Physics

Plasma lamp (plasma globe) Plasma lamp A plasma lamp is a decorative gas-discharge device that creates glowing, interactive filaments of ionized gas. It demonstrates dielectric-barrier discharge and is used for educatio…

Plasma window: a magnetically confined plasma interface between vacuum and atmosphere Plasma window A plasma window uses a confined, heated plasma to separate a vacuum from air while allowing beams and radiation to pass. It enables vacuum-based processes to operate with open-air…

Platonic crystal: periodic plate structures that control flexural (bending) waves Platonic crystal Platonic crystals are periodic patterns in thin elastic plates that manipulate flexural waves. Studied under 'platonics', they produce band gaps, guided modes, filters and metamate…

Plum pudding model (Thomson's early atomic model) Plum pudding model Overview of the plum pudding model: its origin, assumptions, experimental tests that refuted it, and its role in the development of atomic theory.

Poisson's ratio Poisson's ratio quantifies how a material expands or contracts perpendicular to an applied axial strain. It is fundamental to elasticity, material selection, and the behavior of st…

Polar moment of inertia (polar second moment of area) Polar moment of inertia Overview of the polar second moment of area: definition, units, relation to planar moments, common formulas, role in torsion and shear, limitations (warping), computation and engin…

Polarity: physical and chemical orientation of opposing properties Polarity Polarity denotes a two-sided or asymmetric property found in physics and chemistry — from electric charge and magnetic poles to polar molecules and polarized light — with distinct…

Polarization of Waves: Definition, Types, Causes, and Applications Polarization Polarization describes the orientation of oscillations in transverse waves (like light) and has distinct types, causes, measurement methods, and practical applications in optics, r…

Polypropylene (PP) Polypropylene Polypropylene is a versatile thermoplastic polymer used in packaging, textiles, automotive parts and more. It is lightweight, chemically resistant, and produced by polymerizing pro…

Positron (antielectron): properties, behavior, and applications Positron A positron is the antimatter counterpart of the electron. This article explains its physical properties, creation and annihilation, the bound state positronium, historical discover…

Positronium Positronium is a short-lived exotic atom composed of an electron and its antiparticle, the positron; it is used to test quantum electrodynamics and to probe materials via annihilat…

Potential (physics, mathematics, and general use) Potential Overview of the concept of potential: latent capacity and scalar (and vector) potentials in physics, mathematics, thermodynamics and everyday use, including key properties and appl…

Potential energy Stored energy of a system due to position, configuration, or state within a force field; includes gravitational, elastic, electric and chemical forms and underpins many physical an…

Pound (unit of mass) Pound (mass) The pound is a traditional unit of mass used chiefly in the United States and formerly across the British Empire; the modern international avoirdupois pound is defined as exactly 0…

Pound‑force (lbf): the imperial/US unit of force Pound-force The pound‑force (lbf) is the force equal to the weight of one avoirdupois pound under standard gravity. Common in engineering and everyday measures; distinct from pound‑mass (lb).

Powder: definition, properties, production, uses and safety Powder Powder: a collection of fine dry particles. Overview of characteristics, common production methods, historical uses, applications across industries, and safety considerations.

Power (physics) Power is the rate at which work is performed or energy is transferred. This article explains definitions, units, common formulas, historical context, applications, and distinctions…

Power factor: definition, causes, correction and practical significance Power factor Power factor is the fraction of apparent electrical power that does real work. This article explains its meaning, calculation, common causes of low PF, correction methods and pract…

Power spectrum How a signal's power or energy is distributed across frequency: definitions, estimation methods, examples, history, and applications in science and engineering.

Pressure (force per unit area) Pressure Pressure is the amount of force applied perpendicular to a surface per unit area. It is fundamental in physics, engineering, meteorology and physiology, and is measured in pascals…

Principle of Relativity: Galilean and Einsteinian Perspectives Principle of relativity The principle that the laws of physics take the same form in appropriate reference frames. Covers inertial frames, Galilean relativity, Newtonian limits, and the extension by Einst…

Optical Prism: how it bends, splits and redirects light Prism (optics) An optical prism is a transparent object that refracts and/or internally reflects light, producing dispersion, spectra, image rotation or beam steering in scientific and everyday d…

Projectile (physics, arms, and applications) Projectile An object launched, thrown, or fired through space under its own momentum; covers physical behavior, historical weapons, modern uses, and distinctions between types and delivery sy…

Proton decay: theory, predictions and experimental searches Proton decay Proton decay is a hypothetical process in which a proton transforms into lighter particles. Predicted by some unified theories, it has not been observed; experiments impose very lo…

Pseudovector (Axial Vector): Definition, Properties, and Examples Pseudovector A pseudovector (axial vector) behaves like an ordinary vector under rotations but reverses sign under improper transformations; common in physics for angular momentum and magnetic…

QED: proofs and quantum electrodynamics QED QED commonly denotes the Latin phrase meaning a proven statement, or Quantum Electrodynamics, the quantum field theory of electromagnetic interactions.

Quantum chromodynamics (QCD): the theory of the strong interaction Quantum chromodynamics Overview of quantum chromodynamics: basic principles, particles involved, historical development, methods of calculation, experimental evidence, and its role in particle and nuclea…

Quantum electrodynamics (QED) Quantum electrodynamics Quantum electrodynamics (QED) is the quantum field theory of electromagnetic interactions: the rules for how charged particles and photons interact and how light behaves at subatom…

Quantum entanglement: overview, history, and practical significance Quantum entanglement Quantum entanglement is a nonclassical correlation between quantum systems that ties their properties even when separated; it underpins quantum technologies and challenged classica…

Quantum fluctuation Short-term variations in energy and fields in quantum systems, manifesting as vacuum fluctuations, virtual particles, and measurable effects like the Casimir effect and Lamb shift.

Quantum foam: the microscopic structure of spacetime Quantum foam Quantum foam is a theoretical picture of spacetime at the Planck scale in which quantum uncertainty makes geometry fluctuate. Proposed by John Wheeler, it is central to discussions…

Quantum gravity Study of a theoretical framework reconciling general relativity with quantum mechanics, exploring approaches, conceptual issues, and why a unified description of spacetime and quan…

Quantum mechanics: principles, history, and applications Quantum mechanics An accessible overview of quantum mechanics covering its basic principles, historical development, mathematical framework, major applications, and key conceptual distinctions from…

Quantum state: definitions, representations, and significance Quantum state A concise encyclopedia entry explaining what a quantum state is, how it is represented, its behavior under measurement and evolution, and its role in applications like computing an…

Quantum teleportation: transferring quantum states using entanglement and classical channels Quantum teleportation A clear introduction to quantum teleportation: what it transfers, how the standard protocol works, experimental milestones, applications in quantum networks and computing, and its…

Quantum theory: principles, history, and applications Quantum theory Overview of quantum theory, its core principles, mathematical structure, historical development, practical applications, and key distinctions from classical physics.

Quantum tunnelling: barrier penetration, mechanism, and applications Quantum tunnelling Quantum tunnelling is a quantum effect in which particles penetrate classically forbidden energy barriers, central to nuclear decay, semiconductor devices, microscopy and some chem…

Quark (elementary particle) Quark Quarks are fundamental constituents of matter that combine to form hadrons such as protons and neutrons. They carry fractional electric charge, color charge, and participate in str…

Qubit (quantum bit) Qubit A qubit is the fundamental unit of quantum information. This article explains its mathematical description, superposition and phase, measurement, entanglement, physical implementat…

Radiation: forms, history, uses, and safety Radiation Radiation is the emission or transfer of energy as waves or particles. This article explains its main types, origins, common uses, biological effects, and how different forms are d…

Radiation pressure Force per area exerted by electromagnetic radiation; small for everyday light but important in astrophysics, laser physics, optical manipulation, and spacecraft propulsion.

Radio waves: properties, propagation, history and practical uses Radio wave Radio waves are long-wavelength electromagnetic radiation used for communication, radar, and astronomy; they have distinctive propagation behaviors, band classifications, and requi…

Rankine scale — the absolute temperature scale using Fahrenheit degrees Rankine scale An absolute temperature scale that begins at absolute zero and uses Fahrenheit-sized degrees; used historically in engineering and still seen in some US thermodynamic contexts.

Rare-earth magnets: composition, properties, history, and uses Rare-earth magnet Comprehensive overview of rare-earth permanent magnets: what they are, how they are made, their history, key properties, applications, and handling considerations.

Rayleigh scattering Elastic scattering of electromagnetic radiation by particles much smaller than the wavelength; explains blue skies, red sunsets, and wavelength-dependent attenuation in optics.

Réaumur temperature scale Réaumur scale An overview of the Réaumur temperature scale: definition, origins, how it works, historical development, conversions, uses and why it was largely superseded.

Recoil (firearms): causes, effects, and mitigation Recoil Recoil is the rearward motion and muzzle rise of a firearm when it is discharged. This article explains the physics, measurement, historical development, practical effects, and com…

Refraction — bending of waves at boundaries between media Refraction Refraction is the change in direction of a wave when it passes between media with different propagation speeds. It explains effects from straw-bending to lens focus and prism dispe…