magnetismen

Magnetism is a class of physical phenomena related to magnetic fields and the forces that arise from magnets and moving electric charges. A magnetic field exerts forces on magnetic materials and on charges in motion, and magnetic materials can be magnetized to produce a magnetic dipole moment. At the atomic level magnetism mainly originates from electron spin and orbital motion, and the collective alignment of many moments gives rise to stronger effects in certain materials.

Materials respond to magnetic fields in several ways: diamagnetic materials are weakly repelled by fields, paramagnetic materials are weakly attracted, and ferromagnetic, ferrimagnetic, or antiferromagnetic materials exhibit stronger interactions and, in some cases, spontaneous magnetization. Ferromagnets, such as iron or certain alloys, show magnetic domains that can align to produce permanent magnets. Permanent magnets retain magnetization, while electromagnets generate controllable fields by passing current through coils.

Magnetic fields are described by Maxwell's equations; the Lorentz force governs the motion of charges in fields, and changing magnetic fields can induce electric currents (Faraday's law). In atoms, exchange interactions and collective effects lead to various magnetic orders and, in some materials, to strong, temperature-dependent magnetization.

Common phenomena and applications include attraction or repulsion between magnets, the Earth's magnetic field that guides compasses, and magnetic phenomena used in data storage, medical imaging, and sensing. Magnetic materials underpin electrical machines (motors and generators), transformers, and sensors such as Hall effect devices.

Historically, magnetism arose from lodestones and natural magnets. Early work by William Gilbert treated magnetism as a property of matter; experiments by Oersted, Faraday, and others linked electricity and magnetism and led to the formulation of Maxwell's equations, unifying the theory.