Category: Physics Dictionary

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

convection : The transfer of heat by the actual transfer of matter.

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

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

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

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.

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.

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

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

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.

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.

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

Centigrade : See Celsius scale.

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

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

Celsius scale: Also centigrade scale.

A scale and unit of measurement of temperature.

Cation: A positively charged ion. Contrast anion.

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.

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.

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

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

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

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.

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

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.

Brownian motion: Also called pedesis.

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

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.

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

Brewster’s angle: Also called the polarization angle.

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

Bremsstrahlung : Radiation emitted by the acceleration of unbound charged particles.

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.

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

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.

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

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.

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.

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.

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.

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

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.

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

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

beta decay: Also β-decay.

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

Bessel function : A canonical solution y(x) of Friedrich 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 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.

Bernoulli’s principle : In fluid dynamics, a principle which states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.^{[20]: Ch.3 [21]: 156–164, § 3.5}

bending moment : The reaction induced in a structural element when an external force or moment is applied to the element, causing the element to bend.^[18][19] The simplest structural element subjected to bending moments is the beam.

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.

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.

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

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

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

Balmer series: Also Balmer lines.

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

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

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.

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.

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 number : The total number of individual molecules in one mole of a substance, by definition equaling exactly 6.02214076×10²³.

Avogadro constant : The ratio of the number of constituent particles in a substance, usually atoms or molecules, to the amount of substance, of which the SI unit is the mole. It is defined as exactly 6.02214076×10²³ mol⁻¹.

Atomic weight (A): The sum total of protons (or electrons) and neutrons within an atom.

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

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.

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.

Archimedes’ principle: A physical principle which 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.

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.

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 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 radioactive decay. 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 comprising all matter with which humans usually interact), and the other (usually given the prefix “anti-“) as antimatter.

Antineutron: 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.

Anti-gravity: 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.

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.

Annihilation: In particle physics, 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] 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]

Anion : A negatively charged ion. Contrast cation.

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.

Angular momentum: Also (rarely) moment of momentum or rotational momentum.

The rotational equivalent of linear momentum. 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 frequency (ω)

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

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

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

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

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

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

Amplifier : Also electronic amplifier or (informally) amp.

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

Ampere (A)Often abbreviated as amp.

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

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

Ammeter : An instrument that is used to measure electric current.

Alternating current (AC) : A form of electric current in which the movement of electric charge periodically reverses direction. Contrast direct current.

Alpha decay : Also symbolized by α²⁺, He²⁺, and ⁴₂He²⁺
A type of subatomic particle consisting of two protons and two neutrons bound together into a particle identical to the nucleus of a helium-4 ion. It has a charge of +2 e and a mass of 4 u. Alpha particles are classically produced in the process of radioactive alpha decay, but may also be produced in other ways and given the same name.

Alpha decay : Also α-decay.

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

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

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

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.

Air mass: 1.  In meteorology, a volume of air that is defined by its temperature and water vapor content. Air masses may cover many hundreds or thousands of square miles and generally adapt to the characteristics of the surface below them. They are often classified according to their latitude and their source regions.2.  In astronomy, the “amount of air that one is looking through”^[8] when observing a star or other celestial source from a vantage point that is within Earth’s atmosphere. It is formulated as the integral of air density along the light ray.

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

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

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

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

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

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

Acceleration due to gravity: The acceleration on an object caused by the force of gravitation.

Acceleration : The rate at which the velocity of a body changes with time.

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

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

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.