Glossary of Physics

What's inside

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 magnitude

Is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.

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

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

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

Refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, 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, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

accelerating expansion of the universe

Is 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]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-NYT-20170220-1)[[2]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-AM-20170818-2)[[3]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-ES-20180311-3)[[4]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-4)

acceleration

The rate at which the velocity 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

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

adiabatic process

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]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Carathéodory-5)[[6]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Bailyn_21-6) 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

Is 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

In optics an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length.[[7]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-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 (d = f1+f2).

air mass (astronomy)

In astronomy, air mass or airmass is the "amount of air that one is looking through"[[8]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-8) when seeing a star or other celestial source below Earth's atmosphere. It is formulated as the integral of air density along the light ray.

air mass (meteorology)

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of miles and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass' characteristics, as well as modification.

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

Or α-decay, is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) 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. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. It has a charge of +2 *e* and a mass of 4 u.

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. 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 geometrical shape. Or its non-crystalline solid.

ampere (A)

Often abbreviated as amp.

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

amplifier

An amplifier, electronic amplifier or (informally) amp is 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]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Crecraft-9)[[10]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Agarwal-10)[[11]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Glisson-11)

amplitude

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

angle of incidence

In geometric optics, the angle of incidence is 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

Reflection is 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 meters (one ten-billionth of a meter) or 0.1 nanometers.

angular acceleration

Is 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]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-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

Angular displacement of a body is the angle in radians (degrees, revolutions) through which a point revolves around a centre or line has been rotated in a specified sense about a specified axis.

angular frequency

Angular frequency ω (also referred to by the terms angular speed, radial frequency, circular frequency, orbital frequency, radian frequency, and pulsatance) is 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 [angular velocity](https://en.wikipedia.org/wiki/Angular_velocity). The term angular frequency vector

ω
→

{\displaystyle {\vec {\omega }}}

is sometimes used as a synonym for the vector quantity angular velocity.[[13]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-UP1-13)

One revolution is equal to 2π radians, hence[[13]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-UP1-13)[[14]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-14)

ω
=

2
π

T

=

2
π
f

,

{\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*).

angular momentum

(Rarely, moment of momentum or rotational momentum) is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a closed system remains constant.

angular velocity

Refers to 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 symbol omega (ω, sometimes Ω). By convention, positive angular velocity indicates counter-clockwise rotation, while negative is clockwise.

anion

A negatively charged ion. Contrast cation.

annihilation

In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons.[[15]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-15) The total energy and momentum 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.[[16]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Particleadventure.org-16)

anode

The electrode through which a conventional electric current flows into a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of electron 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 electrical circuit. However, in an electrolytic cell, the anode is the wire or plate having excess positive charge, so named because negatively charged anions tend to move towards it. Contrast cathode.

anti-gravity

(Also known as non-gravitational field) is a theory of creating a place or object that is free from the force of gravity. 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.

antineutron

Is the antiparticle of the neutron with symbol
n
. It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, 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 quarks. The antineutron consists of one up antiquark and two down antiquarks.

antiparticle

In particle physics, every type of particle has an associated antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the antielectron (which is often referred to as positron). While the electron has a negative electric charge, the positron has a positive electric charge, and is produced naturally in certain types of radioactive decay. The opposite is also true: the antiparticle of the positron is the electron. Some particles, such as the photon, are their own antiparticle. Otherwise, for each pair of antiparticle partners, one is designated as normal matter (the kind all matter usually interacted with is made of), and the other (usually given the prefix "anti-") as [antimatter](https://en.wikipedia.org/wiki/Antimatter).

antiquark

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

Archimedes' principle

States that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.[[17]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-khanacademy.org-17)

astronomical unit

(Symbol: au,[[18]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-IAUresB2-18)[[19]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-mnras_style-19)[[20]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-20) ua,[[21]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-21) or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149,597,870,700 metres, or about 150 million kilometres (93 million miles).[[22]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-22) The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

astrophysics

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

atom

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

atomic mass unit

One-twelfth the mass of an atom of the isotope 12
6C.

atomic number (Z)

The number of protons found in the nucleus of an atom. It is most often used to classify elements within the periodic table.

atomic physics

A branch of physics that studies atoms as isolated systems of electrons and an atomic nucleus. Compare nuclear physics.

atomic weight (A)

The sum total of protons (or electrons) and neutrons within an atom.

Avogadro constant

(6.02214086 × 1023 mol−1) In chemistry and physics, the Avogadro constant (named after the scientist Amedeo Avogadro) is the number of constituent particles, usually atoms or molecules, that are contained in the amount of substance given by one mole.

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.

Avogadro's number

The number of molecules in exactly 12g of carbon-12, equaling 6.022 x 1023.

axion

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

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 ionising radiation to which the general population is exposed.

Balmer series

Or Balmer lines, in atomic physics, is 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, made of (usually) three quarks. Nearly all matter you 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

Is the reaction induced in a structural element when an external force or moment is applied to the element causing the element to bend.[[23]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-23)[[24]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-24) The most common or simplest structural element subjected to bending moments is the beam.

Bernoulli's principle

In fluid dynamics, Bernoulli's principle 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.[[25]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Clancy1975-25)(Ch.3)[[26]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Batchelor2000-26)(§ 3.5)

Bessel function

Bessel functions, first defined by the mathematician Daniel Bernoulli and then generalized by Friedrich Bessel, are canonical solutions y(x) of Bessel's differential equation

x

2

d

2

y

d

x

2

+
x

d
y

d
x

+

(

x

2

−

α

2

)

y
=
0

{\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](https://en.wikipedia.org/wiki/Cylindrical_harmonics) because they appear in the solution to Laplace's equation in cylindrical coordinates. [Spherical Bessel functions](https://en.wikipedia.org/wiki/Spherical_Bessel_functions) with half-integer α are obtained when the Helmholtz equation is solved in spherical coordinates.

beta decay

In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) 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.

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.

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

Describes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a 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.

boson

A type of 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

The volume of a given mass of a gas at constant temperature is inversely proportional to the pressure.

bremsstrahlung

Radiation emitted by the acceleration of unbound charged particles.

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 1055 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 the fast-moving atoms or molecules in the gas or liquid.

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.

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

Is the ratio of the change in an electric charge in 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

A capacitor consists of two conductors separated by an insulator, also known as a dielectric.
Capacitive reactance is an opposition to the change of voltage across an element. Capacitive reactance

X

C

{\displaystyle \scriptstyle {X_{C}}}

is inversely proportional to the signal frequency

f

{\displaystyle \scriptstyle {f}}

(or angular frequency ω) and the capacitance

C

{\displaystyle \scriptstyle {C}}

.[[27]](https://en.wikipedia.org/wiki/Glossary_of_physics#cite_note-Irwin-27)

Carnot cycle

Is a theoretical ideal thermodynamic cycle proposed by French physicist 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

Is 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 its [origin](https://en.wikipedia.org/wiki/Origin_(mathematics%29), 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.

cation

A positively charged ion. Contrast anion.

Celsius scale

A scale and unit of measurement of temperature, also known as Centigrade.

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 the gravity acts downward as a parallel force field, the center of gravity and the center of mass are the same.

center of mass

A distribution of mass in space is the unique point where the weighted relative position of the distributed mass sums to zero.

centigrade

A scale and unit of measurement of temperature, also known as Celsius.

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.

chain reaction

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

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.

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.

cohesion

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

condensed matter physics

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

convection

The transfer of heat by the actual transfer of matter.

coulomb (C)

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

crest

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

critical mass

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

cyclotron

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

DC motor

A mechanically commutated electric motor powered by direct current.

deflection

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

deformation

1. (mechanics)
2. (engineering)

density

Also called mass density.

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

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 fluid displaced can then be measured, and from this the volume of the immersed object can be deduced (the volume of the immersed object will be exactly equal to the volume of the displaced fluid).

displacement (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.

Doppler effect

The change in frequency of a wave (or other periodic event) for an observer moving relative to its source.The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is 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 friction, which is nearly independent of velocity, drag forces depend on velocity.

ductility

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

dynamics

The branch of classical mechanics that studies forces and torques and their effects on motion, as opposed to kinematics, which studies motion without reference to these forces.

elasticity

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

electric charge

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

electric circuit

An electrical network consisting of a closed loop, giving a return path for the current.

electric current

A flow of electric charge through a conductive medium.

electric field

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

electric power

The rate at which electric energy is transferred by an electric circuit.

electrical conductor

Any material which contains movable electric charges and therefore can conduct an electric current under the influence of an electric field.

electrical insulator

Any material whose internal electric charges do not flow freely and which therefore does not conduct an electric current under the influence of an electric field.

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 electric current through an electrical element.

electricity

The set of physical phenomena associated with the presence and flow of electric charge.

electromagnet

A type of magnet in which the magnetic field is produced by the flow of electric current.

electromagnetic field

Also abbreviated EM field or EMF.

A physical field produced by moving electrically charged objects.

electromagnetic radiation

Also abbreviated EM radiation or EMR.

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

electromotive force (

E

{\displaystyle {\mathcal {E}}}

)

Also abbreviated emf.

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

electron

A subatomic particle with a negative elementary electric charge.

electron paramagnetic resonance

Also called electron spin resonance (ESR) and electron magnetic resonance (EMR).

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

electronvolt (eV)

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

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 electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies.

energy

The ability to do work.

endothermic

An adjective used to refer to a process or reaction in which a system absorbs energy from its surroundings, usually in the form of heat but also in the form of light, electricity, or sound. Contrast exothermic.

entropy

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

escape velocity

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

exothermic

An adjective used to refer to a process or reaction that releases energy from a system, usually in the form of heat but also in the form of light, electricity, or sound. Contrast endothermic.

falling bodies

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

fermion

A type of particle that behaves according to Fermi–Dirac statistics, obeys the Pauli exclusion principle, and possesses half-integer spin. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these (such as all baryons and many atoms and nuclei). Fermions constitute one of two main classes of particles, the other being bosons.

FIRST

An organization founded by inventor Dean Kamen in 1989 in order to develop ways to inspire students in engineering and technology fields.

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 a, relatively, very large amount of energy.

force (F)

Any interaction that, when unopposed, will change the motion of a physical body. A force has both magnitude and direction, making it a vector quantity. The SI unit used to measure force is the newton.

free fall

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

freezing point

The temperature at which a substance changes state from liquid to solid.

fundamental forces

Also called fundamental interactions.

fusion

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

gamma ray

A form of electromagnetic radiation of high frequency and therefore high energy.

gravitation

Also called gravity.

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

gravitational constant (G)

Also called the universal gravitational constant and Newton's constant.

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

gravitational energy

The potential energy associated with the gravitational field.

gravitational field

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

gravitational potential

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

gravitational wave

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

gravity

See gravitation.

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 radioactive decay, but may be refer to any quantity which follows an exponential decay.

heat

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

hertz

The SI unit of frequency defined as the number of cycles per second of a periodic phenomenon.

homeokinetics

The physics of complex, self-organizing systems.

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 current when a voltage is applied.

inertia

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

International System of Units

The International System of Units (abbreviated SI) is the modern form of the metric system. It comprises a system of units of measurement devised around seven base units and the convenience of the number ten.

ion

An atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electric charge.

ionic bond

A type of chemical bond formed through an electrostatic attraction between two oppositely charged ions.

ionization

The process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.

isotope

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

joule

A derived unit of energy, work, or amount of heat in the International System of Units.

Kelvin

A scale and unit of measurement of temperature. The Kelvin scale is an absolute thermodynamic temperature scale which uses absolute zero as its null point.

kinematics

The branch of classical mechanics that describes the motion 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 energy that a physical body possesses due to its motion, defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. The body continues to maintain this kinetic energy unless its velocity changes. Contrast potential energy.

Kirchhoff's circuit laws

Two approximate equalities that deal with the current and voltage in electrical circuits.also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).

Kirchhoff's equations

In fluid dynamics, the Kirchhoff equations describe the motion of a rigid body in an ideal fluid.

laminar flow

Also called streamline flow.

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

Laplace–Runge–Lenz vector

Also abbreviated LRL vector.

A vector 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.

lepton

An elementary particle which does not undergo strong interactions but is subject to the Pauli exclusion principle. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons) and neutral leptons (better known as neutrinos).

lever

A type of machine consisting of a beam or rigid rod pivoted at a fixed hinge or fulcrum; one of six classical simple machines.

light

A form of electromagnetic radiation that occupies a certain range of wavelengths within the electromagnetic spectrum. In physics, the term sometimes refers collectively to electromagnetic radiation of any wavelength, in which case light includes gamma rays, X-rays, microwaves, and radio waves, but in common usage "light" more often refers specifically to visible light.

linear actuator

A form of motor that generates a linear movement directly.

linear algebra

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

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 continuum mechanics.

Liouville's theorem

Phase space volume is conserved.

liquid

One of four classical states of matter having a definite volume but no fixed shape.

liquid crystal (LC)

A matter in a state that 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 molecules may be oriented in a crystal-like way.

M-theory

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

Mach number

A dimensionless quantity representing the ratio of the speed of an object moving through a fluid to the local speed of sound.

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 machine. 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 magnetic influence of electric currents 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 vector field.

magnetism

A property of materials that respond to an applied magnetic field.

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