Glossary of physics

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.

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

Abbe number: In and lens design, a measure of a transparent material's (a variation of  versus ). 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 of the sample.

absolute motion:

absolute pressure: Is zero-referenced against a perfect vacuum, using an, 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, understood by international agreement as equivalent to 0 or -273.15 C. More formally, it is the theoretical lower limit of the scale, at which  and  of a cooled ideal gas reach their minimum values and the fundamental particles of nature have minimal vibrational motion.

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

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.

acceleration: The rate at which the 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 on an object caused by the force of.

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

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

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.

adiabatic cooling:

adiabatic heating:

adiabatic process: A process which occurs without transfer of or of substances between a  and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as. 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.

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 and gas dynamics, and many aspects of aerodynamics theory are common to these fields.

afocal system: An system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length. 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 ($d = f_1 + f_2$).

air mass: 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. In astronomy, the "amount of air that one is looking through" 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.

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 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:

A type of in which an emits an  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):

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

alternating current (AC): A form of in which the movement of periodically reverses direction. Contrast.

ammeter: An instrument that is used to measure.

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

ampere (A):

The base unit of, defined as one of  per second.

amplifier:

An electronic device that can increase the of a (a time-varying  or ). It is a two-port electronic circuit that uses electric power from a power supply to increase the 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 : the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.

amplitude: The height of a 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:,, , , 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,, 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 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. In three dimensions, it is a pseudovector. In SI units, it is measured in radians per second squared (rad/s$2$), and is usually denoted by the Greek letter alpha (α). 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. 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):

A scalar measure of rotation rate. It refers to the 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. The term angular frequency vector $\vec{\omega}$ is sometimes used as a synonym for the vector quantity angular velocity.

One revolution is equal to 2π radians, hence


 * $\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).|undefined

angular momentum:

The rotational equivalent of. It is an important quantity in physics because it is a conserved quantity–that is, the total angular momentum of a closed system remains constant.

angular velocity (ω): How fast an object rotates or revolves relative to another point, i.e. how fast the angular position or orientation of an object changes with time. There are two types of angular velocity: orbital angular velocity and spin angular velocity. Spin angular velocity refers to how fast a rigid body rotates with respect to its centre of rotation. Orbital angular velocity refers to how fast a rigid body's centre of rotation revolves about a fixed origin, i.e. the time rate of change of its angular position relative to the origin. In general, angular velocity is measured in angle per unit time, e.g. radians per second. The SI unit of angular velocity is expressed as radians/sec with the radian having a dimensionless value of unity, thus the SI units of angular velocity are listed as 1/sec. Angular velocity is usually represented by the Greek letter omega (ω, sometimes Ω). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.

anion: A negatively charged. Contrast.

annihilation: In, the process that occurs when a collides with its respective antiparticle to produce other particles, such as an colliding with a  to produce two. The total and  of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of such an original pair are zero. Hence, any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy and conservation of momentum are obeyed.

anode: The electrode through which a conventional flows into a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of flow, and so electrons flow out of the anode. In a galvanic cell, the anode is the negative terminal or pole which emits electrons toward the external part of an. However, in an, the anode is the wire or plate having excess positive charge, so named because negatively charged tend to move towards it. Contrast.

anti-gravity: A theory of creating a place or object that is free from the force of. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift.

antimatter:

antineutron: The antiparticle of the, with symbol. It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same as the neutron, and no net, but has opposite baryon number (+1 for neutron, −1 for the antineutron). This is because the antineutron is composed of antiquarks, while neutrons are composed of. The antineutron consists of one up antiquark and two down antiquarks.

antiparticle: In, every type of has an associated antiparticle with the same but with opposite  such as. For example, the antiparticle of the is the antielectron (which is often referred to as the positron). While the electron has a negative electric charge, the positron has a positive electric charge, and is produced naturally in certain types of. Some particles, such as the, are their own antiparticle. Otherwise, for each pair of antiparticle partners, one is designated as "normal" matter (the kind comprising all matter with which humans usually interact), and the other (usually given the prefix "anti-") as.

antiproton: It is a subatomic particle of the same mass as a proton but having a negative electric charge and oppositely directed magnetic moment. It is the proton’s antiparticle. Antiprotons were first produced and identified in 1955 by Emilio Segrè, Owen Chamberlain

antiquark: For every flavor there is a corresponding type of known as an antiquark that differs from the quark only in that some of its properties (such as the ) have equal magnitude but opposite sign.

arc length:

Archimedes' principle: A physical principle which states that the upward that is exerted on a body immersed in a, whether fully or partially submerged, is equal to the of the fluid that the body  and acts in the upward direction at the center of mass of the displaced fluid.

area moment of inertia:

astrophysics: The branch of astronomy that deals with the physics of the Universe, especially with the compositional nature of celestial bodies rather than their positions or motions in space.

attenuation coefficient: The measure of how much the incident energy beam (e.g. ultrasound or x-rays) is weakened by the material it is passing through.

atom: A basic unit of that consists of a dense central nucleus surrounded by a cloud of negatively charged. The atomic nucleus contains a mix of positively charged and electrically neutral.

atomic line filter:

atomic mass:

atomic mass unit: A deprecated term, usually referring to the, a carbon-based standard, but historically referring to an oxygen-based standard.

atomic number (Z): The number of found in the of an. It is most often used to classify elements within the periodic table.

atomic orbital:

atomic packing factor:

atomic physics: A branch of physics that studies as isolated systems of and an. Compare.

atomic structure:

atomic weight (A): The sum total of (or ) and within an.

audio frequency: A periodic vibration whose frequency is in the band audible to the average human, the human hearing range. It is the property of sound that most determines pitch, with a generally accepted standard hearing range for humans is 20 to 20,000 Hz. Also known as audible frequency (AF)

Avogadro constant: The ratio of the number of constituent in a substance, usually or, to the amount of substance, of which the  unit is the mole. It is defined as exactly $6.022 mol-1$.

Avogadro number: The total number of individual molecules in one mole of a substance, by definition equaling exactly $6.022$.

Avogadro's law: A physical law which states that volumes of gases which are equal to each other at the same temperature and pressure will contain equal numbers of molecules.

axion: A hypothetical postulated to account for the rarity of processes that break charge-parity symmetry. It is very light, electrically neutral, and pseudoscalar.

azimuthal quantum number: A quantum number for an atomic orbital that determines its orbital angular momentum and describes the shape of the orbital.

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 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:

In, 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 such as a or a, each of which is made of (usually) three. Nearly all humans are likely to encounter is baryonic matter.

battery: A combination of two or more electrical cells which produces.

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: 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 or is applied to the element, causing the element to bend. The simplest structural element subjected to bending moments is the.

Bernoulli equation:

Bernoulli's principle: In fluid dynamics, a principle which states that an increase in the of a occurs simultaneously with a decrease in  or a decrease in the fluid's.

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


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

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:

In, a type of in which a is emitted from an, transforming the original  to its.

beta particle: A high-energy, high-speed or emitted by certain types of  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 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, regardless of or. Perfect black bodies are imagined as substitutes for actual physical bodies in many theoretical discussions of, and the construction of nearly perfect black bodies in the real world remains a topic of interest for materials engineers. Contrast.

black-body radiation: The type of within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a (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 with a rope or cable threaded between them, usually used to lift or pull heavy loads.

Bohr model:

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

boiling point elevation: The phenomenon by which the boiling point of a (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 relating the average of the particles in a  with the  of the gas. It is the gas constant R divided by the NA.

Bose–Einstein condensate (BEC):

boson: A type of that behaves according to Bose–Einstein statistics and possesses integer. Bosons include such as,, , , and the hypothetical , as well as certain  such as  and  of even. Bosons constitute one of two main classes of particles, the other being. Unlike fermions, there is no limit to the number of bosons that can occupy the same.

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: emitted by the of unbound charged particles.

Brewster's angle:

The angle of incidence at which with a particular is completely transmitted through a transparent  surface, with no. 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 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. 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 when subjected to. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.

Brownian motion:

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

Bubble:

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

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

C
calculus: A branch of 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 of a system to the corresponding change in its. 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 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 across an electrical circuit element. Capacitive reactance $\scriptstyle{X_C}$ is inversely proportional to the signal $\scriptstyle{f}$ (or, ω) and the  $\scriptstyle{C}$.

capacitor: An element consisting of two separated by an  (also known as a ).

Carnot cycle: A theoretical ideal 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 into, 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 flows out of a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of 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. However, in an, the cathode is the wire or plate having excess negative charge, so named because positively charged tend to move towards it. Contrast.

cathode ray:

cation: A positively charged. Contrast.

celestial mechanics:

Celsius scale:

A scale and unit of measurement of.

center of curvature:

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

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

center of pressure:

centigrade: See.

central-force problem: A classic problem in potential theory involving the determination of the motion of a particle in a single. The solutions to such problems are important in, since many naturally occurring forces, such as and, 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 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 existing theories of,, and , particularly by envisioning the three universal constants fundamental to each field – the ($c$), the  ($G$), and the  ($h$) – as the edges of a three-dimensional cube, at each corner of which is positioned a major sub-field within  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:.

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.

chromatic aberration:

circular motion:

classical mechanics:

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

coefficient of friction:

coherence:

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

cold fusion:

complex harmonic motion:

composite particle:

Compton scattering: A type of light–matter interaction in which a is scattered by a, usually an, 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 by the actual transfer of.

convex lens:

coulomb (C): The derived unit of, defined as the charge transported by a constant of one  in one second.

Coulomb's law:

converging lens:

cosmic background radiation:

creep:

crest: The point on a 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.

cube of theoretical physics: See.

Curie temperature:

current density:

current length:

curvilinear motion: The 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 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 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.

decibel:

definite integral:

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

deformation: (mechanics) (engineering)

density:

A physical property of a substance defined as its per unit volume.

derivative: For a mathematical function of a real variable, a measurement of the sensitivity to change of the function value (output) with respect to a change in its argument (input); e.g. the derivative of the position of a moving object with respect to time is the object's and measures how quickly the position of the object changes as time changes. Derivatives are a fundamental tool of.

destructive interference:

diamagnetism:

dielectric: An electrical that can be by an applied. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they would in a but only shift slightly from their equilibrium positions, with positive charges displaced in the direction of the field's flow and negative charges displaced in the opposite direction; this creates an internal electric field that reduces the larger field within the dielectric material.

diffraction:

direct current (DC):

dispersion:

displacement: (fluid) Occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the immersed object will be exactly equal to the volume of the displaced fluid, so that the volume of the immersed object can be deduced if the volume of the displaced fluid is measured. (vector) The shortest distance from the initial to the final position of a point. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by.

distance: A numerical description of how far apart objects are.

drift velocity:

Doppler effect: The change in of a (or other periodic event) for an observer moving relative to its source. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

drag: Forces which act on a solid object in the direction of the relative fluid flow velocity. Unlike other resistive forces, such as dry, which is nearly independent of velocity, drag forces depend on velocity.

ductility: A solid material's ability to under tensile stress; this is often characterized by the material's ability to be stretched into a wire.

dynamics: The branch of that studies and  and their effects on, as opposed to, which studies motion without reference to these forces.

dyne:

E
econophysics:

elastic collision:

elastic energy:

elastic instability:

elastic modulus:

elasticity: The tendency of a material to return to its original shape after it is.

electric charge: A physical property of that causes it to experience a when near other electrically charged matter. There are two types of electric charge: positive and negative.

electric circuit: An consisting of a closed loop, giving a return path for the.

electric current: A flow of through a conductive medium.

electric displacement field:

electric field: The region of space surrounding electrically particles and time-varying. The electric field represents the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding.

electric field gradient:

electric field intensity:

electric generator:

electric motor:

electric potential:

electric power: The rate at which electric energy is transferred by an.

electrical conductor: Any material which contains movable and therefore can conduct an under the influence of an.

electrical insulator: Any material whose internal do not flow freely and which therefore does not conduct an under the influence of an.

electrical potential energy:

electrical and electronics engineering:

electrical network: An interconnection of electrical elements such as resistors, inductors, capacitors, voltage sources, current sources, and switches.

electrical resistance: The opposition to the passage of an through an electrical element.

electricity: The set of physical phenomena associated with the presence and flow of.

electro-optic effect:

electrochemical cell:

electrodynamics:

electrolytic cell:

electromagnet: A type of magnet in which the is produced by the flow of.

electromagnetic field:

A physical field produced by moving electrically charged objects.

electromagnetic induction:

electromagnetic radiation:

A form of emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space.

electromagnetic spectrum:

electromagnetic wave equation:

electromagnetism:

electromechanics:

electromotive force ($\mathcal{E}$):

The electrical intensity or "pressure" developed by a source of electrical energy such as a or and measured in. Any device that converts other forms of into electrical energy provides electromotive force as its output.

electron: A with a negative.

electron capture:

electron cloud:

electron pair:

electron paramagnetic resonance:

A method for studying materials with unpaired which makes use of the. It shares some basic principles with nuclear magnetic resonance (NMR).

electronvolt (eV): A unit of equal to approximately 1.6×10−19. By definition, it is the amount of energy gained by the charge of a single moved across an electric potential difference of one.

electronegativity: A chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

electronics: A field that deals with that involve active electrical components such as vacuum tubes, transistors, diodes, and integrated circuits as well as associated passive interconnection technologies.

electrostatics:

electrostriction:

elementary charge:

elementary particle:

emission spectrum:

emissivity:

energy: The ability to do.

energy level:

endothermic: An adjective used to refer to a process or reaction in which a system absorbs from its surroundings, usually in the form of but also in the form of,, or. Contrast.

engineering physics:

enthalpy:

entropy: A quantity which describes the randomness of a substance or system.

equilibrant force:

equipartition:

escape velocity: The at which the plus the gravitational  of an object is zero. It is the speed needed to "escape" from a gravitational field without further propulsion.

excited state:

exothermic: An adjective used to refer to a process or reaction that releases from a system, usually in the form of but also in the form of,, or. Contrast.

experimental physics:

F
farad:

falling bodies: Objects that are moving towards a body with greater gravitational influence, such as a planet.

faraday:

Faraday constant:

Fermat's principle:

Fermi surface:

fermion: A type of that behaves according to Fermi–Dirac statistics, obeys the, and possesses half-integer. Fermions include all and, as well as all  made of an odd number of these (such as all  and many  and ). Fermions constitute one of two main classes of particles, the other being.

ferrimagnetism:

ferromagnetism:

field line:

first law of thermodynamics:

fission: Either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays) and releasing relatively large amounts of energy.

flavour:

fluid:

fluid mechanics:

fluid physics:

fluid statics:

fluorescence:

flux:

flux density:

focal length:

focus:

force (F): A push or pull. Any interaction that, when unopposed, will change the of a physical body. A force has both magnitude and direction, making it a quantity. The unit used to measure force is the.

force carrier:

Force field (physics):

frame of reference:

Fraunhofer lines:

free body diagram:

frequency:

frequency modulation:

free fall: Any motion of a body where its own is the only force acting upon it.

freezing point: The temperature at which a substance changes state from to.

friction:

function:

fundamental forces:

fundamental frequency:

fundamental theorem of calculus:

fusion: A nuclear reaction in which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus.

G
gamma ray: A form of of very high frequency and therefore very high energy.

gas:

general relativity:

geophysics:

gluon:

Graham's law of diffusion:

gravitation:

A natural phenomenon by which physical bodies attract each other with a proportional to their.

gravitational constant (G):

A physical constant involved in the calculation of between two bodies.

gravitational energy: The associated with the.

gravitational field: A model used to explain the influence that a massive body extends into the space around itself, producing a on another massive body. Thus, a gravitational field is used to explain and represent gravitational phenomena. It is measured in per kilogram (N/kg).

gravitational potential: The gravitational potential at a location is equal to the ( transferred) per unit that is done by the force of  to move an object to a fixed reference location.

gravitational wave: A ripple in the curvature of that propagates as a and is generated in certain gravitational interactions, travelling outward from their source.

graviton:

gravity: See.

ground:

ground reaction force:

ground state:

group velocity:

H
hadron: A composite particle made from three quarks or three antiquarks baryon, or one quark and one antiquark meson.

half-life: The time required for a quantity to fall to half its value as measured at the beginning of the time period. In physics, half-life typically refers to a property of, but may refer to any quantity which follows an exponential decay.

Hamilton's principle:

Hamiltonian mechanics:

harmonic mean:

heat: A form of transferred from one body to another by thermal interaction.

heat transfer:

Helmholtz free energy:

hertz: The unit of, defined as the number of cycles per second of a periodic phenomenon.

Higgs boson:

homeokinetics: The physics of complex, self-organizing systems.

horsepower (hp):

Huygens–Fresnel principle:

hydrostatics:

I
ice point: A physical process that results in the phase transition of a substance from a liquid to a solid.

impedance: The measure of the opposition that a circuit presents to a when a is applied.

Implosion:

impulse: The change in momentum, which is equal to the average net external force multiplied by the time this force acts.

indefinite integral:

inductance:

infrasound:

inertia: The resistance of any physical object to a change in its state of or, or the tendency of an object to resist any change in its motion.

inductive reactance:

integral:

integral transform:

International System of Units (SI): The modern form of the metric system, comprising a system of units of measurement devised around seven base units and the convenience of the number ten.

invariant mass:

ion: An or in which the total number of  is not equal to the total number of, giving the atom a net positive or negative.

ionic bond: A type of chemical bond formed through an electrostatic attraction between two oppositely.

ionization: The process of converting an or into an  by adding or removing charged particles such as  or other ions.

ionization chamber:

ionizing radiation:

isotope: A variant of a particular chemical element. While all isotopes of a given element share the same number of, each isotope differs from the others in its number of.

J
Josephson effect:

joule: A derived unit of,, or amount of in the.

K
Kelvin: A scale and unit of measurement of. The Kelvin scale is an absolute scale which uses as its null point.

kinematics: The branch of that describes the of points, bodies (objects), and systems of bodies (groups of objects) without consideration of the causes of motion. The study of kinematics is often referred to as the "geometry of motion".

kinetic energy: The that a physical body possesses due to its, defined as the needed to  a body of a given  from rest to its stated. The body continues to maintain this kinetic energy unless its velocity changes. Contrast.

Kirchhoff's circuit laws:

Two approximate equalities that deal with the and in. See Kirchhoff's laws for other meanings of the term.

Kirchhoff's equations: In, a set of equations which describe the of a rigid body in an ideal.

L
Lagrangian mechanics:

laminar flow: Occurs when a fluid flows in parallel layers with no disruption between the layers.

Laplace transform:

Laplace–Runge–Lenz vector:

A used chiefly to describe the shape and orientation of the orbit of one astronomical body around another, such as a planet revolving around a star. For two bodies interacting by Newtonian gravity, the LRL vector is a constant of motion, meaning that it is the same no matter where it is calculated on the orbit; equivalently, the LRL vector is said to be conserved.

laser: A device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation"

law of universal gravitation:

LC circuit: A circuit consisting of an inductor (with inductance L) and a capacitor (with capacitance C).

Lenz's law:

lepton: An elementary particle which does not undergo but is subject to the. Two main classes of leptons exist: charged leptons (also known as the -like leptons) and neutral leptons (better known as ).

lever: A type of consisting of a beam or rigid rod pivoted at a fixed hinge or fulcrum; one of six classical.

levitation (physics):

light: A form of that occupies a certain range of within the. In physics, the term sometimes refers collectively to electromagnetic radiation of any wavelength, in which case light includes,, , and , but in common usage "light" more often refers specifically to.

linear actuator: A form of motor that generates a linear movement directly.

linear algebra: The branch of concerning, often finite or countably infinite dimensional, as well as linear mappings between such spaces.

line of force:

linear elasticity: The mathematical study of how solid objects deform and become internally stressed due to prescribed loading conditions. Linear elasticity is a simplification of the more general nonlinear theory of elasticity and is a branch of.

Liouville's theorem: Phase space volume is conserved.

liquid: One of four classical having a definite but no fixed shape.

liquid crystal (LC): A which has properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its may be oriented in a crystal-like way.

longitudinal wave:

M
M-theory: An extension of that attempts to unify seemingly contradictory mathematical formulations and which identifies 11 dimensions.

Mach number: A dimensionless quantity representing the ratio of the of an object moving through a fluid to the local.

Mach's principle: The proposition that the existence of absolute rotation (the distinction of local inertial frames vs. rotating reference frames) is determined by the large-scale distribution of matter.

machine: Any powered tool consisting of one or more parts that is constructed to achieve a particular goal. Machines are usually powered by mechanical, chemical, thermal or electrical means, and are frequently motorised.

machine element: An elementary component of a. There are three basic types: structural components, mechanisms, and control components.

Maclaurin series: A representation of a function as an infinite sum of terms that are calculated from the values of the function's derivatives at a single point.

magnetic field: A mathematical description of the influence of and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a field.

magnetism: A property of materials that respond to an applied.

magnetostatics:

mass:

mass balance:

An application of the law of to the analysis of physical systems.

mass density: See.

mass flux: The rate of mass flow per unit area. The common symbols are j, J, φ, or Φ, sometimes with subscript m to indicate mass is the flowing quantity. Its SI units are kg s−1 m−2.

mass moment of inertia: A property of a distribution of mass in space that measures its to rotational acceleration about an axis.

mass number:

The total number of and (together known as ) in an atomic nucleus.

mass spectrometry:

material properties:

materials science: An interdisciplinary field incorporating elements of physics, chemistry, and engineering that is concerned with the design and discovery of new materials, particularly.

mathematical physics: The application of to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories.

mathematics: The abstract study of topics encompassing quantity, structure, space, change, and other properties.

matrix: A rectangular array of numbers, symbols, or expressions arranged in rows and columns. The individual items in a matrix are called its elements or entries.

matter: Any substance (often a particle) that has and (usually) also.

Maxwell's equations: A set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. Maxwell's equations describe how and are generated and altered by each other and by  and.

measure of central tendency: A term which relates to the way in which quantitative data tend to cluster around some value. A measure of central tendency is any of a number of ways of specifying this "central value".

mechanical energy:

mechanical filter:

mechanical equilibrium:

mechanical wave:

mechanics: The branch of science concerned with the behaviour of physical bodies when subjected to or displacements and the subsequent effects of the bodies on their environment.

melting: A physical process that results in the phase transition of a substance from a solid to a liquid.

meson: A type of composed of one  and one  bound together by the. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond.

modulus of elasticity: The mathematical description of an object's or substance's tendency to be elastically (i.e. non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its in the elastic deformation region. As such, a material will have a higher elastic modulus.

molar concentration:

molar mass: A physical property of defined as the of a given substance divided by the amount of substance and expressed in grams per mole.

molecule: An electrically neutral group of two or more held together by covalent chemical bonds. Molecules are distinguished from by having a net  equal to zero.

molecular physics: A branch of physics that studies the physical properties of and the chemical bonds between as well as their molecular dynamics. It is closely related to atomic physics and overlaps greatly with theoretical chemistry, and.

moment:

moment of inertia: A property of a distribution of in space that measures its to rotational acceleration about an axis.

momentum: A vector quantity consisting of the product of the mass and velocity of an object.

monochromatic light:

motion: Any change in the position of an object over. Motion can be mathematically described in terms of,, , , , and , and is observed by attaching a to an observer and measuring the change in an object's position relative to that frame. An object's motion cannot change unless it is acted upon by a.

muon: An elementary particle, technically classified as a, that is similar to the, with unitary negative electric charge (−1) and a spin of 1⁄2. Muons are not believed to have any sub-structure.

N
nanoengineering: The practice of engineering on the nanoscale. Nanoengineering is largely a synonym for, but emphasizes the applied rather the field.

nanotechnology:

The manipulation of matter on an and scale; a more generalized description by the National Nanotechnology Initiative is "the manipulation of matter with at least one dimension sized from 1 to 100 nanometres".

Navier–Stokes equations:

neurophysics:

neutrino: A type of electrically neutral denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have but that their mass is tiny even by the standards of subatomic particles. Their mass has never been measured accurately.

neutron: Subatomic particle with no charge


 * prompt neutron:
 * Immediate emission of neutrons after a nuclear fission event


 * delayed neutron:
 * Delayed emission of neutrons after a nuclear fission event, by one of the fission products (actually, a fission product daughter after beta decay)

neutron cross-section:

newton (N):

Newton's laws of motion: A set of three physical laws which describe the relationship between the acting on a body and its motion due to those forces. Together they form the basis for.

Newton's law of universal gravitation:

Newtonian fluid:

Newtonian mechanics:

normal force:

nuclear force:

nuclear physics: The branch of physics that studies the constituents and interactions of.

nuclear reaction:

nuclear transmutation:

nucleon: Either a or a in its role as a component of an.

nucleus:

nuclide:

An species characterized by the specific composition of its, i.e. by its number of, its number of , and its nuclear.

O
Ohm: The derived unit of.

Ohm's law: The through a between two points is directly proportional to the  difference across the two points.

optical tweezers: An optomechanical device used for the capture, analysis, and manipulation of objects or particles, which operates via the application of by the  of light.

optically detected magnetic resonance: An optical technique for the initialisation and readout of in some crystal defects.

optics: The branch of physics which involves the behaviour and properties of, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light; however, other forms of such as, microwaves, and radio waves exhibit similar properties.

P
paraffin:

parallel circuit:

parity: (mathematics) (physics)

particle:

particle accelerator:

particle displacement:

particle physics: A branch of physics that studies the nature of, which are the constituents of what is usually referred to as and.

Pascal's law: A principle in which states that exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the initial pressure variations remain the same.

Pauli exclusion principle:

pendulum:

periodic table of the elements:

A tabular display of the chemical elements organised on the basis of their, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number (number of protons).

phase (matter):

phase (waves):

phase equilibrium:

phenomenology:

phosphorescence:

photoelectric effect:

photon: An elementary particle, the of and all other forms of, and the force carrier for the.

photonics:

physical chemistry: The study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of laws and concepts of physics.

physical constant:

physical quantity:

physics: The natural science that involves the study of and its motion through and, along with related concepts such as  and. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.

piezoelectricity:

pion:

Planck constant ($h$): A fundamental universal that is the of action in.

Planck units:

Planck's law:

plasma:

plasma physics:

plasticity:

pneumatics: The study and control of mechanical force and movement generated by the application of compressed gas.

positron:

potential energy:

power:

pressure: The ratio of to the area over which that force is distributed.

principle of relativity:

probability: A measure of the expectation that an event will occur or that a statement is true. Probabilities are given a value between 0 (will not occur) and 1 (will occur). The higher the probability of an event, the more certain one can be that the event will occur.

probability distribution:

probability theory:

proton:

psi particle:

pulley: A that is designed to support movement of a cable or belt along its circumference; one of six classical. Pulleys are used in a variety of ways to lift loads, apply, and transmit.

pulse:

pulse wave:

Q
quantization:

quantum:

quantum chromodynamics:

quantum electrodynamics (QED): The of. In essence, it describes how and  interact and is the first theory where full agreement between  and  is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of and represents the quantum counterpart of, giving a complete account of matter and light interaction.

quantum field theory: A theoretical framework for constructing quantum mechanical models of in and  in.

quantum gravity:

quantum mechanics: A branch of physics dealing with physical phenomena at microscopic scales, where the action is on the order of the. Quantum mechanics departs from at and  length scales, and provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter that occur at this scale.

quantum number:

quantum physics:

quantum state:

quark: An elementary and a fundamental constituent of. Quarks combine to form composite particles called, the most stable of which are and, the components of.

quasiparticle:

R
radiant energy:

radiation:

radioactive decay:

radionuclide:

Any possessing excess nuclear to the point that it is unstable. Such excess energy is emitted through any of several processes of, resulting in a or sometimes another unstable radionuclide which can then undergo further decay. Certain radionuclides occur naturally; many others can be produced artificially in nuclear reactors,, , or radionuclide generators.

radius of curvature:

redshift: A phenomenon which occurs when seen coming from an object that is moving away from the observer is proportionally increased in or "shifted" to the red end of the  spectrum.

refraction: The change in direction of a as it passes from one to another or as a result of a gradual change in the medium. Though most commonly used in the context of refraction of, other waves such as waves and fluid waves also experience refraction.

refractive index:

relative atomic mass:

relativistic mechanics:

relativity:

rest frame:

rigid body: An idealization of a solid body in which is neglected. In other words, the between any two given points of a rigid body remains constant in time regardless of the external forces exerted on it. Even though such an object cannot physically exist due to, objects can normally be assumed to be perfectly rigid if they are not moving near the.

rotational energy:

The due to the rotation of an object, which forms part of its total kinetic energy.

rotational speed:

The number of complete rotations or revolutions a rotating body makes per unit time.

Rydberg formula: A formula used in atomic physics to describe the wavelengths of spectral lines of many chemical elements.

S
scalar: Any simple physical quantity that can be described by a single number (as opposed to,, etc., which are described by several numbers such as magnitude and direction) and is unchanged by coordinate system rotations or translations (in Newtonian mechanics) or by Lorentz transformations or central-time translations (in relativity).

scattering: The general physical process by which some forms of, such as, , or moving particles, are forced to deviate from a straight by one or more localised non-uniformities in the medium through which they pass.

science: A systematic enterprise that builds and organises knowledge in the form of testable explanations and predictions about the universe.

screw: A mechanism that converts rotational motion to linear motion, and a (rotational force) to a linear force; one of six classical.

second law of thermodynamics:

Seebeck effect:

series circuit:

shadow matter:

shear modulus:

shear strength:

shear stress:

shortwave radiation (SW): of the with in the, near-ultraviolet, and near-infrared spectra, the broadest definition of which includes all radiation with a wavelength between 0.1 μm and 5.0 μm.

Schrödinger equation: A mathematical equation which describes the time evolution of wave functions in.

simple harmonic motion:

simple machine: A mechanical device that changes the direction or magnitude of a. In general, a set of six classical simple machines identified by Renaissance scientists drawing from Greek texts on technology are collectively defined as the simplest mechanisms that can provide mechanical advantage (also called leverage).

siphon: A tube in an inverted U shape that causes a liquid to flow uphill without pumps, powered by the fall of the liquid as it flows down the tube under the pull of. The term may also more generally refer to a wide variety of devices involving the flow of liquids through tubes.

Snell's law:

solar cell:

solid:

solid mechanics:

solid-state physics:

solubility: The tendency of a solid, liquid, or gaseous chemical substance (called a solute) to dissolve in another solid, liquid, or gaseous substance (called a solvent) to form a homogeneous solution of the solute in the solvent. The solubility of a solute fundamentally depends on the specific solvent as well as on and.

Sonoluminescence:

sound: A mechanical that is an oscillation of transmitted through a solid, liquid, or gas and composed of frequencies within the range of human hearing.

special relativity:

specific activity:

speed:

speed of light ($c$): A fundamental universal defined as exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. When not otherwise qualified, the term "speed of light" usually refers to the speed of in, as opposed to the speed of light through some physical medium.

speed of sound:

spherical aberration:

spin quantum number:

stable isotope ratio: The relative abundances of the atomically of a given element as they occur in nature or in a particular experimental context.

stable nuclide: Any that is not radioactive and does not spontaneously undergo, as opposed to a. When such nuclides are referred to in relation to specific elements, they are usually termed.

standard atomic weight:

Standard Model: The theory of which describes three of the four known (the, the, and the , but not the ) and classifies all known.

standing wave:

state of matter:

statics: The branch of mechanics concerned with the analysis of loads ( and, or "moment") on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant.

statistical mechanics:

stiffness: The rigidity of an object, i.e. the extent to which it resists in response to an applied.

strain: The transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body.

strain hardening:

strength of materials:

stress: An applied or system of forces that tends to or  a physical body. A measure of the internal forces acting within a deformable body. A quantitative measure of the average force per unit area of a surface within a body on which internal forces act.

stress–strain curve:

string duality:

string theory:

structural load:

subatomic particle: Any particle that is smaller than an.

sublimation: The physical process by which matter is transformed directly from the solid phase to the gas phase without passing through an intermediate liquid phase. Sublimation is an phase transition that occurs at temperatures and pressures below a substance's in its phase diagram.

superconductivity:

superconductor: A phenomenon of exactly zero and expulsion of occurring in certain materials when cooled below a characteristic critical temperature.

superhard material:

superposition principle:

supersymmetry (SUSY):

surface tension:

T
temperature: A physical property of that quantitatively expresses the common notions of hot and cold.

tensile modulus:

tensile strength:

tesla (T):

test particle:

theoretical physics: A branch of physics that employs mathematical models and abstractions of physical objects and systems in order to rationalize, explain, and predict natural phenomena, as opposed to, which relies on data generated by experimental observations.

theory of everything (ToE):

theory of relativity:

thermal conduction:

thermal equilibrium: A state in which there is no net flow of between two physical systems when the systems are connected by a path permeable to heat. A system may also be said to be in thermal equilibrium with itself if the within the system is spatially and temporally uniform. Systems in are always in thermal equilibrium, but the converse is not always true.

thermal radiation:

thermionic emission:

thermodynamic equilibrium:

thermodynamic free energy:

thermodynamics:

thermometer: An instrument used to measure.

third law of thermodynamics:

threshold frequency:

torque:

The tendency of a to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object.

total internal reflection:

toughness: The ability of a material to absorb and plastically without fracturing. Material toughness is defined as the amount of energy per unit volume that a material can absorb before rupturing. It is also defined as the resistance to fracture of a material when.

trajectory: The path that a moving object follows through as a function of.

transducer:

transmission medium:

transverse wave:

trigonometry: A branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides.

trimean:

triple point: The and at which the three phases (gas, liquid, and solid) of a given substance coexist in.

truncated mean:

U
Unbalanced forces: When there is unbalanced force(s); and as such, the object changes its state of motion. The object is not at equilibrium and subsequently accelerates.

uncertainty principle: Any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a, such as position x and momentum p, cannot be known simultaneously.

unified atomic mass unit: One dalton: one-twelfth the mass of an isolated neutral atom of the in its ground state.

uniform motion:

uniform circular motion:

unit vector:

utility frequency: The of the oscillations of (AC) in an electric power grid transmitted from a power plant to the end-user.

V
vacuum: An area of which contains no.

valence electron: An that is associated with an and can participate in the formation of a chemical bond.

valence shell: The outermost electron shell of an.

valley of stability:

Van de Graaff generator:

variable capacitor:

variable resistor:

vector: Any quantity that has both magnitude and direction.

vector space: A mathematical structure formed by a collection of elements called, which may be added together and multiplied ("scaled") by numbers called.

velocity ($v$): A quantity defined as the rate of change of the position of an object with respect to a given. Velocity specifies both an object's and direction of  (e.g. 60 kilometres per hour to the north).

virtual image:

virtual particle:

viscoelasticity:

viscosity:

visible light: A form of generally defined as the range of visible to the average human eye.

volt (V): The derived unit for,, and , defined as the difference in electric potential between two points of a wire when an  of one  dissipates one  of  between those two points.

Volta potential:

voltage:

voltmeter: An instrument used for measuring the difference in between two points in an. Analog voltmeters move a pointer across a scale in proportion to the of the circuit.

volt per metre:

volume:

W
W and Z bosons:

watt (W): A derived unit of in the International System of Units (SI) defined as one joule per second. The watt measures the rate of energy conversion or transfer.

wave: A disturbance or oscillation that travels through accompanied by a transfer of.

wave equation:

wave function:

wave function collapse:

wave–particle duality:

wavelength: A measure of the distance traversed by a single spatial period of a sinusoidal, i.e. the distance over which the wave's shape repeats.

weak interaction:

One of the four fundamental forces of nature, along with the,, and. It is responsible for the of and initiates the process known as  in stars.

weber (Wb):

wedge: A triangular round tool in the form of a compound and portable inclined plane; one of six classical.

weight:

wheel and axle: A wheel attached to an axle in such a way that the two parts rotate together and transfer forces between them; one of six classical.

white body: A hypothetical idealized physical body that reflects all incident completely and uniformly in all directions; the opposite of a.

wind: The flow of gases on a large scale.

work:

work function:

X
X-ray: A high-energy (between 100 and 100 keV) with a wavelength shorter than that of ultraviolet radiation and longer than that of gamma radiation.

Y
Young’s modulus: A measure of the of a solid material which defines the relationship between mechanical and.

Z
Zeeman effect: The effect of splitting a spectral line into several components in the presence of a static magnetic field by the lifting of degeneracy in electronic states.