This glossary of physics is a list of definitions of terms and concepts relevant to physics, its sub-disciplines, and related fields, including mechanics, materials science, nuclear physics, particle physics, and thermodynamics. For more inclusive glossaries concerning related fields of science and technology, see Glossary of chemistry terms, Glossary of astronomy, Glossary of areas of mathematics, and Glossary of engineering.

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A

ab initio

A mathematical model which seeks to describe atomic nuclei by solving the non-relativistic Schrödinger equation for all constituent nucleons and the forces that exist between them. Such methods yield precise results for very light nuclei but become more approximate for heavier nuclei.

Abbe number

Also called the V-number or constringence.

In optics and lens design, a measure of a transparent material's dispersion (a variation of refractive index versus wavelength). High values of V indicate low dispersion.

absolute electrode potential

In electrochemistry, the electrode potential of a metal measured with respect to a universal reference system (without any additional metal–solution interface).

absolute humidity

The ratio of the water vapor in a sample of air to the volume of the sample.

absolute motion

absolute pressure

Is zero-referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure.

absolute scale

Any system of measurement that begins at a minimum, or zero point, and progresses in only one direction. The zero point of an absolute scale is a natural minimum, leaving only one direction in which to progress, whereas an arbitrary or "relative" scale begins at some point selected by a person and can progress in both directions.

absolute zero

The theoretical lowest possible temperature, understood by international agreement as equivalent to 0 Kelvin or −273.15 °C (−459.67 °F). More formally, it is the theoretical lower limit of the thermodynamic temperature scale, at which enthalpy and entropy of a cooled ideal gas reach their minimum values and the fundamental particles of nature have minimal vibrational motion.

absorption spectroscopy

Any of various spectroscopic techniques that measure the absorption of electromagnetic radiation due to its interaction with a sample. The sample absorbs energy, i.e. photons, from the radiating field. The intensity of the absorption varies as a function of frequency or wavelength, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

absorptivity

accelerating expansion of the universe

The observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.^[1][2][3][4]

acceleration

The rate at which the velocity of a body changes with time, also the rate of change of the rate at which the position of a body changes with time.

acceleration due to gravity

The acceleration on an object caused by the force of gravitation.

accelerometer

An instrument used to measure the proper acceleration of a body irrespective of other forces.

acoustics

The branch of physics dealing with the production, transmission, and effects of sound.

adhesion

adhesion is what makes things stick together. It's the force that allows tape to stick to a surface or glue to hold two objects together. Contrast cohesion.

adiabatic cooling

adiabatic heating

adiabatic process

A process which occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work.^[5][6] The adiabatic process provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics, and as such it is a key concept in thermodynamics.

aerodynamics

The study of the motion of air, particularly its interaction with a solid object, such as an airplane wing. It is a sub-field of fluid dynamics and gas dynamics, and many aspects of aerodynamics theory are common to these fields.

afocal system

An optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length.^[7] This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's focal length (${\displaystyle d=f_{1}+f_{2}}$).

air mass

1.  In meteorology, a volume of air that is defined by its temperature and water vapor content. Air masses may cover many hundreds or thousands of square miles and generally adapt to the characteristics of the surface below them. They are often classified according to their latitude and their source regions.

2.  In astronomy, the "amount of air that one is looking through"^[8] when observing a star or other celestial source from a vantage point that is within Earth's atmosphere. It is formulated as the integral of air density along the light ray.

air mass coefficient

Defines the direct optical path length through the Earth's atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. The air mass coefficient can be used to help characterize the solar spectrum after solar radiation has traveled through the atmosphere.

albedo

The fraction of the total light incident on a reflecting surface, especially a celestial body, which is reflected back in all directions.

alloy

A chemical mixture of a metal with one or more other metals or other elements.

alpha decay

Also α-decay.

A type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or "decays" into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two.

alpha particle (α)

Also symbolized by α²⁺, He²⁺
, and ⁴
₂He²⁺
.

A type of subatomic particle consisting of two protons and two neutrons bound together into a particle identical to the nucleus of a helium-4 ion. It has a charge of +2 e and a mass of 4 u. Alpha particles are classically produced in the process of radioactive alpha decay, but may also be produced in other ways and given the same name.

alternating current (AC)

A form of electric current in which the movement of electric charge periodically reverses direction. Contrast direct current.

ammeter

An instrument that is used to measure electric current.

amorphous solid

A type of solid which does not have a definite geometric shape.

ampere (A)

Often abbreviated as amp.

The SI base unit of electric current, defined as one coulomb of electric charge per second.

amplifier

Also electronic amplifier or (informally) amp.

An electronic device that can increase the power of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.^[9][10][11]

amplitude

The height of a wave as measured from its center (normal) position.

angle of incidence

In geometric optics, the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal. The ray can be formed by any wave: optical, acoustic, microwave, X-ray, etc.

angle of reflection

The change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound, and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.

ångström (Å)

A unit of length primarily used to measure subatomic particles that is equal to 10⁻¹⁰ metres (one ten-billionth of a metre) or 0.1 nanometres.

angular acceleration

The time rate of change of angular velocity. In three dimensions, it is a pseudovector. In SI units, it is measured in radians per second squared (rad/s²), and is usually denoted by the Greek letter alpha (α).^[12] Just like angular velocity, there are two types of angular acceleration: spin angular acceleration and orbital angular acceleration, representing the time rate of change of spin angular velocity and orbital angular velocity, respectively. Unlike linear acceleration, angular acceleration need not be caused by a net external torque. For example, a figure skater can speed up her rotation (thereby obtaining an angular acceleration) simply by contracting her arms inwards, which involves no external torque.

angular displacement

The angle (in radians, degrees, or revolutions) through which a point revolving around a centre or line has been rotated in a specified sense about a specified axis.

angular frequency (ω)

Also angular speed, radial frequency, circular frequency, orbital frequency, radian frequency, and pulsatance.

A scalar measure of rotation rate. It refers to the angular displacement per unit time (e.g. in rotation) or the rate of change of the phase of a sinusoidal waveform (e.g. in oscillations and waves), or as the rate of change of the argument of the sine function. Angular frequency (or angular speed) is the magnitude of the vector quantity that is angular velocity. The term angular frequency vector ${\displaystyle {\vec {\omega }}}$ is sometimes used as a synonym for the vector quantity angular velocity.^[13] One revolution is equal to 2π radians, hence^[13][14]

${\displaystyle \omega ={{2\pi } \over T}={2\pi f},}$

where:

ω is the angular frequency or angular speed (measured in radians per second),

T is the period (measured in seconds),

f is the ordinary frequency (measured in hertz) (sometimes symbolised with ν).

Also (rarely) moment of momentum or rotational momentum.

B

Babinet's principle

A theorem concerning diffraction which states that the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity.

background radiation

The ubiquitous ionizing radiation to which the general human population is exposed.

Balanced Forces

When all the forces acting upon an object balance each other, the object will be at equilibrium; it will not accelerate.

ballistics

Balmer series

Also Balmer lines.

In atomic physics, one of a set of six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered by Johann Balmer in 1885.

barometer

A scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short-term changes in the weather.

baryon

A subatomic particle such as a proton or a neutron, each of which is made of (usually) three quarks. Nearly all matter humans are likely to encounter is baryonic matter.

battery

A combination of two or more electrical cells which produces electricity.

beam

A structural element that is capable of withstanding load primarily by resisting bending. Beams are traditionally descriptions of building or civil engineering structural elements, but smaller structures such as truck or automobile frames, machine frames, and other mechanical or structural systems contain beam structures that are designed and analyzed in a similar fashion.

bending

Also known as flexure.

The behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element.

bending moment

The reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend.^[20][21] The simplest structural element subjected to bending moments is the beam.

Bernoulli equation

Bernoulli's principle

In fluid dynamics, a principle which states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.^{[22]: Ch.3 [23]: 156–164, § 3.5}

Bessel function

A canonical solution y(x) of Friedrich Bessel's differential equation

${\displaystyle x^{2}{\frac {d^{2}y}{dx^{2}}}+x{\frac {dy}{dx}}+\left(x^{2}-\alpha ^{2}\right)y=0}$

for an arbitrary complex number α, the order of the Bessel function. Although α and −α produce the same differential equation, it is conventional to define different Bessel functions for these two values in such a way that the Bessel functions are mostly smooth functions of α. The most important cases are when α is an integer or half-integer. Bessel functions for integer α are also known as cylinder functions or the cylindrical harmonics because they appear in the solution to Laplace's equation in cylindrical coordinates. Spherical Bessel functions with half-integer α are obtained when the Helmholtz equation is solved in spherical coordinates.

beta decay

Also β-decay.

In nuclear physics, a type of radioactive decay in which a beta particle is emitted from an atomic nucleus, transforming the original nuclide to its isobar.

beta particle

A high-energy, high-speed electron or positron emitted by certain types of radioactive atomic nuclei.

Big Bang

The prevailing cosmological model that describes the early development of the Universe.

binding energy

The mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than the sum of its constituent parts.

binomial random variable

biocatalysis

biophysics

An interdisciplinary science using methods of and theories from physics to study biological systems.

black body

A hypothetical idealized physical body that completely absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. Perfect black bodies are imagined as substitutes for actual physical bodies in many theoretical discussions of thermodynamics, and the construction of nearly perfect black bodies in the real world remains a topic of interest for materials engineers. Contrast white body.

black-body radiation

The type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body.

block and tackle

A system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.

Bohr model

boiling point

The temperature at which a liquid undergoes a phase change into a gas; the vapour pressure of liquid and gas are equal at this temperature.

boiling point elevation

The phenomenon by which the boiling point of a liquid (a solvent) increases when another compound is added, meaning that the resulting solution has a higher boiling point than the pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such as water. The boiling point can be measured accurately using an ebullioscope.

Boltzmann constant

A physical constant relating the average kinetic energy of the particles in a gas with the temperature of the gas. It is the gas constant R divided by the Avogadro constant NA.

Bose–Einstein condensate (BEC)

boson

A type of subatomic particle that behaves according to Bose–Einstein statistics and possesses integer spin. Bosons include elementary particles such as photons, gluons, W and Z bosons, Higgs bosons, and the hypothetical graviton, as well as certain composite particles such as mesons and stable nuclides of even mass number. Bosons constitute one of two main classes of particles, the other being fermions. Unlike fermions, there is no limit to the number of bosons that can occupy the same quantum state.

Boyle's law

A chemical law which states that the volume of a given mass of a gas at constant temperature is inversely proportional to its pressure.

Bra–ket notation

Bragg's law

bremsstrahlung

Radiation emitted by the acceleration of unbound charged particles.

Brewster's angle

Also called the polarization angle.

The angle of incidence at which light with a particular polarization is completely transmitted through a transparent dielectric surface, with no reflection. When unpolarized light is incident at this angle, the light that is reflected is consequently perfectly polarized.

british thermal unit (btu)

An Imperial unit of energy defined as the amount of energy needed to heat one pound of water by one degree Fahrenheit; 1 btu is equal to about 1,055 joules. In scientific contexts the btu has largely been replaced by the SI unit of energy, the joule.

brittleness

The tendency of a material to break without significant plastic deformation when subjected to stress. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.

Brownian motion

Also called pedesis.

The presumably random movement of particles suspended in a fluid (liquid or gas) resulting from their bombardment by fast-moving atoms or molecules in the gas or liquid.

Bubble

Bulk modulus

A measure of a substance's resistance to uniform compression defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. Its base unit is the pascal.

buoyancy

An upward force exerted by a fluid that opposes the weight of an immersed object.

C

calculus

A branch of mathematics that studies change and has two major sub-fields: differential calculus (concerning rates of change and slopes of curves), and integral calculus (concerning accumulation of quantities and the areas under and between curves). These two branches are related to each other by the fundamental theorem of calculus.

capacitance

The ratio of the change in the electric charge of a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors).

capacitive reactance

An opposition to the change of voltage across an electrical circuit element. Capacitive reactance ${\displaystyle \scriptstyle {X_{C}}}$ is inversely proportional to the signal frequency ${\displaystyle \scriptstyle {f}}$ (or angular frequency, ω) and the capacitance ${\displaystyle \scriptstyle {C}}$.^[24]

capacitor

An electrical circuit element consisting of two conductors separated by an insulator (also known as a dielectric).

Carnot cycle

A theoretical ideal thermodynamic cycle proposed by French physicist Nicolas Léonard Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference by the application of work to the system. It is not an actual thermodynamic cycle but is a theoretical construct.

Cartesian coordinate system

A coordinate system that specifies each point uniquely in a plane by a set of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis (plural axes) of the system, and the point where they meet is called the origin, at ordered pair (0, 0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

cathode

The electrode through which a conventional electric current flows out of a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of electron flow, and so electrons flow into the cathode. In a galvanic cell, the cathode is the positive terminal or pole which accepts electrons flowing from the external part of an electrical circuit. However, in an electrolytic cell, the cathode is the wire or plate having excess negative charge, so named because positively charged cations tend to move towards it. Contrast anode.

cathode ray

cation

A positively charged ion. Contrast anion.

celestial mechanics

Celsius scale

Also centigrade scale.

A scale and unit of measurement of temperature.

center of curvature

center of gravity

The point in a body around which the resultant torque due to gravity forces vanish. Near the surface of the earth, where gravity acts downward as a parallel force field, the center of gravity and the center of mass are the same.

center of mass

Within a given distribution of mass, the unique point in space at which the weighted relative position of the distributed mass sums to zero.

center of pressure

centigrade

See Celsius scale.

central-force problem

A classic problem in potential theory involving the determination of the motion of a particle in a single central potential field. The solutions to such problems are important in classical mechanics, since many naturally occurring forces, such as gravity and electromagnetism, are central forces.

centrifugal force

The apparent outward force that draws a rotating body away from the centre of rotation. It is caused by the inertia of the body as the body's path is continually redirected.

centripetal force

A force which keeps a body moving with a uniform speed along a circular path and is directed along the radius towards the centre.

cGh physics

Any attempt in mainstream physics to unify existing theories of relativity, gravitation, and quantum mechanics, particularly by envisioning the three universal constants fundamental to each field – the speed of light (${\displaystyle c}$), the gravitational constant (${\displaystyle G}$), and the Planck constant (${\displaystyle h}$) – as the edges of a three-dimensional cube, at each corner of which is positioned a major sub-field within theoretical physics according to which of the three constants are accounted for by that sub-field and which are ignored. One corner of this so-called "cube of theoretical physics", where all three constants are accounted for simultaneously, has not yet been satisfactorily described: quantum gravity.

chain reaction

A sequence of reactions in which a reactive product or byproduct causes additional similar reactions to take place.

change of base rule

charge carrier

chemical physics

A branch of chemistry and physics that studies chemical processes from the point of view of physics by investigating physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics.

chromatic aberration

circular motion

classical mechanics

Also called Newtonian mechanics.

A sub-field of mechanics concerned with the set of physical laws describing the motion of bodies under the collective actions of a system of forces.

coefficient of friction

coherence

cohesion

The tendency of similar particles or surfaces to cling to one another. Contrast adhesion.

cold fusion

complex harmonic motion

composite particle

Compton scattering

A type of light–matter interaction in which a photon is scattered by a charged particle, usually an electron, which results in part of the energy of the photon being transferred to the recoiling electron; a resulting decrease in the energy of the photon is called the Compton effect. The opposite phenomenon occurs in inverse Compton scattering, when a charged particle transfers part of its energy to a photon.

concave lens

condensation point

condensed matter physics

A branch of physics that studies the physical properties of condensed phases of matter.

conservation of momentum

conservation law

constructive interference

continuous spectrum

continuum mechanics

convection

The transfer of heat by the actual transfer of matter.

convex lens

coulomb (C)

The SI derived unit of electric charge, defined as the charge transported by a constant current of one ampere in one second.

Coulomb's law

converging lens

cosmic background radiation

creep

crest

The point on a wave with the maximum value or upward displacement within a cycle.

crest factor

critical angle

critical mass

The smallest amount of fissile material needed for a sustained nuclear chain reaction.

cube of theoretical physics

See cGh physics.

Curie temperature

current density

current length

curvilinear motion

The motion of a moving particle or object that conforms to a known or fixed curve. Such motion is studied with two coordinate systems: planar motion and cylindrical motion.

cyclotron

A type of particle accelerator in which charged particles accelerate outwards from the center along a spiral path.

D

Dalton's law

damped vibration

Damping ratio

Any influence upon or within an oscillatory system that has the effect of reducing, restricting, or preventing its oscillations. Damping is a result of processes that dissipate the energy stored in the oscillation.

Darcy–Weisbach equation

dark energy

dark matter

DC motor

A mechanically commutated electric motor powered by direct current.

decibel

definite integral

deflection

The degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

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