Physics

Bell's theorem: implications for quantum mechanics and hidden variables Bell's theorem Bell's theorem shows that no local hidden-variable theory can reproduce all quantum predictions. It shaped experiments on entanglement, clarified locality and realism, and underpin…

Bernoulli's principle Principle in fluid dynamics relating pressure, velocity and energy for ideal flow: higher fluid speed corresponds to lower static pressure along a streamline under specified assump…

Beta decay Beta decay is a radioactive transformation in which a nucleus emits an electron or positron (a beta particle), converting a neutron to a proton or a proton to a neutron via the wea…

Beta ferrite (β‑ferrite): the paramagnetic BCC form of iron Beta ferrite Beta ferrite is the body-centered cubic (BCC) form of iron that is paramagnetic above the Curie temperature. Covers structure, temperature behaviour, metallurgical relevance, and a…

Beta particle (β): electrons and positrons emitted in radioactive decay Beta particle Beta particles are high-speed electrons or positrons produced by nuclear beta decay. They are a type of ionizing radiation with characteristic interactions, detection methods, shie…

Birefringence (double refraction): causes, properties, and applications Birefringence Birefringence is the optical property of anisotropic materials that splits light into two polarized rays with different speeds, causing double images and polarization effects used…

Blackbody Radiation Thermal electromagnetic emission from an object in thermal equilibrium: principles, key laws (Planck, Wien, Stefan–Boltzmann), history, and major applications such as stars and the…

Blast injury: causes, classification, effects and clinical management Blast injury An overview of blast injuries from explosive events: mechanisms, the main injury types, organs affected, acute care and long-term physical and psychological consequences.

Bohr model of the atom Bohr model Early quantum model that introduced quantized electron orbits to explain atomic spectra, especially the hydrogen lines; historically important though superseded by modern quantum m…

Boltzmann constant The Boltzmann constant (k or kB) links microscopic energy to macroscopic temperature. It has the exact SI value 1.380649×10⁻²³ J·K⁻¹ and underpins statistical mechanics and thermod…

Bomb (explosive device) Bomb A bomb is a device that releases energy rapidly to produce a blast, heat, or fragments. This article explains types, components, history, uses, and key distinctions between chemica…

Born's rule Fundamental quantum principle that links the wavefunction to measurement outcomes by assigning probabilities equal to the squared magnitude of amplitudes; extends to density matric…

Bose gas: quantum ideal gas of bosons and Bose–Einstein condensation Bose gas An overview of the Bose gas: definition, quantum statistics, historical development, physical properties, distinctions from classical gases and fermions, and experimental and pract…

Bose–Einstein statistics: occupancy rules and physical consequences Bose-Einstein statistics Bose–Einstein statistics describe how indistinguishable bosons occupy energy states, the distribution formula, historical origin, physical examples (photons, BECs) and limits compa…

Bose–Einstein condensate (BEC) Bose–Einstein condensate A Bose–Einstein condensate is a macroscopic quantum state formed when bosonic particles occupy the same ground state at ultralow temperatures, producing coherence and superfluid-li…

Boson: integer-spin particles and force carriers in quantum physics Boson Bosons are particles with integer spin that obey Bose–Einstein statistics. They include force carriers (gauge bosons), composite bosons and the Higgs scalar; they enable forces and…

Bottom quark (beauty): properties, role in particle physics, and applications Bottom quark The bottom (beauty) quark is a third‑generation elementary quark with charge −1/3. It forms B hadrons, decays by the weak force, and is central to studies of CP violation and the S…

Boyle's law — the inverse pressure–volume relation for gases Boyle's law Boyle's law describes how, for a fixed amount of gas at constant temperature, pressure and volume vary inversely: PV is constant. It is a foundation of gas behavior and a special c…

British thermal unit (BTU): unit of heat and energy British thermal unit Concise reference on the British thermal unit (BTU): definition, approximate conversions, history, common uses in heating and cooling, and its relation to SI units and fuel measure…

Brownian motion: random particle motion, theory and applications Brownian motion Random jittery motion of small particles in fluids caused by molecular impacts; foundational to kinetic theory, diffusion, stochastic processes and applied models in science and fi…

Bulk modulus (measure of resistance to uniform compression) Bulk modulus The bulk modulus quantifies how resistant a material is to uniform compression. It appears in elasticity, acoustics and fluid mechanics and links pressure change to fractional volu…

Bullet‑proof glass (bullet‑resistant glazing): construction, uses, and limits Bullet-proof glass Bullet‑proof glass (bullet‑resistant glazing) is layered transparent material designed to slow or stop projectiles. This article explains how it is made, rated, used, and what limi…

Buoyancy: Principles, Causes, and Practical Applications Buoyancy Buoyancy is the upward force exerted by a fluid on an immersed object. This article covers its causes, Archimedes' principle, formula, examples, and engineering uses.

Butterfly effect: sensitive dependence and consequences in complex systems Butterfly effect The butterfly effect describes how tiny differences in initial conditions can produce widely divergent outcomes in nonlinear systems. It explains limits to prediction in weather, e…

Calorie (unit of energy) Calorie The calorie is a pre-SI unit of energy used in heat and nutrition. This article explains its definitions, history, conversions, biological role, measurement, and common distinction…

Candela — SI unit of luminous intensity Candela The candela (symbol cd) is the SI base unit measuring luminous intensity in a given direction, tied to human photopic sensitivity and used to relate radiometric power to perceived…

Capacitance: definition, principles, history, and practical uses Capacitance Capacitance measures an object's ability to store electric charge per unit voltage. This article explains basic formulas, physical origins, common types of capacitors, and typical…

Capacity Capacity is the measurable ability of an object, system, or organization to contain, produce, or perform; used in physical, technical, legal, and business contexts.

Capillary action Capillary action is the movement of liquids through narrow spaces or porous materials driven by surface tension, adhesion, and cohesion; it explains wicking, plant water transport,…

Carat (unit of mass) Carat (mass) The carat is the standard unit of mass used for gem stones and pearls. One carat equals 0.2 grams and is subdivided into 100 points (2 mg each); the unit was internationally standa…

Celsius (temperature scale) Celsius Celsius, formerly centigrade, is a temperature scale used worldwide for everyday and scientific measurements; it defines 0°C as water’s freezing point and 100°C as its boiling poin…

Center of Mass (Barycenter): Definition, Calculation, and Applications Center of mass The center of mass (barycenter) is the average position of mass in a system. This article explains its definition, calculation for discrete and continuous bodies, physical properti…

Centimetre–gram–second (CGS) system of units Centimetre-gram-second system of units An overview of the centimetre–gram–second (CGS) system: base units, common derived measures (dyne, erg, barye), electrical variants, history, conversions and how CGS differs from S…

Centripetal force: causes, formula, examples, and relation to centrifugal force Centripetal force Centripetal force is the inward-directed force required to keep an object moving along a curved path. This article explains its physics, sources, formula, examples, history, and di…

Charge — meanings in physics, law, warfare, finance, and everyday use Charge Comprehensive overview of the word “charge”: its principal senses in physics, law, military action, finance, mechanics and everyday language, with examples and distinctions.

Charge conjugation (C symmetry) Charge conjugation Charge conjugation is the operation that replaces every particle by its antiparticle, reversing additive charges while leaving mass and spin unchanged; it is a central concept in p…

Charm quark (c quark) Charm quark A second-generation quark with electric charge +2/3. The charm quark appears in charmed hadrons (J/ψ, D mesons, Λc), played a central role in 1970s particle physics and remains imp…

Cherenkov radiation (Vavilov–Cherenkov effect) Cherenkov radiation Electromagnetic radiation emitted when a charged particle travels faster than light's phase velocity in a medium; known for its blue glow and used in reactors, particle detectors a…

Classical mechanics: laws, scope, and applications Classical mechanics An encyclopedic overview of classical mechanics: core principles, main formulations, conservation laws, typical problems, applications in engineering and astronomy, and limits with…

Classical physics: principles, scope, history and limits Classical physics Survey of classical physics: core concepts, main branches and laws, historical development, practical applications, and why modern physics replaced it at very small and very fast s…

Closed system (thermodynamics) Closed system A thermodynamic system that does not exchange matter with its surroundings but can exchange energy (heat and work); used in modeling engines, calorimeters and many engineering prob…

Coefficient of friction Dimensionless measure of the resistive force between surfaces in contact; relates frictional force to the normal force and distinguishes static and kinetic behavior.

Coefficient of thermal expansion How a material's dimensions change with temperature. Defines linear, area and volumetric coefficients, measurement methods, causes, examples and engineering implications.

Coherence (physics) Coherence describes the fixed phase relationship and correlation between waves or quantum states. It distinguishes perfect, partial, temporal and spatial coherence and underpins in…

Cold fusion — the low‑temperature fusion hypothesis Cold fusion Cold fusion: a disputed claim that nuclear fusion can occur at near‑ambient conditions. Overview of principles, historical events, ongoing research, challenges, and potential impli…

Collimated light: definition, production, uses, and limits Collimated light Light whose rays travel nearly parallel so the beam changes little with distance; used in optics, lasers, telescopes and measurement, but limited by diffraction and practical diver…

Collision (Physics): Types, Principles, and Examples Collision An overview of collisions in physics, covering conservation laws, elastic and inelastic types, typical examples, historical context, and practical importance in engineering and sci…

Colour (Color): Perception, Models, Uses Colour An overview of colour: how it arises from light, how humans perceive and measure it, common colour names, primary systems, uses in art and industry, and variations such as colour b…