Physics

Gravitino A theoretical spin-3/2 fermion predicted by supergravity: the supersymmetric partner of the graviton with important implications for particle physics and cosmology.

Graviton: the hypothetical quantum of the gravitational field Graviton A concise, neutral overview of the graviton: its theoretical role as a quantum mediator of gravity, main properties, history of ideas, experimental challenges, and significance for…

Gravity (Gravitation) Gravity Gravity is the universal attraction between masses. Described by Newton's inverse-square law for orbits and by Einstein's curvature of spacetime, it shapes motion from falling appl…

Gravity wave (fluid dynamics) Gravity wave A gravity wave is a fluid wave restored by gravity or buoyancy, occurring on surfaces or within stratified fluids such as the ocean or atmosphere. Distinct from gravitational waves…

Ground state (quantum lowest-energy configuration) Ground state The ground state is the lowest-energy quantum configuration of a particle, atom, molecule, or system; it defines stability, spectra, and how the system responds when energy is adde…

Group velocity: speed of a wave packet and its physical significance Group velocity Group velocity is the rate at which a wave packet's envelope—and often energy or information—propagates. It differs from phase velocity and is central to dispersion, pulse spreadin…

Gyroscope: principles, types, history and applications Gyroscope A gyroscope is a device that measures or preserves orientation using a spinning rotor and angular momentum. This article explains how gyroscopes work, their types, history, uses, a…

Hadron: composite particles of quarks and the strong interaction Hadron Hadrons are composite subatomic particles made of quarks and bound by the strong interaction. This article explains their structure, classification, history, examples, and signific…

Hamiltonian mechanics A reformulation of classical mechanics centered on the Hamiltonian function, describing dynamics in phase space and bridging to modern concepts in physics, including quantum theory…

Hardness (materials): definition, measurement methods and applications Hardness Hardness describes a material's resistance to shape change, wear, or scratching. This article explains types of hardness, common test methods and typical applications across minera…

Harmonic: integer-multiple frequency components in waves and signals Harmonic A harmonic is a frequency component whose frequency is an integer multiple of a waveform's fundamental frequency. Article covers definition, math, acoustics, musical examples, inte…

Hawking radiation: quantum emission from black holes Hawking radiation Hawking radiation is the quantum thermal emission predicted to arise from black holes. It leads to slow mass loss (evaporation), ties gravity to thermodynamics, and raises question…

Heat (thermal energy): definition, transfer, measurement, and uses Heat Heat is energy transferred between bodies due to temperature differences. This article explains what heat is, how it moves, how it is measured, its role in thermodynamics and commo…

Heat capacity Amount of heat required to change an object's temperature. Covers extensive and specific forms, units, Cp vs Cv, typical values, measurement, and common applications.

Heat conduction (thermal conduction) Heat conduction Heat conduction is energy transfer through matter caused by temperature differences; it depends on material properties, geometry and microscopic carriers like phonons and electrons…

Heat index: how humidity and temperature shape perceived heat Heat index The heat index combines air temperature and relative humidity to estimate how hot conditions feel to humans, describing the apparent temperature and guiding heat-safety actions.

Heat lamp: function, types, uses, and safety Heat lamp A heat lamp produces directed radiant heat—commonly infrared—for warming, drying, or therapeutic uses. Covers types, applications in animal care and industry, installation, and saf…

Heat transfer: modes, principles, history and practical applications Heat transfer Heat transfer is the movement of thermal energy between systems. This article explains the modes (conduction, convection, radiation), governing principles, history, and common appl…

Height (vertical measurement) Height Height is the vertical distance between an object's lowest and highest points. This article explains definitions, measurement methods, human variation, uses, history, and distincti…

Heisenberg's uncertainty principle A fundamental quantum principle limiting simultaneous precision of certain pairs of observables (notably position and momentum), with broad implications for measurement, technology…

Helium-4 (4He): Properties, Origin, and Superfluid Behavior Helium-4 Helium-4 is the dominant stable isotope of helium. This article outlines its atomic structure, cosmic and terrestrial origins, unique quantum properties (including superfluidity),…

Henry (unit) SI unit of electrical inductance, symbol H; measures how a changing current induces electromotive force. Named for Joseph Henry; related to volts, webers, and vacuum magnetic const…

Hertz: the SI unit of frequency Hertz Hertz (Hz) is the SI-derived unit for frequency, equal to one event per second. Used across acoustics, electronics and radio, it quantifies cycles per second and relates to period…

Higgs boson Elementary particle associated with the Higgs field; gives mass to other particles in the Standard Model and was discovered by experiments at CERN in 2012.

Higgs field: the quantum field behind particle masses Higgs field The Higgs field is a pervasive scalar field in the Standard Model whose nonzero vacuum value breaks electroweak symmetry and endows many elementary particles with mass; its quantum…

Hooke's law: linear elasticity and simple springs Hooke's law Hooke's law states that, within the elastic limit, the deformation of many solids is proportional to the applied force; it underpins springs, oscillators, and linear elasticity mod…

Horsepower: unit of power, origins, definitions, and common uses Horsepower Horsepower is a traditional unit of power (about 746 watts). Originating with James Watt to compare steam engines to horses, it remains common in vehicles and machinery with severa…

Hydrostatic equilibrium — balance of gravity and pressure Hydrostatic equilibrium State in which gravity is exactly balanced by a pressure-gradient force in a fluid or gas; fundamental to liquids, atmospheres, planets and stars.

Hyperon: Strange baryons in particle physics Hyperon Hyperons are baryons that contain strange quarks. This article explains their composition, decay, experimental study, and roles in particle physics and astrophysics.

Hypersonic speed Speeds greater than about five times the speed of sound. Hypersonic flight produces extreme aerodynamic heating and chemical changes in air, creating special engineering and scient…

Hysteresis: memory and lag in physical, biological, and engineered systems Hysteresis Hysteresis is the dependence of a system's output on both present input and its past history, producing loops, memory effects and rate-dependent behaviors across physics, materials…

Ideal gas (idealized gas model) Ideal gas An ideal gas is a theoretical model in which pointlike, non‑interacting particles undergo elastic collisions and obey PV = nRT; useful as an approximation for many real gases under…

Ideal gas law The ideal gas law relates pressure, volume, temperature and amount of a hypothetical ideal gas (pV = nRT). Covers its meaning, assumptions, history, applications, and limitations v…

Impulse (physics) Impulse is the time integral of force that produces a change in momentum. It links force and motion, with applications from collisions to rocketry and safety design.

Inch of mercury (inHg): a historical unit of pressure Inch of mercury The inch of mercury (inHg) is a traditional, non‑SI unit of pressure based on a 1‑inch column of mercury at 0 °C and standard gravity; once common in weather reports and aviation.

Inclined Plane (Simple Machine) Inclined plane An inclined plane is a sloping surface that reduces the force needed to raise or lower loads. It underlies ramps, wedges, screws and many cutting tools, trading force for distance…

Inductively Coupled Plasma: principles, components, and applications Inductively coupled plasma An inductively coupled plasma (ICP) is a high-density, radio‑frequency plasma generated by electromagnetic induction, widely used in analytical chemistry, materials processing, and…

Inductor (electrical component) Inductor An inductor stores energy in a magnetic field created by electric current. Used in filters, chokes, power supplies and tuned circuits, inductors differ by core, winding and intende…

Inertia (physics): resistance to changes in motion Inertia Inertia is the tendency of matter to resist changes in its state of motion. This article explains the concept, its physical basis, historical development, common examples, and its…

Infrared radiation Infrared Infrared (IR) radiation is electromagnetic energy with wavelengths longer than visible red light. It spans near to far infrared and is used in imaging, sensing, communication, heat…

Infrasound: low-frequency sound below the range of human hearing Infrasound Sound waves with frequencies below the typical human hearing threshold (roughly below 20 Hz). Produced by natural events, animals and machines; travels long distances and is monito…

Insertion device (magnet assembly for synchrotron light sources) Insertion device A magnetic assembly installed in a straight section of a particle accelerator that forces charged particles to oscillate and emit synchrotron radiation; includes undulators and wig…

Electrical insulator (electricity) Insulator (electricity) An electrical insulator is a material or device that resists electric current flow. This article explains common materials, electrical properties, uses in equipment, historical dev…

Intensity (physics): energy flux, measurement, and significance Intensity (physics) Physical intensity measures energy, particles, or radiation per unit area (and sometimes per frequency or solid angle). Covers definitions, inverse-square behavior, related quantit…

Interference (wave phenomena) Interference Interference is the superposition of waves that produces regions of reinforcement and cancellation. It underlies patterns in light, sound, water waves and quantum systems and requi…

Intermolecular force Forces between molecules or macromolecular groups that determine many physical, chemical and biological properties; includes dispersion, dipole, hydrogen bonding and electrostatic…

Internal energy (thermodynamics) Internal energy Internal energy is the total microscopic energy contained within a system—kinetic and potential at the molecular level—important for heat, work, phase changes, and thermodynamic re…

International Nuclear Event Scale (INES) International Nuclear Event Scale A concise guide to the International Nuclear Event Scale: purpose, structure, history, typical uses, limits, and notable events classified on the scale.