric current. James Clerk Maxwell synthesized these findings in 1865 with his equations of the electromagnetic field, predicting the existence of electromagnetic radiation including light.

Quantum and relativistic developments

The twentieth century brought the understanding that magnetism is fundamentally a quantum mechanical and relativistic effect. Albert Einstein's special theory of relativity (1905) showed that magnetic forces can be viewed as relativistic consequences of electric forces seen from a moving reference frame. Quantum mechanics explained the origin of atomic magnetic moments through electron spin and orbital angular momentum, concepts formalized in quantum mechanics during the 1920s.

Physical principles

Magnetism originates from two microscopic sources: the orbital motion of electrons around atomic nuclei and the intrinsic spin angular momentum of electrons and other subatomic particles. Both sources give rise to magnetic dipole moments. When these moments align — spontaneously or under an external field — macroscopic magnetic effects become observable.

Magnetic fields

A magnetic field, represented by the vector quantities B (magnetic flux density) and H (magnetic field intensity), permeates the space around magnets and moving charges. The SI unit of magnetic flux density is the tesla (T). Field lines conventionally emerge from the north pole of a magnet and re-enter at the south pole, forming closed loops.

Maxwell's equations

Maxwell's four equations collectively describe how electric and magnetic fields are generated by charges and currents, and how the two fields interact and propagate. Gauss's law for magnetism states that magnetic monopoles do not exist: every magnetic field line forms a closed loop. As of the time of writing, no magnetic monopole has been experimentally confirmed, although some theories beyond the Standard Model of particle physics predict their existence.

Magnetic force on moving charges

The force on a charged particle moving through a magnetic field is given by the Lorentz force law: F = q(v × B), where q is the particle's charge, v is its velocity, and B is the magnetic flux density. This force is always perpendicular to the particle's velocity, doing no work but curving the particle's trajectory. This principle underpins devices such as cyclotrons and mass spectrometers.

Types of magnetic materials

Materials respond to external magnetic fields in characteristic ways, classified into several categories based on their microscopic magnetic structure.

Diamagnetism

Diamagnetic materials develop a weak magnetization opposing an applied field, resulting in a slight repulsion. All materials exhibit diamagnetism to some degree. Examples include bismuth, copper, and water. Superconductors display perfect diamagnetism — the Meissner effect — expelling all magnetic flux from their interior below a critical temperature.

Paramagnetism

Paramagnetic materials contain atoms with unpaired electrons whose magnetic moments align partially with an applied field, producing a weak attraction. The alignment is disrupted by thermal agitation and is described by the Curie law, which states that magnetic susceptibility is inversely proportional to absolute temperature.

Ferromagnetism

Ferromagnetic materials — including iron, nickel, and cobalt — exhibit strong, spontaneous magnetization arising from quantum mechanical exchange interactions that cause large numbers of atomic moments to align parallel to one another in regions called magnetic domains. Above the Curie temperature, thermal energy overcomes exchange coupling and ferromagnetism is lost, the material becoming paramagnetic.

Antiferromagnetism and ferrimagnetism

In antiferromagnetic materials, adjacent atomic moments align antiparallel, producing no net magnetization. Ferrimagnetic materials, such as magnetite, also have antiparallel sublattice moments, but of unequal magnitude, yielding a net magnetization. Ferrimagnetism underlies the magnetic behavior of many ceramic magnets used in electronics.

Applications of magnetism

Magnetism has transformative applications across technology and science.

Electrical power generation and motors

Virtually all large-scale electrical power generation exploits Faraday's law: rotating coils in magnetic fields generate alternating current. Conversely, electric motors convert electrical energy to mechanical motion through the force exerted by magnetic fields on current-carrying conductors.

Data storage

Magnetic recording media — from early magnetic tape to modern hard disk drives — store data by orienting microscopic magnetic domains. Although solid-state storage has gained dominance in consumer devices, magnetic hard drives remain widely used for bulk data storage as of the early 2020s.

Medical imaging

Magnetic resonance imaging (MRI) uses strong magnetic fields and radio-frequency pulses to align and then perturb the nuclear magnetic moments of hydrogen atoms in the body. The relaxation signals are reconstructed into detailed cross-sectional images without ionizing radiation.

Particle accelerators and fusion research

Strong superconducting magnets steer and focus particle beams in accelerators such as the Large Hadron Collider at CERN, Geneva, Switzerland. In nuclear fusion research, tokamak devices confine hot plasma using powerful magnetic fields to prevent contact with reactor walls.

Everyday technology

Magnets feature in loudspeakers, microphones, sensors, door latches, credit card strips, and inductive charging systems. Permanent rare-earth magnets — particularly those made from neodymium-iron-boron alloys — are prized for their exceptional strength-to-volume ratio and are critical components in electric vehicle motors and wind turbines.

Magnetism in Earth and space

Earth's geomagnetic field is generated by convective motion of molten iron in the outer core, a mechanism described by dynamo theory. The geomagnetic field deflects the solar wind and shields the biosphere from harmful cosmic radiation. Paleomagnetic records preserved in ancient rocks document repeated reversals of Earth's magnetic poles throughout geologic history.

Many other planets, stars, and galaxies possess magnetic fields. Neutron stars called magnetars possess surface magnetic fields estimated to be on the order of 10¹⁰ to 10¹¹ tesla, far exceeding any field produced in a laboratory.

Frequently asked questions

Is magnetism the same as electricity?

Magnetism and electricity are distinct but inseparable aspects of the unified force called electromagnetism. A changing electric field produces a magnetic field and vice versa, as described by Maxwell's equations.

Is a magnetic monopole possible?

No isolated magnetic monopole has ever been detected experimentally. Gauss's law for magnetism asserts that magnetic poles always occur in north–south pairs, though some theoretical frameworks beyond the Standard Model permit monopoles.

Is the Curie temperature the same for all ferromagnets?

No; each ferromagnetic material has its own characteristic Curie temperature. For iron it is approximately 770 °C, for nickel approximately 358 °C, and for cobalt approximately 1,115 °C.

Is Earth's magnetic field stable?

Earth's magnetic field slowly changes in both intensity and direction over geologic time and has reversed polarity many times in the past. It is not stable on million-year timescales, though it is effectively stable for most practical purposes on human timescales.

Is magnetism relevant to modern computing?

Yes. Beyond traditional hard disk drives, magnetism underlies emerging technologies such as spintronics, which exploits electron spin — a quantum magnetic property — to develop faster, lower-power memory and logic devices.