Standard physics glossary

thermo · intro

Calorimetry

The experimental technique used to measure the amount of heat released or absorbed during a physical or chemical process, typically using an insulated device called a calorimeter. By tracking the temperature change of a known quantity of a substance, such as water, scientists can calculate the heat exchanged based on the substance's specific heat capacity. Calorimetry underlies measurements of reaction energies, specific heats, and latent heats across physics and chemistry.

em · intro

Capacitance

A measure of a component's ability to store electric charge for a given voltage applied across it, measured in farads. A capacitor with higher capacitance stores more charge at the same voltage. Capacitance depends on the geometry of the conductors involved and the insulating material, called a dielectric, between them; larger plate area and smaller separation both increase capacitance.

em · intro

Capacitor

An electronic component that stores electric energy in an electric field between two conductive plates separated by an insulator. When connected to a voltage source, charge accumulates on the plates until the capacitor reaches that voltage. Capacitors are used to smooth power supplies, store energy for quick release, and filter electronic signals, and their behavior is characterized by their capacitance.

nuclear · intro

Carbon-14 Dating

Also called: radiocarbon dating

A method for estimating the age of organic material by measuring the remaining proportion of carbon-14, a radioactive isotope that decays at a known, steady rate with a half-life of about 5,730 years. Living organisms continuously exchange carbon with their environment, keeping carbon-14 levels roughly constant, but this exchange stops at death, after which the isotope steadily decays. Comparing remaining carbon-14 to stable carbon-12 gives an age estimate for samples up to roughly 50,000 years old.

thermo · intermediate

Carnot Cycle

An idealized thermodynamic cycle that represents the most efficient possible heat engine operating between two temperatures, consisting of two isothermal and two adiabatic processes. No real engine can exceed the efficiency of a Carnot cycle operating between the same hot and cold reservoirs, making it a theoretical upper bound used to evaluate real engines. It was proposed by French engineer Sadi Carnot in 1824.

mechanics · intro

Center of Mass

The point in a system of objects, or within a single object, where its mass can be considered to be concentrated for the purpose of analyzing motion. For a symmetric object of uniform density, the center of mass sits at its geometric center. Under gravity, an object behaves as though all its weight acts at this point, which simplifies the study of complex motions like a spinning wrench thrown through the air.

mechanics · intro

Centripetal Force

The net force that keeps an object moving along a curved path, directed toward the center of curvature at every point along the motion. For an object moving in a circle at constant speed, this force is what continuously changes the direction of the object's velocity without changing its speed. Gravity provides the centripetal force that keeps planets in orbit, while tension provides it for a ball on a string swung overhead.

thermo · intro

Charles's Law

A gas law stating that, at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature: heating a gas causes it to expand, and cooling it causes it to contract, as long as pressure stays fixed. Named after French scientist Jacques Charles, it is one of the basic relationships combined to form the ideal gas law.

optics · intermediate

Chromatic Aberration

An optical distortion in which a lens fails to focus all wavelengths of light at exactly the same point, because the refractive index of the lens material varies slightly with wavelength. This causes colored fringes to appear around the edges of objects in images produced by simple lenses. Photographers and telescope makers reduce chromatic aberration using combinations of lenses made from different materials, called achromatic doublets.

mechanics · intro

Classical Mechanics

The branch of physics that describes the motion of everyday objects, from falling apples to orbiting planets, using the laws formulated primarily by Isaac Newton in the seventeenth century. It successfully predicts motion for objects much larger than atoms and moving much slower than the speed of light, and remains the practical foundation for engineering. At very small scales or very high speeds, it is superseded by quantum mechanics and relativity.

particle · intro

Cloud Chamber

A particle detector consisting of a sealed container filled with a supersaturated vapor, in which charged particles passing through leave visible trails of condensed droplets, similar to the way an airplane leaves a contrail. It was one of the earliest tools used to observe and photograph subatomic particles, including the discovery of the positron in 1932. Modern particle detectors have largely replaced it, but it remains a popular tool for physics education.

optics · intermediate

Coherence

A property of waves, especially light waves, in which they maintain a constant, predictable phase relationship with each other over time and space. Coherent light sources, such as lasers, produce waves that can interfere in stable, predictable patterns, which is essential for applications like holography and interferometry. Ordinary light bulbs produce incoherent light, with rapidly and randomly varying phase relationships between different light waves.

cosmology · intermediate

Coincidence Problem

An open question in cosmology asking why the present-day energy densities of matter and dark energy happen to be of roughly the same order of magnitude, given that the two components evolve very differently over cosmic time, with matter density dropping much faster than dark energy density as the universe expands. We appear to be observing the universe at a special epoch when the two are comparable, rather than a time when one overwhelmingly dominates, and the standard Lambda-CDM model does not explain this timing from first principles.

quantum · intermediate

Compton Scattering

The scattering of a photon by a charged particle, usually an electron, in which the photon loses energy and increases in wavelength, while the electron gains kinetic energy. Discovered by Arthur Compton in 1923, this effect provided strong evidence that light behaves as discrete particles, photons, carrying momentum, rather than only as a continuous wave. The shift in wavelength depends on the scattering angle.

thermo · intro

Conduction (Thermal)

Also called: heat conduction

The transfer of heat through a material by direct molecular and atomic collisions, without any bulk movement of the material itself. Metals conduct heat efficiently because their free electrons carry thermal energy quickly through the material, while materials like wood or air conduct heat poorly and are used as insulators. Thermal conduction is described quantitatively by Fourier's law, relating heat flow to temperature gradient.

em · intro

Conductor

A material that allows electric charge, or in some contexts heat, to flow through it easily, because it contains charge carriers, typically electrons, that are free to move. Metals such as copper and silver are excellent electrical conductors due to their loosely bound outer electrons. Conductors are distinguished from insulators, which strongly resist the flow of charge, and semiconductors, whose conductivity lies between the two and can be controlled.

mechanics · intro

Conservation of Energy

A fundamental principle stating that the total energy of an isolated system remains constant over time, though it can change form, such as converting between kinetic, potential, thermal, and other types of energy. No known physical process violates this law when all forms of energy, including mass-energy, are properly accounted for. It underlies analysis across every branch of physics, from simple falling objects to nuclear reactions.

mechanics · intro

Conservation of Momentum

A fundamental principle stating that the total momentum of an isolated system remains constant unless an external force acts on it. This applies to collisions, explosions, and rocket propulsion alike; when a rocket expels exhaust gas backward, the rocket itself is pushed forward so that total momentum is preserved. Conservation of momentum follows directly from Newton's third law when applied to a system of interacting objects.

waves · intro

Constructive Interference

The reinforcement of waves that occurs when two or more waves overlap while in phase, so that their crests align with crests and troughs align with troughs, producing a combined wave of greater amplitude than either wave alone. It is one of the two basic outcomes of wave superposition, the other being destructive interference. Constructive interference produces the bright fringes seen in patterns from double-slit experiments and thin films.

thermo · intro

Convection

The transfer of heat through the bulk movement of a fluid, such as air or water, as warmer, less dense regions rise and cooler, denser regions sink, creating a circulating flow. Convection drives weather patterns in the atmosphere, currents in the ocean, and the boiling of water in a pot. It differs from conduction, which transfers heat without bulk motion of the material.

mechanics · intermediate

Coriolis Effect

An apparent deflection of moving objects observed within a rotating reference frame, such as the surface of the rotating Earth, even though no real sideways force is acting on the object. It causes winds and ocean currents to curve, deflecting to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is named after French scientist Gaspard-Gustave de Coriolis, who described it mathematically in 1835.

cosmology · intro

Cosmic Microwave Background

Also called: CMB

Faint microwave radiation that permeates the entire observable universe almost uniformly, representing the earliest light able to travel freely once the universe cooled enough for neutral atoms to form, about 380,000 years after the start of the standard hot early expansion phase. First detected in 1965, the cosmic microwave background carries tiny temperature variations that encode detailed information about the universe's composition, geometry, and history, and it stands as one of the strongest pieces of evidence supporting the standard Big Bang cosmological model.

particle · intro

Cosmic Ray

A high-energy particle, mostly protons and atomic nuclei, that travels through space and strikes Earth's atmosphere, originating from sources such as the sun, supernova remnants, and other high-energy astrophysical events. When cosmic rays collide with atoms in the upper atmosphere, they produce cascades of secondary particles that can be detected at ground level. Cosmic rays are also a natural source of background radiation on Earth.

cosmology · intermediate

Cosmological Constant

Also called: lambda

A term introduced into Einstein's field equations of general relativity, representing a constant energy density that fills space uniformly and is the simplest theoretical explanation for the observed accelerating expansion of the universe within the standard Lambda-CDM model. Einstein originally introduced the term in 1917 to allow for a static universe, later calling it his greatest blunder once the universe was found to be expanding, though it was revived decades later to explain cosmic acceleration.

cosmology · intermediate

Cosmological Constant Problem

A major unresolved discrepancy in theoretical physics between the extremely small value of the cosmological constant inferred from cosmological observations and the vastly larger value predicted by calculating the vacuum energy density using quantum field theory, a mismatch that can span more than one hundred and twenty orders of magnitude depending on the calculation. This enormous discrepancy is often described as one of the worst theoretical predictions in the history of physics and remains a significant unsolved problem connecting cosmology and quantum theory.

em · intro

Coulomb's Law

A law describing the electric force between two stationary charged objects, stating that the force is proportional to the product of the two charges and inversely proportional to the square of the distance between them. Like charges repel while opposite charges attract. Formulated by French physicist Charles-Augustin de Coulomb, this law forms the foundation of electrostatics and closely parallels Newton's law of universal gravitation in mathematical form.

cosmology · intermediate

Critical Density

The precise average density of matter and energy that a universe would need to have for its overall spatial geometry to be exactly flat, according to the equations of general relativity applied to cosmology. Comparing the universe's actual measured density to this critical value determines its overall geometric shape: a density above critical would produce positively curved space, below critical would produce negatively curved space, and observations show the actual universe is very close to critical density and therefore very close to flat.