magnetism

magnetism
Name	Magnetism
Caption	Field lines of a bar magnet

magnetism is a class of physical phenomena mediated by magnetic fields. Electric currents and the intrinsic magnetic moments of elementary particles, most notably the electron, give rise to a magnetic field, which then exerts forces on other currents and magnetic moments. The interaction between magnetic fields and moving electric charges underpins a vast range of technologies, from electric motors to data storage, and is a cornerstone of the unified theory of electromagnetism.

Fundamental concepts

The origins of magnetism lie in the motion of electric charges. The Lorentz force law describes the force exerted by a magnetic field on a moving charge, a principle fundamental to devices like the cyclotron. At the atomic level, magnetism arises from two primary sources: the orbital motion of electrons around an atomic nucleus and the intrinsic spin of the electrons themselves. These microscopic magnetic moments are quantified by the Bohr magneton. The foundational laws governing static magnetic fields were formulated by Carl Friedrich Gauss and André-Marie Ampère, whose work on Ampère's circuital law connected magnetic fields to electric currents. The behavior of materials in magnetic fields is characterized by their magnetic permeability and magnetic susceptibility.

Magnetic materials

Materials respond to external magnetic fields in distinct ways, classified by their magnetic ordering. Diamagnetism is a weak, repulsive effect present in all materials, famously observed in the Meissner effect in superconductors. Paramagnetism, exhibited by materials like aluminum and oxygen, involves a weak attraction as unpaired electron spins align with the field. Strong, permanent magnetism arises from ferromagnetism, where domains of aligned magnetic moments create materials like iron, cobalt, and nickel; this alignment persists even after the external field is removed, a property known as hysteresis. Other orderings include antiferromagnetism, where adjacent moments oppose each other as in chromium, and ferrimagnetism, exhibited by magnetite and other ferrites.

Magnetic fields and forces

A magnetic field is a vector field that exerts a force on moving charges and magnetic dipoles. It is visually represented by magnetic field lines, which emerge from the north pole and enter the south pole of a magnet. The strength and direction of the field are described by the magnetic flux density, often denoted by B. The force between two magnets or between a magnet and a ferromagnetic material is a manifestation of this field interaction. The field produced by a steady current is described by the Biot–Savart law, while Ampère's force law quantifies the force between current-carrying wires. The Earth itself generates a planetary magnetic field, the geomagnetic field, which shields the planet from the solar wind.

Electromagnetism

Magnetism is inextricably linked to electricity, unified in the 19th century by the work of James Clerk Maxwell. His Maxwell's equations describe how changing electric fields create magnetic fields and vice-versa, a phenomenon known as electromagnetic induction discovered by Michael Faraday. This principle is the basis for electric generators and transformers. The complete symmetry of the phenomena is demonstrated in the production of electromagnetic radiation, such as radio waves and light. Landmark experiments, including those by Hans Christian Ørsted and Faraday, established the fundamental connections between electricity and magnetism.

Applications of magnetism

Magnetic phenomena are harnessed in countless technologies. Electric motors and loudspeakers convert electrical energy into motion using magnetic forces, while dynamos and alternators perform the reverse. Magnetic resonance imaging (MRI) uses powerful superconducting magnets for medical diagnostics. Information storage relies on magnetism in hard disk drives and magnetic tape. Magnetic levitation is used in high-speed Maglev train systems like the Shanghai Maglev Train. Other critical applications include magnetic separators in industry, induction cooking hobs, and the magnetron in microwave ovens.

Measurement and units

The science of measuring magnetic fields is known as magnetometry. The SI unit for magnetic flux density is the tesla (T), named for Nikola Tesla, while the older cgs unit is the gauss (G), named for Carl Friedrich Gauss; one tesla equals ten thousand gauss. Magnetic field strength, or H-field, is measured in amperes per meter (A/m). Instruments for measurement include the magnetometer, the Hall effect sensor, and the fluxgate compass. The National Institute of Standards and Technology (NIST) maintains standards for magnetic measurements.

Category:Electromagnetism Category:Force Category:Physical phenomena