Physics

Absolute Space and Time: Historical Concept and Role in Physics Absolute space and time The idea that space and time exist independently of observers, central to Aristotelian and Newtonian thought and superseded by Einstein's spacetime in relativity.

Absolute temperature (thermodynamic temperature) Absolute temperature Absolute temperature is the thermodynamic measure of thermal energy using scales with zero at absolute zero (Kelvin or Rankine); it underlies physical laws, conversions, and cryoge…

Absolute zero: concept, physical meaning, experiments and practical effects Absolute zero Absolute zero is the lowest theoretical temperature (0 K). This article describes its physical meaning, temperature scales, cooling techniques, experimental approaches, application…

Acceleration (physics) Acceleration Change in velocity per unit time; a vector quantity describing how quickly speed or direction changes, with uses from free fall to vehicle design and measurement by accelerometers.

Acceleration due to gravity The acceleration that a mass acquires because of a gravitational field, commonly denoted g on Earth. A vector quantity with a standard value 9.80665 m/s² that varies with location…

Visual accommodation (eye) — how the lens changes focus Accommodation (eye) Accommodation is the eye's process of changing lens power to keep near and distant objects sharp; it involves the lens, zonules and ciliary muscle and declines with age, causing pr…

Adiabatic process An adiabatic process is a thermodynamic change in which a system exchanges negligible heat with its surroundings, producing temperature changes through work and pressure changes ra…

Aerostatics: Principles, History, and Practical Uses Aerostatics Aerostatics examines gases at rest, their pressure and buoyancy effects, and how these principles enable balloons and airships and support meteorology, tethered platforms, and ligh…

Albedo (Reflectivity of Surfaces and Planetary Bodies) Albedo Albedo is the fraction of incoming electromagnetic radiation reflected by a surface or body. It affects climate, remote sensing, and planetary observations and varies with waveleng…

Alpha particle (helium nucleus) Alpha particle An alpha particle is a helium nucleus emitted in alpha decay. This article explains its composition, properties, biological effects, history, and practical uses, with examples and…

Ammeter: instrument for measuring electric current Ammeter An ammeter measures electric current in amperes. This article explains how ammeters work, their types, key parts, proper connections, typical uses, safety considerations and differ…

Amorphous solid: structure, properties, formation and applications Amorphous solid An amorphous solid lacks long-range atomic order. This article explains its structure, formation (glass transition), properties, common examples, production methods, characterizati…

Ampere — SI unit of electric current Ampere The ampere (A) is the SI base unit for electric current, equal to one coulomb of charge passing a point per second. Covers definition, history, practical realization, examples and…

Ampère's circuital law A basic law of electromagnetism linking the circulation of a magnetic field around a closed loop to the electric current it encloses, later extended by Maxwell to cover changing el…

Amplitude: size and measures of a wave's oscillation Amplitude Amplitude describes the maximum displacement of a periodic signal or wave from equilibrium. Common measures include peak, peak-to-peak, and RMS values; it relates to energy and man…

Angular frequency Angular frequency (ω) measures how fast an angle changes in time, given in radians per second. It relates to ordinary frequency, period, angular velocity, and appears widely in osc…

Angular momentum Measure of rotational motion: the product of an object's moment of inertia and its angular velocity; conserved in isolated systems and appearing in many classical and quantum conte…

Angular velocity Angular velocity describes how quickly an object rotates and the axis and direction of that rotation; it links rotational motion to linear motion and appears across physics and eng…

Annihilation (particle–antiparticle interaction) Annihilation Annihilation is the process in which a particle and its antiparticle collide and convert their mass into other particles or radiation, governed by conservation laws and producing p…

Antimatter Antimatter: particles composed of antiparticles that mirror ordinary matter in mass but have opposite charges and quantum numbers, notable for annihilation on contact and roles in…

Antimatter fuel: principles, applications, and technical challenges Antimatter fuel Antimatter fuel uses particle–antiparticle annihilation to release mass-energy. This article explains how it works, storage and production issues, propulsion and power concepts, sa…

Antineutron — the neutron's antiparticle Antineutron The antineutron is the neutral antiparticle of the neutron, built from antiquarks. It shares mass and many properties with the neutron but has opposite quantum numbers and annihila…

Antiparticle: definition, properties, production, and significance Antiparticle An antiparticle has the same mass but opposite additive quantum numbers to a particle. This article explains properties, pair production and annihilation, history, uses, and open q…

Antiproton: the proton’s antiparticle, properties, history and uses Antiproton An antiproton is the antiparticle of the proton: same mass, negative charge and opposite magnetic moment. Produced in accelerators, it is central to antimatter research, precision…

Applied Mechanics Branch of physics and engineering that studies how solid and fluid bodies respond to forces; links theory with technological design, analysis, and prediction.

Archimedes number A dimensionless parameter in fluid mechanics that compares buoyancy (gravitational) forces to viscous forces, used to characterize particle settling, bubble rise, and buoyancy-driv…

Archimedes' principle: buoyancy, displaced fluid, and practical applications Archimedes' principle Archimedes' principle states that a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces. Explains concept, origin, limits, an…

Astrodynamics: Motion, Orbits, and Spacecraft Navigation Astrodynamics Astrodynamics studies the motion of spacecraft and natural bodies under gravity and other forces, covering orbital mechanics, maneuvers, trajectory design, and applications from sa…

ATLAS experiment (A Toroidal LHC ApparatuS) ATLAS experiment ATLAS is a large general-purpose particle detector at CERN's Large Hadron Collider. It records proton collisions to study the Higgs boson, test the Standard Model and search for ne…

Atmosphere (unit) The atmosphere (symbol: atm) is a historical unit of pressure defined as exactly 101325 pascals, used as a reference for sea-level air pressure and for practical conversions to tor…

Atmospheric pressure The force exerted by the weight of air above a surface; varies with altitude and weather and is measured by barometers in units such as pascals or hectopascals.

Atomic clock — ultra-precise timekeeping using atomic transitions Atomic clock A clock that uses the natural frequency of atomic energy transitions as its timekeeping reference; the basis of modern time standards, navigation, telecom and precision science.

Atomic nucleus The atomic nucleus is the small, dense central region of an atom that contains protons and neutrons; it determines mass, isotopes, stability, and many nuclear phenomena.

Atomic orbital A summary of atomic orbitals: definition, quantum numbers, shapes, history, and their role in chemistry and spectroscopy, written in accessible encyclopedic style.

Atomic physics: structure, methods, history, and applications Atomic physics Study of atoms as electron–nucleus systems: electronic structure, transitions, experimental methods, history, and practical applications from clocks to quantum technologies.

Attraction: forces, psychology, and cultural meanings Attraction Overview of attraction as a tendency for things to draw together, covering physical forces, chemical and biological attraction, interpersonal dynamics, cultural uses, measurement,…

Background radiation: sources, measurement, variation, and health context Background radiation Background radiation is the continuous low-level ionizing radiation from natural and artificial sources. This article explains its origins, measurement, variation by location, heal…

Ball lightning — a persistent luminous atmospheric phenomenon Ball lightning Ball lightning is a rare, spherical luminous phenomenon reported during thunderstorms. This article reviews its appearance, reported behavior, scientific theories, historical obser…

Ballistic coefficient (BC) Ballistic coefficient Measure of a projectile's ability to overcome air resistance; depends on mass, cross-sectional area and shape. Used to predict trajectory, retained velocity, wind drift, and energy…

Ballistics: the science of projectile motion and effects Ballistics Ballistics is the study of the motion, behavior and effects of projectiles—from barrel dynamics and external trajectories to terminal impacts—applied in engineering, forensics, def…

Band gap: energy ranges and their role in solids Band gap A band gap is the energy range in a solid where no electronic states exist between valence and conduction bands; it determines electrical, optical and thermal behaviour and underpi…

Bar (unit of pressure) Bar (unit) The bar is a non-SI unit of pressure commonly used in meteorology, engineering and oceanography. It is closely related to atmospheric pressure and is convenient for everyday pressu…

Barometer Instrument that measures atmospheric pressure; types include mercury, aneroid and water barometers. Used in weather forecasting, altitude measurement and scientific research; recor…

Baryon (subatomic particle) Baryon A baryon is a composite fermion made from three valence quarks (or more exotic combinations). Includes protons and neutrons; covers properties, classification, history, and roles i…

Beam: structural member and directed flow of waves, light or particles Beam A beam is either an elongated structural member that resists loads or a concentrated, directed propagation of energy or particles. Covers structural types, analysis, materials, opt…

Beamline: definition, components, history, and uses in research Beamline A beamline guides and conditions particle, photon or neutron beams from a source to an experimental station; used in accelerators, synchrotrons, neutron facilities and many laborat…

Becquerel: the SI unit of radioactivity Becquerel The becquerel (Bq) is the SI derived unit of radioactivity, defined as one nuclear decay per second. This article explains its meaning, relation to decay laws, multiples, uses, mea…

Beer–Lambert law: absorption of light by matter Beer-Lambert law The Beer–Lambert law describes how a material attenuates light as it travels through it. It links absorbance to concentration and path length and underpins spectrophotometry and ma…