magnetisme

Magnetisme is a physical phenomenon associated with magnetic fields and forces arising from moving electric charges and from intrinsic magnetic moments of elementary particles. It is a manifestation of electromagnetism, one of the fundamental interactions in nature. A magnetic field exerts force on moving charges (Lorentz force) and on magnetic dipoles, tending to align them with the field. In materials the response to an external field is described by magnetization M, and the field B is related to the applied field H by B = μ0(H + M).

Different materials show various types of magnetism: diamagnetism, which slightly weakens an applied field; paramagnetism, which is weakly attracted; ferromagnetism, ferrimagnetism and antiferromagnetism, which involve ordered alignment of magnetic moments. Ferromagnets display spontaneous magnetization and magnetic domains; at high temperatures (Curie temperature) they lose order.

On the microscopic level magnetism arises from electron spin and orbital motion. The exchange interaction between neighboring spins can lead to cooperative alignment, producing large macroscopic magnetization in ferromagnets. Quantum mechanics governs these phenomena.

Maxwell's equations describe how changing magnetic fields propagate and couple to electric fields. Practical uses include electric motors and generators, transformers, data storage, magnetic resonance imaging, compasses, and various sensors. Magnets are characterized by properties such as magnetic moment, coercivity, retentivity, and susceptibility.