Physics Topics

1.054571596e-34 J s: Dirac′s constant, also known as h bar.
1.0973731568549e7m^-1: Rydberg Constant.
1.38062e-23JK^-1: Boltzmanns constant.
1.60217733e-19 J: Electron Volt.
1.602e-19 C: Electron charge.
1.672e-27kg: Proton mass.
1.759e11Ckg^-1: Electron charge to mass ratio.
101325 Pa: Standard atmospheric pressure.
1836.1526675: Proton to electron mass ratio.
2.9979e8 ms^-1: Velocity of light in vacuo.
3.289841960368e15 Hz: Rydberg Constant multiplied by the speed of light.
376.73 Ohm: Impedance of free space.
4.13566727e-15 eV s: Plancks Constant.
5.67e-8Wm^-2K^-4: Stefan-Boltzmann constant.
6.022e23mol^-1: Avogadro′s number
6.58211889e-16 eV s: Known as h bar or Dirac′s constant.
6.626e-34 Js: Planck′s constant.
6.67e-11 Nm²kg^-2: Gravitational constant.
8.314 JK^-1mol^-1: Molar gas constant.
8.85e-12Fm^-1: Permittivity of free space.
9.109e-31 kg: Electron rest mass.
9.648456e4Cmol^-1: Faraday constant
9.80665 ms^-2: Standard acceleration due to gravity.
Abbe Number: The ratio of refractivity to dispersion in an optical medium.
Ablate: To carry away.
Ablating Material: A material designed to provide thermal protection to a body in a fluid stream through loss of mass.
Absolute Accuracy: A measure of the uncertainty of an instrument reading compared to that of a primary standard traceable to a National standard.
Absolute Altitude: Altitude above the actual surface, either land or water, of a planet or natural satellite.
Absolute Humidity: The total mass of water vapour present in the air per unit volume, generally given in g/m³.
Absolute Motion: Motion relative to a fixed point.
Absolute Pressure: Pressure measured from a starting point of 0 in perfect vacuum.
Absolute Space: Space that is not affected by what occupies it or occurs within it and that provides a standard for distinguishing inertial systems from other frames of reference.
Absolute Temperature: Temperature measured on a scale that sets absolute zero as zero.
Absolute Unit: A unit defined in terms of fundamental quantities.
Absolute Vacuum: A volume which contains no matter.
Absorb: To transform radiant energy into a different form, with a resultant rise in temperature.
Absorption: Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength.
Absorption Band: A dark band in the absorption spectrum of a substance, corresponding to a range of wavelengths for which the substance absorbs more strongly than at adjacent wavelengths.
Absorption Lines: Dark lines in a spectrum that are produced when light or other electromagnetic radiation passes through a gas cloud or similar object closer to the observer.
Absorption Spectroscopy: A technique for determining the concentration and structure of a substance by measuring the amount of electromagnetic radiation the sample absorbs at various wavelengths.
Absorption Spectrum: The collection of wavelengths missing from a continuous distribution of wavelengths.
AC Line: The set of conductors that route ac voltage from one point to another.
AC Line Filter: A circuit filter placed in the ac line to condition or smooth out variations that are higher in frequency than the line frequency.
AC: An electric current whose direction changes direction with a frequency independent of circuit components.
Acausal: If a system is acausal it means the output begins before the input.
Accelerating Electrode: An electrode in an electron tube which is maintained at a positive potential with respect to the cathode and any other electrodes situated between the cathode and the accelerating electrode, thus imparting acceleration to electrons in the direction away from the cathode.
Accelerating Grid: An accelerating electrode in the form of a grid.
Accelerator: A machine used to accelerate particles to high speeds.
Accelerator Valve: A thermionic valve employed as a particle accelerator.
Acceptor: An atom which is likely to take on one or more electrons when placed in a crystal.
Accretion: The process by which the terrestrial planets grew.
Accretion Disc: An accretion disc is a disc-shaped rotating mass formed by gravitational attraction.
Accuracy: Accuracy is the correctness of a single measurement.
Acoustic Microscopy: A general term referring to the use of high resolution, high frequency ultrasonic techniques to produce images of surface or near surface features.
Acoustics: From the Greek akouein ( "to hear") a term sometimes used for the science of sound in general.
Acquisition: The process of locating the orbit of a satellite or trajectory of a space probe so that tracking or telemetry data can be gathered.
Actinic: Pertaining to electromagnetic radiation capable of initiating photochemical reactions.
Actinic Ray: A ray of light of short wavelengths, as ultraviolet or violet, that produces photochemical effects.
Actinometer: An actinometer is an instrument for measuring heat radiation.
Actinometry: The science of measurement of radiant energy.
Action: Refers to an effective motion or mechanism as for example the breech action of a gun.
Active Satellite: A satellite which transmits a signal, in contrast to passive satellite.
Active Sun: The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radio frequency radiation are at a maximum.
Activity: The rate at which a collection of radioactive nuclei decay.
Additive Colour System: An additive colour system is a colour reproduction system in which an image is displayed by mixing appropriate amounts of red, green and blue light, as for example in a cathode-ray tube.
Additive Primaries: Red, green, and blue are the primary colours of light from which all other colours can be made.
Adhesive Force: The attractive force exerted on a liquid molecule by the molecules in the surface of the solid.
Adiabat: A line on an indicator diagram that represents an adiabatic process.
Adiabatic Ionization Energy: The lowest energy required to remove an electron from an atom, ion, or molecule in the gas phase.
Adiabatic Plasma: Part of plasma confinement system which involves particles where the orbit radius and orbit period are small compared to the characteristic scales of length and time.
Advection: The transfer of matter such as water vapour or heat through the atmosphere as a result of horizontal movement of an air mass.
Aeolipile: A round vessel caused to rotate by the force of tangentially escaping steam: an early example of jet propulsion.
Aeolotropy: Aeolotropy is the antithesis of isotropy.
Aerodynamics: Lists all Aerodynamics topics in the Encyclopaedia
Aerolite: A stony meteorite, composed primarily of silicates.
Aerolithology: The science of aerolites - stony meteorites.
Aeronomy: The study of the upper regions of the atmosphere where ionization, dissociation, and chemical reactions take place.
Aerosol: A colloid in which solid particles or liquid droplets are suspended in a gas.
Aether: An invisible medium that was thought to suffuse all space.
After Image: An image seen after the eye′s retina has been exposed for a time to an intense or stationary light source.
Afterheat: The heat generated in a reactor core after shutdown by continuing radioactive decay of fission products.
Agonic Line: A line joining points at which the magnetic variation is zero.
Agravic: Or pertaining to a condition of no gravitation.
Air Capacitor: A capacitor, which uses air as the dielectric between the plates.
Air Equivalent: A measure of the effectiveness of a material in absorbing nuclear radiation, expressed as the thickness of an air layer (at 0°C and 1 atmosphere) causing the same absorption.
Air Spring: A simple mass on an air spring.
Airflow: A flow or stream of air.
Airflow Resistance: The quotient of the air pressure difference across a specimen divided by the volume velocity of airflow through the specimen. The pressure difference and the volume velocity may be either steady or alternating.
Airfoil: A structure designed to obtain a useful reaction on itself in its motion through the air.
Airy Disc: A pattern of illumination caused by diffraction at the edge of a circular aperture.
Albedo: The ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it.
Albedometer: An instrument used for the measurement of the reflecting power, the albedo, of a surface.
Alfven, Hannes: Swedish plasma physicist who won the 1970 Nobel Prize in Physics for his work developing the theory of magnetohydrodynamics.
Alfven Waves: Waves moving perpendicularly through a magnetic field.
Aliasing: The erroneous interpretation of high-frequency signals as lower-frequency signals.
Alidade: That part of an optical measuring instrument comprising the optical system, indicator, vernier, etc.
Allobar: A form of an element that has isotopic abundances that are different from the naturally occuring form.
Allomer: Substances with different chemical composition but the same crystalline form.
Allotropy: The reversible phenomenon by which certain metals may exist in more than one crystal structure.
Alloy: A metal produced by mixing other metals.
Alpha: The first letter of the Greek alphabet.
Alpha Particle: The nucleus of helium consisting of two protons and two neutrons.
Alpha Radiation: A stream of alpha particles. Alpha rays rapidly dissipate their energy as they pass through materials, and are far less penetrating than beta particles and gamma rays.
Alternating Current: An electric current whose direction changes direction with a frequency independent of circuit components.
Altitude: Height in space of an object or point relative to sea level or ground level.
Alvarez, Luis Walter: American physicist who produced free protons with a particle accelerator.
Ambient Environment: The conditions characterizing the air or other medium that surrounds materiel.
Ambient Temperature: The average temperature of the surroundings.
Ammeter: Instrument for measuring the current in amps, milliamps or microamps.
Amor Asteroids: Have orbits that cross the orbit of Mars and approach Earth′s orbit.
Amorphography: The branch of science concerned with the determination of amorphous solid structures and their systemmatic classification.
Amorphous: Non-crystalline, without long-range order.
Amorphous Solar Cell: These cells are not as efficient as the monocrytalline cells but they do perform better in low light levels.
Ampere-Turn: The magnetomotive force developed by 1 ampere of current flowing through a coil of one turn.
Amplitude: The maximum distance from the equilibrium position that occurs in periodic motion.
Analogue: Any representation of a physical quantity in terms of a continuous variation of a second physical quantity.
Analyser: An Analyser is another name for a secondary polariser in an optical system.
Anemometer: A sensor that measures wind speed.
Aneroid Barometer: An instrument used to measure atmospheric pressure consisting of an evacuated container with a flexible wall.
Angle of Incidence: Angle between direction of motion of waves and a line perpendicular to the surface the waves are striking.
Angle of Reflection: Angle between direction of motion of waves and a line perpendicular to surface the waves are reflected from.
Angle of Refraction: The angle at which a refracted ray of energy leaves the interface at which the refraction occurred.
Angstrom: It is an old unit used for measuring the wavelength of electromagnetic radiation including visible light and X-rays.
Angular Momentum Quantum Number: A quantum number that labels the subshells of an atom.
Anisotropic: Unequal physical properties along different axes.
Annihilation: A process in which a particle meets its corresponding antiparticle and both disappear.
Annual Aberration: The component of stellar aberration resulting from the motion of the Earth about the Sun.
Anode: The electrode where electrons are lost (oxidized) in redox reactions.
Anomalous Dispersion: A sudden change in the refractive index of a material for wavelengths in the vicinity of absorption bands in the spectrum of the material.
Anthropic Principle: Principles describing what conditions are necessary for the development of intelligent life.
Anti-Aliasing Filter: A device that attenuates signal content outside the desired bandwidth.
Antibaryon: The antiparticle of a baryon.
Antibonding Molecular Orbital: Also known as Antibonding Orbital.
Antibonding Orbital: A molecular orbital that can be described as the result of destructive interference of atomic orbitals on bonded atoms.
Antielectron: The anti particle of an electron.
Antimatter: Material made from antifermions. We define the fermions that are common in our universe as matter and their antiparticles as antimatter.
Antineutrino: The antiparticle of a neutrino.
Antiparticle: A subatomic particle with the same-size properties as those of the particle although some may have the opposite sign.
Antiquark: The antiparticle of a quark.
Antistatic: Of or pertaining to the ability to either prevent the accumulation of or to enable the dissipation of static charge.
Anyon: An elementary particle or particle-like excitation having properties intermediate between those of bosons and fermions.
Aphelion: The point in a planet's orbit furthest from the Sun.
Apocentre: The point on a spacecraft′s orbit at which it is furthest away from the body it is orbiting.
Apollo Asteroids: Have orbits that cross the Earth′s orbit.
Apostilb: A unit of surface luminance used when defining diffusing surfaces equal to 1 lumenm^-2
Apparent Horizon: Where the shy appears to meet the Earth.
Apparent Power: That power apparently available for use in an ac circuit containing a reactive element. It is the product of effective voltage times effective current expressed in volt-amperes. It must be multiplied by the power factor to obtain true power available.
Arago Spot: A bright spot that appears in the shadow of a uniform disc being backlit by monochromatic light emanating from a point source.
Archimedes: Forever to be known for the Archimedean principle: "a body plunged in a fluid loses as much weight as... "
Archimedes′ Principle: The buoyant force is equal to the weight of the displaced fluid.
Aristotle: A Greek philosopher who stressed the importance of direct observations in securing facts and data.
Arjuna Asteroids: Asteroids that orbit the Sun in a near circular path and are no more than 100m in diameter.
Armature: In a relay, the movable portion of the relay. The windings in which the output voltage is generated in a generator or in which input current creates a magnetic field that interacts with the main field in a motor.
Armature Losses: Copper losses, eddy current losses, and hysteresis losses that act to decrease the efficiency of armatures.
Armature Reaction: The effect in a dc generator of current in the armature creating a magnetic field that distorts the main field and causes a shift in the neutral plane.
Artificial Gravity: Gravity established within a space vehicle by rotation or acceleration.
Artificial Satellite: A man made satellite.
Artificial Transmission Line: An LC network that is designed to simulate characteristics of a transmission line.
Astable: A circuit that has no stable state and thus oscillates at a frequency dependent on component values.
Asterism: A group of stars which are traditionally imagined to represent a pattern.
Asteroids: These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter.
Astigmatism: An aberration, or defect, in a mirror or lens that causes the image of a point to spread out into a line.
Astrolabe: An instrument that was used to determine the altitude of objects in the sky.
Astronomical Unit: Defined as the mean distance of the Earth from the Sun.
Astronomy: The generic name for the study of the universe around us.
Astrophysics: The physics of astronomical objects such as stars and galaxies.
Asynchronous Orbit: One where the satellite does not rotate or move at the same speed as the earth.
Aten Asteroids: Have orbits that lie inside the Earth′s orbit.
Atmometry: The science of measuring the rate at which water evaporates.
Atmosphere: The gases around a planet or star.
Atmospheric Pressure: Atmospheric pressure is the force exerted by the weight of the atmosphere of the Earth at ground level.
Atom: The smallest particle of an element that can exist and still retain the ordinary chemical properties of that element.
Atom Percent: Concentration specification on the basis of the number of moles or atoms of a particular element relative to the total number of moles or atoms of all elements within an alloy.
Atomic Clock: A timekeeping device controlled by the frequency of the natural vibrations of certain atoms.
Atomic Mass Unit: Defined as one twelfth of the mass of the most abundant isotope of carbon (¹²C)
Atomic Nucleus: A tiny, incredibly dense positively charged mass at the heart of the atom - normally abbreviated Nucleus.
Atomic Number: The atomic number (Z) of an element is the number of protons each atom of that element has in its nucleus.
Atomic Orbital: A wavefunction that describes the behavior of an electron in an atom.
Atomic Packing Factor: The fraction of the volume of a unit cell that is occupied by ′hard sphere′ atoms or ions.
Atomic Particle: One of the particles of which an atom is constituted.
Atomic Radius: One half the distance between nuclei of atoms of the same element, when the atoms are bound by a single covalent bond or are in a metallic crystal.
Atomic Theory: An explanation of chemical properties and processes that assumes that tiny particles called atoms are the ultimate building blocks of matter.
Atomic Time Scale: A time scale based on an atomic definition of the second.
Atomic Unit: A system of non-SI units used in quantum chemistry to simplify calculations and mathematical expressions.
Atomic weight: The weighted average mass of the atoms in a naturally occurring element.
Attenuation: The reduction of a physical quantity.
Attenuation of Sound in Air: The attenuation of sound in air at 20�C due to viscous, thermal and rotational loss mechanisms is 1.6E-10f�dB/m.
Atto: Prefix representing a multiplication of 10^-18
Attraction: The force that tends to make two objects approach each other.
Atwood′s Machine: A weight-and-pulley system devised to measure the acceleration due to gravity at the Earth′s surface
AU: Defined as the mean distance of the Earth from the Sun.
Aufbau Principle: An approximate procedure for writing the ground state electronic configuration of atoms.
Aurora: A faint visual phenomenon associated with geomagnetic activity, which occurs mainly in the high-latitude night sky.
Aurora Borealis: Latin for Northern Dawn, an old name for the Aurora.
Auroral Kilometric Radiation: Intense radio waves whose wavelength is of the order of a kilometre, emitted from regions above the ionosphere where the aurora is accelerated.
Auroral Zone: The region on Earth where auroras are common.
Avalanche: A cumulative ionization process.
Average Speed: The distance traveled divided by the time taken.
Averaging: In any process it is often necessary to average a number of measurements to gain any confidence in the measured value.
Avogadro′s Law: Equal volumes of an ideal gas contain equal numbers of molecules, if both volumes are at the same temperature and pressure.
Avogadros Number: N = 6.022045x10²³ mol^-1
Axial Load: A load applied along or parallel to and concentric with the primary axis.
Axis: The line, real or imaginary, passing through the centre of an object about which it could rotate; a point of reference.
Azimuth: In land navigation it is defined as a horizontal angle measured clockwise from a north base line or meridian.
Azimuthal Quantum Number: Also known as Angular Momentum Quantum Number.
Backscattered Electron: Produced by an incident electron colliding with an atom in the specimen which is nearly normal to the incident′s path.
Backscattering: Primary radiation deflected or secondary radiation emitted in the general direction of the incident radiation beam.
Balmer Series: A series of lines in the emission spectrum of hydrogen that involve transitions to the n=2 state from states with n>2.
Band: A set of closely spaced energy levels in an atom, molecule, or metal.
Band Gap Energy: For semiconductors and insulators, the energies that lie between the valence and conduction bands.
Band Spectrum: An emission spectrum that contains groups of sharp peaks that are so close together that they are not distinguishable separately.
Bandgap: The range of energies between existing energy bands where no energy levels exist.
Bandwidth: The bandwidth of a filter is the separation between the lower and upper frequencies at which the amplitude of a sinusoidal signal is attenuated by a factor of 2.
Barometer: A device for measuring atmospheric pressure.
Barometric Pressure: The total pressure exerted by the atmosphere.
Baromil: The unit of length used in graduating a mercury barometer in the centimetre-gram-second system.
Barretter: A type of bolometer characterized by an increase in resistance as the dissipated power rises.
Bartels Rotation Number: The serial number assigned to 27-day recurrence periods of solar and geophysical parameters.
Barycentre: This is the centre of gravity of 2 or more masses.
Baryon: A particle with spin 1/2 (or 3/2) which consists of three elementary particles known as quarks.
Base Terminal: That electrode of a transistor which compares generally to the grid of a vacuum tube.
Base Unit: Base units are units that are fundamental building blocks in a system of measurement.
Battery: A device for converting chemical energy into electrical energy.
Battery Capacity: The amount of energy available from a battery. Battery capacity is expressed in ampere-hours.
bcc: Abbreviation of body centred cubic. About 15% of all the elements crystallize into this structure.
Beam Steering: The method of steering the main lobe of a transducer to a certain direction.
Beam Width: The width of the main beam lobe, in degrees, of the transducer. It is usually defined as the width between the "half power point" or "-3dB" point.
Bearing Resolution: Ability of a radar to distinguish between targets that are close together in bearing.
Beats: Periodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.
Bells′ Inequality: A quantum mechanical theorem which demonstrates that quantum mechanics must have nonlocal properties.
Beta Particle: An electron emitted by a radioactive nucleus.
Bhabda Scattering: Scattering of positrons by electrons.
Big Bang Theory: A theory that suggests that the universe started with a giant explosion.
Bimetallic Strip: Two metals with different thermal expansion coefficients are bonded together and wound into a spiral.
Binary: Pertaining to a characteristic that involves the selection, choice, or condition in which there are only two possibilities.
Binding Energy: The amount of energy required to take a nucleus apart.
Biot-Savart Law: A law describing the magnetic flux density at a distance r away from an electrical conductor.
Black Box: A unit whose output is a specified function of the input, but for which the method of converting input to output is not necessarily specified.
Black Hole: A massive star that has collapsed to such a small size that its gravitational force is so strong that not even light can escape from its 'surface'.
Blackbody: A hypothetical body that absorbs without reflection all of the electromagnetic radiation incident on its surface.
Blackbody Radiation: The electromagnetic radiation emitted by an ideal black body.
Blackett, Patrick Maynard Stuart: English chemist who developed the cloud chamber.
Blanking Distance: Minimum sensing range in an ultrasonic proximity sensor.
Blue Shift: The apparant (Doppler) shift of the wavelength towards the higher frequency region of radiation emitted by an approaching object.
Blue Supergiant: A supergiant star with spectral type O or B.
B-meson: One of the heaviest mesons, containing a bottom quark.
Body Centred Cubic: About 15% of elements crystallize with a body centred cubic structure.
Bohr Atom: A model of the atom that explains emission and absorption of radiation as transitions between stationary electronic states in which the electron orbits the nucleus at a definite distance.
Bohr Magneton: b = 9.27x10^-24 Am²
Bohr, Niels: While at Copenhagen University, Bohr, in 1922, won the Nobel Prize in Physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them."
Bohr Radius: a₀ = 5.291772x10^-11 m
Boiling: The rapid vapourization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure.
Boiling Point: The temperature at which the vapour pressure of a liquid is equal to the external pressure on the liquid.
Boiling Point Elevation: The boiling point of a solution is higher than the boiling point of the pure solvent.
Bolide: A brilliant meteor, especially one which explodes, a detonating fireball.
Bolometer: A device for measuring minute amounts of radiant energy.
Boltzmann′s Constant: Constant named after Stefan Boltzmann. Used in the determination of energy radiation from bodies.
Bond Order: In Lewis structures, the number of electron pairs shared by two atoms.
Bonding Energy: The energy required to separate two atoms that are chemically bonded to each other.
Born, Max: Won the Nobel Prize in Physics for his fundamental research in quantum mechanics.
Bose-Einstein Statistics: Quantum statistics for particles not obeying the exclusion principle.
Bosons: Particles which do not obey the Pauli exclusion principle, for example, photons.
Bottom: The flavor of the fifth quark.
Bottoming: A transistor in the fully conducting state.
Boundary Layer: The portion of a fluid flowing past a body that is in the immediate vicinity of the body.
Bow Shock: The shock wave caused by the edge our Solar System travelling through deep space.
Boyle, Robert: Known for that scientific law named after him.
Boyles Gas Law: For a fixed mass of gas at constant temperature the product of pressure and volume is constant.
bp: Abbreviation of boiling point, the temperature at which the vapour pressure of a liquid is equal to the external pressure on the liquid.
Brackett Series: The series which describes the emission spectrum of hydrogen when the electron is jumping to the fourth orbital.
Bragg′s Law: A relationship that stipulates the condition for diffraction by a set of crystallographic planes.
Breaking Stress: Also known as the ultimate tensile strength. This is the maximum stress that can be applied to a material.
Bremsstrahlung: X-rays emitted when a charged particle (such as an electron) is decelerated by passing through matter.
Bremsstrahlung Effect: The emission of electromagnetic radiation as a consequence of the acceleration of charged elementary particles.
Brewsters Law: Unpolarised light can be polarised by reflection. When unpolarised light is reflected from the surface of an optical material (glass, say) then preferential reflection occurs for the electric-field vector that is perpendicular to plane of incidence.
Bright Field Imaging: A imaging mode in a TEM that uses only Unscattered Electrons to form the image.
Brittleness: The tendency of a material to fracture without first undergoing significant plastic deformation.
Broadside Array: An array in which the direction of maximum radiation is perpendicular to the plane containing the elements.
Broglie, Louis-Victor de: He demonstrated mathematically that electrons and other subatomic particles exhibit wavelike properties.
Brown Dwarf: These straddle the domain between stars and giant planets.
Brown, Robert: Discovered the brownian movement of minute particles.
Brownian Motion: Small particles suspended in liquid move spontaneously in a random fashion.
Bubble Chamber: A chamber filled with liquid at low pressure chosen so that small bubbles form along the path of any charged particle.
Bulk Modulus: The bulk modulus of a gas is a measure of its compressibility (elastic property).
Buoyant Force: The upward force exerted by a fluid on a submerged or floating object.
Burgess Vector: A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation.
Bushel: A dry or liquid measure equal to eight gallons.
c: Prefix representing a multiplication of 10^-2
Caloric: A postulated elastic fluid associated with heat.
Calorimeter: An insulated vessel for measuring the amount of heat absorbed or released by a chemical or physical change.
Calorimetry: The science of measuring heat flow.
Calutron: A device that separates isotopes by ionizing the sample.
Camera Obscura: A room with a small hole in one wall used by artists to produce images.
Capacitance: The capacitance is defined as the total electric charge on a body divided by its potential.
Capacitive Reactance: The opposition, expressed in ohms, offered to the flow of an alternating current by capacitance.
Capacitor: An electrical component that passes alternating currents but blocks direct currents.
Capillary Action: A phenomenon whereby the narrower the tube the higher the liquid will climb above it's normal bulk level in the container.
Carat Gold: Measure of parts of gold per 24 parts of an alloy and equal to 41.667 milligrams of gold per gram of alloy.
Carat Precious Stones: A standard measure of weight for precious stones.
Carbon-Nitrogen-Oxygen Cycle: In stars more massive than the sun this cycle is the primary process which converts hydrogen into helium.
Carrier Particle: A fundamental boson associated with quantum excitations of the force field corresponding to some interaction.
Carrington Longitude: A system of fixed longitudes rotating with the sun.
Casimir Effect: A quantum mechanical effect, where two very large plates placed close to each other will experience an attractive force, in the absence of other forces.
Cathode: Electrode where electrons are gained (reduction) in redox reactions.
Cathode Ray: An electron emitted from the negative electrode in an evacuated tube.
Cathode Ray Tube: An evacuated tube containing an anode and a cathode that generates cathode rays when operated at a high voltage.
Causality Principle: The principle that cause must always preceed effect. More formally, if an event A ("the cause") somehow influences an event B ("the effect") which occurs later in time, then event B cannot in turn have an influence on event A.
Cavendish, Henry: English physicist and chemist, discovered hydrogen.
Cavitation: A condition which can occur in liquid handling machinery where a system pressure decrease in the suction line and pump inlet lowers fluid pressure and vaporization occurs.
Cavity Resonator: A space totally enclosed by a metallic conductor and supplied with energy in such a way that it becomes a source of electromagnetic oscillations.
Cavity Wavemeter: An instrument used to measure microwave frequencies.
centi: Prefix representing a multiplication of 10^-2
Centre: A point or axis around which anything revolves or rotates.
Centre European pour Rechearche Nucleaire: The major European International Accelerator Laboratory located near Geneva, Switzerland.
Centre Feed Method: Connecting the center of an antenna to a transmission line which is then connected to the output stage of the transmitter.
Centre Frequency: Frequency to which an amplifier is tuned. The frequency half way between the cut-off frequencies of a tuned circuit.
Centre of Mass: The balance point of an object. The location in an object that has the same translational motion as the object if it were shrunk to a point.
Centrifugal Force: A fictitious force arising in a rotating reference system.
Centripetal: An adjective meaning 'centre-fleeing.'
Cepheid Variable: A type of variable star which exhibits a regular pattern of changing brightness as a function of time.
Cerenkov Radiation: A charged particle emits Cerenkov radiation in a cone around its direction of travel when it travels through any medium faster than the speed of light through that medium.
CERN: The major European International Accelerator Laboratory located near Geneva, Switzerland.
CGS Units: Abbreviation for centimetre-gramme-second.
Chadwick, James: English physicist who discovered the neutron.
Chain: The chain is a unit of length.
Chain Reaction: A process in which the fissioning of one nucleus initiates the fissioning of others.
Chandrasekhar Limit: A limit which mandates that no white dwarf can be more massive than about 1.4 solar masses.
Chandrasekhar, Subrahmanyan: Indian astrophysicist reknowned for creating theoretical models of white dwarf stars, among other achievements.
Change of State: The change in a substance between solid and liquid or between liquid and gas.
Characteristic X-ray: An X-ray having a unique energy that is emitted by an atom during its de-energization after ionization of one of its electrons.
Charge: The excess or deficiency of electrons resulting in the body having negative or positive charge.
Charge Conjugation: The technical term for mathematical operations which interchange particles and antiparticles.
Charged: Possessing a net negative or positive charge.
Charged Particle Beam: A projected stream of charged particles.
Charged Particles: Particles with positive or negative charge, for example electrons, protons or ions.
Charles Gas Law: For a fixed mass of gas at constant pressure the volume is directly proportional to the temperature.
Charm: The flavor of the fourth quark.
Charmed: Having a nonzero value of charm.
Charmed Quark: The quark having electric charge 2/3 times the elementary charge and charm C = +1. It is more massive than the up, down, and strange quarks.
Charmonium: Any meson composed of a charmed quark and a charmed antiquark.
Charon: Pluto′s double.
Chemical Differentiation: The separation of different elements, often heavier elements from lighter elements, as a consequence of different chemical reactions.
Chemical Energy: Energy stored in the bonds of atoms and molecules eg coal, petrol, biomass.
chi: A letter of the Greek alphabet.
Chirality: Handedness, the quality of having non-superimposable mirror images.
Choke Joint: A joint between two sections of waveguide that provides a good electrical connection without power losses or reflections.
Chromatic Aberration: A defect in lenses that causes different colors (wavelengths) of light to have different focal lengths.
Chromophore: A group or substructure on a molecule that is responsible for the absorption of light.
Chromosphere: The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona.
Chronology Protection Conjecture: The concept that the formation of any closed timelike curve will automatically be destroyed by quantum fluctuations as soon as it is formed.
Ci: Abbreviation of curie, a unit of radioactivity.
Circuit: Interconnection of components to provide an electrical path between two or more components.
Clark Cell: A voltaic cell that is used as a reference emf.
Classical Electron Radius: A classical distribution of charge totalling the electron′s charge has electrostatic potential energy equivalent to the electron′s rest mass if it were confined to a volume of this radius.
Classical Mechanics: Lists all Classical Mechanics topics in the Encyclopaedia
Classical Physics: Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal.
Clerk-Maxwell, James: Clerk-Maxwell′s greatest work was his initial contribution to electromagnetic radiation.
Clock: A device that generates periodic, accurately spaced signals used for timing applications.
Closed Shell: A shell that contains the maximum number of electrons permitted by the exclusion principle.
Closed System: A system which can exchange only energy with its surroundings.
CNO Cycle: Abbreviation for Carbon-Oxygen-Nitrogen Cycle. In stars more massive than the sun this cycle is the primary process which converts hydrogen into helium.
Coanda Effect: The effect that indicates that a fluid tends to flow along a surface, rather than flow through free space.
Coaxial Line: A type of transmission line that contains two concentric conductors.
Coefficient: A coefficient is a constant multiplicative factor of a certain object.
Coefficient of Coupling: An expression of the extent to which two inductors are coupled by magnetic lines of force.
Coercive Force: Magnetizing force needed to reduce residual magnetism in a material to zero.
Cohesive Force: The attractive force exerted on a liquid molecule by the neighbouring liquid molecules.
Cold Junction: The reference junction of a thermocouple which is kept at a constant temperature.
Cold Plasma Model: A model of a plasma in which the temperature is neglected.
Colebrook Equation: Used to calculate the friction coefficients in different kinds of fluid flows - air ventilation ducts, pipes and tubes with water or oil, compressed air and much more.
Collector: One terminal of a transistor.
Collider: An accelerator in which two beams traveling in opposite directions are steered together
Colligative Properties: Properties of a solution that depend only on the number of particles dissolved in it, not the properties of the particles themselves.
Collimation: The alignment of the direction of the photons, so the beam of radiation can be directed at a well-defined part of a target material.
Collimator: A mechanical device installed along the trajectory of a beam to reduce the size of the beam.
Collision: An encounter between two ojects that changes their existing momentum and energy conditions.
Collision Frequency: The average number of collisions that a molecule undergoes each second.
Colloid: A heterogeneous mixture composed of tiny particles suspended in another material.
Colour: A quality of light, depending on its wavelength. Spectral colour of an emission of light is its place in the rainbow spectrum.
Colour: Property of quarks associated with their binding with gluons.
Columbium: A name sometimes given to Niobium, a platinum-gray, ductile metal with brilliant luster that is used in alloys, especially stainless steels.
Coma: A spherical cloud of material surrounding the head of a comet.
Comet: A chunk of frozen gasses, ice, and rocky debris that orbits the Sun.
Common Base Connection: Same as ground base connection. A mode of operation of a transistor in which the base is common to both the input and output circuits and is usually earthed.
Common Collector Connection: A mode of operation of a transistor in which the collector is common to both the input and the output circuits and is usually connected to one of the power rails.
Common Emitter Connection: A mode of operation for a transistor in which the emitter is common to the input and output circuits. The base is the input terminal and the collector is the output terminal.
Commutator: An electrical switch that periodically reverses the current direction in an electric motor or electrical generator.
Compass: The earliest-known compass dates from China, during the Han Dynasty (2nd century BC - 2nd century AD).
Compensation Windings: Windings embedded in slots in pole pieces, connected in series with the armature, whose magnetic field opposes the armature field and cancels armature reaction.
Complementarity Principle: The principle that a given system cannot exhibit both wave-like behavior and particle-like behavior at the same time.
Complementary Colours: For lights, two colours that combine to form white.
Complementary Transistor: A PNP and NPN pair used in a push-pull circuit.
Complimentary Metal Oxide Semiconductor: Family of logic devices that uses p-type and n-type channel devices on the same IC. It has the advantage of offering medium speed and very low power requirements.
Compressibility: The change in volume of a unit volume of a fluid when subjected to a unit change of pressure.
Compressible Fluids: Compressible flow requires the integration of the equations of conservation of mass and momentum with that of energy conservation.
Compression: A pressing force that squeezes a material together.
Compressional Wave: Wave in an elastic medium which causes an element of the medium to change its volume without undergoing rotation.
Compton Effect: An effect that demonstrates that photons have momentum.
Compton Wavelength: The Compton Wavelength is the wavelength associated with the mass of any particle.
Condensation: The process by which vapour molecules reform a liquid.
Conductance: The electrical conductance of a conductor is the reciprocal of the resistance of the conductor.
Conduction Band: Lowest empty or partially filled band in a semiconductor.
Conductivity: The ease with which a substance transmits electricity.
Conductor: A material that allows the passage of electric charge or the easy transfer of thermal energy.
Confinement: The containment of plasma particles and energy within a container for some extended period of time.
Conical Pendulum: The motion of the wire from which the bob hangs describes the surface of a cone.
Conjecture: A statement that may seem to be true, but has yet to be proven.
Conservation Laws: A law which states that, in a closed system, the total quantity of something will not increase or decrease, but remain exactly the same; that is, its rate of change is zero.
Conservation of Charge: In an isolated system the total charge is conserved.
Conservation of Energy: The total energy of an isolated system does not change.
Conservation of Mass: The total mass in a closed system does not change even when physical and chemical changes occur.
Conserved: This term is used in physics to mean that a number associated with a physical property does not change; it is invariant.
Constancy Principle: One of the postulates of Einstein′s special theory of relativity, which puts forth that the speed of light in vacuum is measured as the same speed to all observers, regardless of their relative motion.
Constellation: A named grouping of fixed stars.
Constructive Interference: When the peaks and troughs of two interfering waves match, the amplitudes add to give the resultant wave a higher amplitude.
Contact EMF: Electromotive force which arises at the contact of dissimilar metals at the same temperature, or the same metal at different temperatures.
Contact Potential: The voltage generated by the contact of two dissimilar metals or materials.
Continuity Equation: An equation which states that a fluid flowing through a pipe flows at a rate which is inversely proportional to the cross-sectional area of the pipe.
Continuous Spectrum: A plot of the relative absorbance or intensity of emitted light vs. wavelength or frequency that shows a smooth variation, rather than a series of sharp peaks or bands.
Convection: The heat is transfered from the solid or liquid by the surrounding gas.
Convergent Beam Electron Diffraction: An electron probe is tightly focused on a specimen and the resulting pattern of diffracted electrons is observed.
Coordinated Universal Time: By international agreement, the local time at the prime meridian, which passes through Greenwich, England.
Copernican Principle: The idea, suggested by Copernicus, that the Sun, not the Earth, is at the centre of the Universe.
Copernican System: A theory of planetary motions, proposed by Copernicus, according to which all planets move in circular orbits around the Sun.
Copernicus, Nicolaus: Polish astronomer who advanced the theory that the Earth and other planets revolve around the Sun.
Copper Loss: The power lost because of the resistance of the conductors.
Cord: An imperial unit of volume used mainly in the timber industry.
Core Electron: Electrons occupying completely filled shells under the valence shell.
Coriolis Force: A fictitious force that occurs in rotating reference frames.
Corona: The outermost layer of the Sun′s atmosphere.
Corona Extinction Voltage: The highest voltage at which continuous corona of specified pulse amplitude no longer occurs as the applied voltage is gradually decreased from above the corona inception value.
Corona Inception Voltage: The lowest voltage at which continuous corona of specified pulse amplitude occurs as the applied voltage is gradually increased.
Coronal Mass Ejection: A huge cloud of hot plasma, occasionally expelled from the Sun.
Correlation: Generally speaking, a similarity between data; the extent to which data are related.
Correspondence Principle: The principle that when a new, more general theory is put forth, it must reduce to the more specialized theory under normal circumstances.
Cosmic Background Radiation: Believed to be the cosmologically redshifted radiation released by the Big Bang itself.
Cosmic Ray: An extremely energetic (relativistic) charged particle.
Cosmological Redshift: An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself.
Cosmology: The study of the history of the universe.
Cosmos: The universe regarded as an orderly, harmonious whole.
Coulombic Force: A force between charged particles, such as ions.
Coulometer: Instrument used for the measurement of electrical charge.
Couple: Two equal forces acting on a body in opposite directions and located at a specific distance apart produce a turning effect on the body.
Covalent Bond: Describes the mutual electrostatic attraction of two or more adjacent nuclei for a shared pair of electrons which occupy the same molecular energy level.
Covalent Radius: The radius of atoms obtained from covalent bond lengths.
C_P: Abbreviation for pressure coefficient, a dimensionless value which acts as a means of indicating the local pressure at some point of interest around a body, and which is independent of velocity.
Creep: The time-dependent permanent deformation that occurs under stress.
Crest: The peak of a wave disturbance.
Crest Factor: Peak value divided by the R.M.S. value, used as a measure of the severity of a segment of data.
Critical Angle: The minimum angle of incidence for which total internal reflection occurs.
Critical Chain Reaction: A chain reaction in which an average of one neutron from each fission reaction initiates another reaction.
Critical Mass: The minimum mass of a substance that will allow a chain reaction to continue without dying out.
Critical Temperature: The temperature at the critical point.
Crookes Dark Space: The dark space between the cathode glow and the negative glow in a vacuum tube, occurring when the pressure is low.
Crookes, William: English chemist and physicist who discovered thallium.
CRT: An evacuated tube containing an anode and a cathode that generates cathode rays when operated at a high voltage.
Cryogenic: Operating at extremely low temperatures.
Cryogenic Gas: A gas that has been liquified by lowering the temperature, usually to a temperature under about -100�C.
Cryogenic Storage: Extreme low-temperature storage.
Cryostat: Container used to isolate thermally a fluid from its environment and maintain it at low temperatures.
Crystal: A material in which the atoms are arranged in a definite geometric pattern.
Crystalline: The state of a solid material characterized by a periodic and repeating three-dimensional arrays of atoms, ions, or molecules.
Crystallization: Act or process of forming crystals or bodies by elements or compounds solidifying.
Curie: Unit of radioactivity equal to 3.7x10¹⁰Bequerels.
Curie Constant: A characteristic constant, dependent on the material in question, which indicates the proportionality between its susceptibility and its thermodynamic temperature.
Curie, Pierre & Marie: French physicists, researched radioactivity.
Curie Temperature: The temperature above which a ferromagnetic or ferrimagnetic material becomes paramagnetic.
Curie′s Law: The susceptibility of an isotropic paramagnetic substance is related to its thermodynamic temperature.
Curie-Weiss Law: More general form of Curie′s law, which states that the susceptibility of a paramagnetic substance is related to its thermodynamic temperature.
Current: Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up.
Curvature: Departure from flatness of a surface. Defined as the reciprocal of the radius of curvature.
Cutoff: A transistor operating mode where very little current flows between the collector and emitter.
Cutoff Frequency: The cutoff frequency of an anechoic wedge or set of wedges is the lowest frequency above which the normal incidence sound absorption coefficient is at least .990
Cycle: One complete repetition of a periodic motion.
Cycles per Second: The rate of repetition of periodic motion measured in hertz (cycles per second).
Cyclotron: A type of particle accelerator in which an ion introduced at the centre is accelerated in an expanding spiral path by use of alternating electrical fields in the presence of a magnetic field.
Cylindrical Wave: A wave in which the surfaces of constant phase are coaxial cylinders.
d: Prefix representing a multiplication of 10^-1
da: Prefix representing a multiplication of 10
Dalton, John: He developed atomic theory that accounts for the law of conservation of mass, law of definite proportions and law of multiple proportions.
Dalton′s Law of Partial Pressures: The total pressure of a mixture of gases, which do not interact chemically, is equal to the sum of the partial pressures.
Damping Factor: The ratio of actual damping in a system to its critical damping.
Damping Ratio: Also known as Damping Factor, the ratio of actual damping in a system to its critical damping.
Danjon Scale: A scale measuring the darkness of lunar eclipses.
Dark Field Imaging: Using a single diffracted beam to form the image in a TEM.
Dark Matter: Matter that is in space but is not visible to us because it emits no radiation by which to observe it.
Data Transmission: The transfer of information from one place to another or from one part of a system to another.
Daughter Isotope: In a nuclear equation the compound remaining after the parent isotope (the original isotope) has undergone decay.
Davisson-Germer Experiment: An experiment that conclusively confirmed the wave nature of electrons.
de Broglie Wavelength: The wavelength associated with a particle or body.
deca: Prefix representing a multiplication of 10.
Decalescence: A decrease in temperature that occurs while heating metal through a range in which change in structure occurs.
Decay Rate: The rate at which a population of radioactive atoms decays into stable daughter atoms.
Decaying Orbit: An unstable orbit from which the orbiting object will gradually spiral into the body it is orbiting.
Deceleration: Negative acceleration is called deceleration .
deci: Prefix representing a multiplication of 10^-1
Decimal Prefixes: The decimal prefixes from pico to tera.
Deduction: A conclusion derived by reasoning.
Deflection Coils: In a cathode-ray tube, coils used to bend an electron beam a desired amount.
Deflection Plates: Two pairs of parallel electrodes, one pair set forward of the other and at right angles to each other, parallel to the axis of the electron stream within an electrostatic cathode-ray tube.
Degenerate Orbital: A set of orbitals are said to be degenerate if they all have the same energy.
Deionization Potential: The potential at which ionization of the gas within a gas-filled tube ceases and conduction stops.
Deionization Time: In a spark gap, the time required for ionized gas to return to its neutral state after the spark is removed.
Delta: A letter of the Greek alphabet.
Depletion Region: The region in a semiconductor where essentially all free electrons and holes have been swept out by the electrostatic field which exists there.
Derived SI Units: These are multiplications and divisions of the basic units to form derived units.
Derived Units: Derived units are units constructed from a base system of units.
Destructive Interference: When the peaks of one wave match the troughs of another, the waves interfere destructively.
Determinism Principle: The principle that if one knows the state to an infinite accuracy of a system at one point in time, one would be able to predict the state of that system with infinite accuracy at any other time, past or future.
Deuteron: Nucleus of a deuterium atom.
Dew Point: The temperature at which water vapour begins to condense.
Dextrorotatory: A chiral molecule which rotates plane-polarized light to the right.
Dielectric: An insulating material. Such as the material between the plates of a capacitor.
Dielectric Breakdown Voltage: Voltage an insulating material will withstand before flashover (arcing) or puncture.
Dielectric Constant: The ratio of the permittivity of a medium to that of a vacuum.
Dielectric Displacement: The magnitude of charge per unit area of capacitor plate.
Dielectric Heating: The phenomenon in which radiowave or microwave electromagnetic radiation heats a dielectric material, especially as caused by dipole rotation.
Dielectric Hysteresis Loss: Power loss of a capacitor because of the changes in orientation of electron orbits in the dielectric; the changes in orientation are caused by rapid reversal in polarity of line voltage.
Dielectric Losses: Power losses due to the conductance of dielectric materials.
Dielectric Strength: The magnitude of an electric field necessary to cause significant current passage through a dielectric material.
Dielectric Withold Volatge: Voltage an insulating material will withstand before flashover (arcing) or puncture.
Differential Amplifier: One whose output is proportional to the difference between two inputs.
Differential Input: Often used to reduce noise picked up by the signal leads.
Diffraction: The spreading of waves passing through an opening or around a barrier.
Diffraction Grating: Grating device that splits light into a spectrum of the component wavelengths.
Diffuse Reflection: The reflection of rays from a rough surface. The reflected rays do not leave at fixed angles.
Diffusion: The mixing of two substances caused by random molecular motions.
Diffusion Bonding: A solid-state process for joining metals by using heat and pressure to achieve atomic bonding.
Diffusion Coefficient: The constant of proportionality between diffusion flux and the concentration gradient in Fick′s first law.
Diffusion Rate: The number of randomly moving molecules that pass through a unit area per second.
Dilatometer: A device for measuring volume changes.
Dimensions: All quantities can be expressed in terms of three basic quantities, mass M, length L and time T.
Dioptre: A measure of the focal length of a mirror or lens.
Dip Angle: The vertical angle between the true horizon and the apparent horizon.
Dip Equator: Another name for Aclinic Line or Magnetic Equator.
Dipole: A pair of equal yet opposite electrical charges that are separated by a small distance.
Dirac Delta Function: The limiting case of a pulse with unit area that is infinitely short and at the same time infinitely high.
Dirac, Paul: English physicist; published Principles of Quantum Mechanics in 1930.
Dirac′s Constant: Equal to Planck′s constant divided by 2 pi.
Direct Current: A current that does not change in direction.
Direct Measurement: A process of obtaining the measurement of some entity by reading a measuring tool, such as a ruler for length, a scale for weight, or a protractor for angle size.
Discontinuity: A break in sequence or continuity of anything.
Dislocation: A linear crystalline defect around which there is an atomic misalignment.
Dislocation Line: The line that extends along the end of the extra half-plane of atoms for an edge dislocation, and along the center of the spiral of a screw dislocation.
Dispersion: The spreading of light into a spectrum of colour. The variation in the speed of a periodic wave due to its wavelength or frequency.
Displacement: Displacement is a vector quantity that specifies the change of position of a body or particle and is usually measured from the mean position or position of rest.
Distortion: Any change in the waveform or harmonic content of an original signal as it passes through a device.
Distributed Constants: The constants of inductance, capacitance, and resistance in a transmission line.
Diurnal Aberration: The component of stellar aberration resulting from the observer′s diurnal motion about the center of the Earth.
Dobson Unit: Unit used in geophysics to measure the ozone in the atmosphere.
Donor: An atom which is likely to give off one or more electrons when placed in a crystal.
Doping: The intentional alloying of semiconductor materials with controlled concentrations of donor or acceptor impurities
Doppler Effect: A change in the frequency of a periodic wave due to the motion of the observer, the source, or both.
Doppler Sonar: An acoustic instrument that measures the change in the acoustic frequency of the scattered sound or echo from that of the transmitted pulse. The magnitude and direction of the shift in frequency is related to the relative motion of the sensor and the scatterer.
Down Quark: The second flavour of quark (in order of increasing mass), with electric charge -1/3.
Drift: Motion of carriers caused by an electric field.
Dry Air: Air without contained water vapour.
Dry Bulb Temperature: The temperature of a gas or mixture of gases indicated by an accurate thermometer after correction for radiation.
Dry Bulb Thermometer: A thermometer used to measure the ambient temperature.
Dry Cell: A electrolytic cell also known as a Leclanch� cell that uses a moist paste rather than a liquid as an electrolyte.
Ductile Fracture: A mode of fracture that is attended by extensive gross plastic deformation.
Ductile-to-Brittle Transition: The transition from ductile to brittle behaviour with a decrease in temperature.
Ductility: A measure of a material′s ability to undergo appreciable plastic deformation before fracture.
Dynamic Equilibrium: Equilibrium which includes inertial forces.
Dynamic Friction: Resistance to relative movement of two bodies that are already in motion.
Dynamic Range: The power range over which a component or system functions properly.
E: Prefix representing a multiplication of 10¹⁸
Earth: The basic Earth data.
Earth Current: A large-scale surge of electric charge within the earth′s crust, associated with a disturbance of the ionosphere.
Earth Shine: Another name for Earthlight.
Earthlight: The illumination of the dark part of the moon′s disk produced by sunlight reflected onto the moon from the earth′s surface and atmosphere.
Echo Location: Determining the location of a target relative to the sensor face by means of measuring the time it takes for a sound wave to travel to the target and be reflected back to the sensor
Eclipse: A chance alignment between the Sun and two other celestial objects within the solar system in which one body blocks the light of the Sun from the other.
Eddington Limit: The theoretical limit at which the photon pressure would exceed the gravitational attraction of a light-emitting body.
Eddy Currents: Circulating currents within inductor core material caused by magnetic lines of force cutting across the core in a direction which induces a voltage in the core material.
Edge Dislocation: A linear crystalline defect associated with the lattice distortion produced in the vicinity of the end of an extra half-plane of atoms within a crystal.
Edison Effect: The phenomenon wherein electrons emitted from a heated element within a vacuum tube will flow to a second element that is connected to a positive potential.
Effective Nuclear Charge: The nuclear charge experienced by an electron when other electrons are shielding the nucleus.
Effective Temperature: An arbitrary index which combines into a single value the effect of temperature, humidity, and air movement on the sensation of warmth or cold felt by the human body.
Efficiency: A measure of how well a machine changes energy into useful energy.
Effusion: Movement of gas molecules through a small opening.
E-Field: Abbreviation of Electric Field.
EHF: Abbreviation of Extremely High Frequency.
Ehrenfest Paradox: The special relativistic paradox involving a rapidly rotating disc.
Eigenvalue: Possible values for a parameter of an equation for which the solutions will be compatible with the boundary conditions.
Einstein, Albert: We may of heard of his Theory of Relativity and his Electromagnetic Theory of Light; but few of us will ever understand them.
Einstein Field Equation: The cornerstone of Einstein′s general theory of relativity.
Elapsed Time: A period of time that has passed, usually between a given starting time and ending time.
Elastic Collisions: When two bodies collide their total momentum is conserved unless external forces act on them.
Electric Charge: A property used to explain attractions and repulsions between certain objects.
Electric Constant: e₀ = 8.854187817x10^-12Fm^-1
Electric Dipole: An object whose centres of positive and negative charge do not coincide.
Electric Dipole Moment: A measure of the degree of polarity of a polar molecule.
Electric Discharge: The flow of electricity through a gas, resulting in the emission of radiation that is characteristic of the gas and of the intensity of the current.
Electrical Conductivity: The capacity of a material to conduct electric current.
Electrical Energy Storage: The storage of electrical energy in a device such as a capacitor.
Electrical Impedance: Impedance of a linear circuit element with two terminals is the ratio of the complex sinusoidally varying voltage applied across the terminals to the complex current that flows in response.
Electrical Insulation: A material used to prevent the leakage of electricity from a conductor and to provide mechanical spacing or support as protection against accidental contact with the conductor.
Electrochemical Energy Storage: The storage of energy in a system such as a battery.
Electrodes: Device that moves electrons into or out of a solution by conduction.
Electrolysis: Changing the chemical structure of a compound using electrical energy.
Electrolyte: A substance that dissociates fully or partially into ions when dissolved in a solvent, producing a solution that conducts electricity.
Electromagnetism: Lists all Electromagnetism topics in the Encyclopaedia
Electromechanical: Converting electrical input into mechanical action.
Electromotive Series: The electrode potential of a material according to decreasing tendency to release electrons with hydrogen as zero.
Electron: The electron charge is 1.602x10^-19 C
Electron Affinity: The measure of an atom′s tendency to gain an electron.
Electron Capture: A decay process in which an inner atomic electron is captured by the nucleus.
Electron Charge: e = 1.602x10^-19 C
Electron Configuration: A list showing how many electrons are in each orbital or subshell.
Electron Flow: The direction in which electrons flow. This is from negative to positive - as electrons are negatively charged.
Electron Gun: An electrode of a CRT that is equivalent to the cathode and control grid of conventional tubes.
Electron Microscope: A type of microscope that uses a particle beam of electrons to illuminate the specimen and produce a magnified image.
Electron Orbital Movement: The movement of an electron around the nucleus of an atom.
Electron Shell: A group of electrons which have a common energy level that forms part of the outer structure of an atom.
Electron Spin: Electrons have an intrinsic angular momentum that is similar to what would be observed if they were spinning.
Electron Spin Pair: Two electrons with opposite spins, usually occupying the same orbital.
Electron Volt: Unit of energy of moving particles. Given by the kinetic energy acquired by an electron losing one volt of potential.
Electronegativity: Measure of a substances' ability to attract electrons.
Electronics: Lists all the Electronics topics
Electron-Positron Annihilation: When an electron and its anti-particle, a positron, collide, they annihilate emitting a pair of gamma-ray photons each with an energy of 511 keV.
Electrophile: A species that loves electrons.
Electrovalence: The valence as determined by the electrons lost or gained by the elements reacting to form a compound.
Electroweak Interaction: In the Standard Model, electromagnetic and weak interactions are related (unified).
Element: A pure substance which cannot be broken down into simpler substances by chemical means.
Elementary Charge: The electric charge on an electron particle.
Elementary Particles: The particles which form the building blocks of atoms and those which carry energy.
ELF: That portion of the radio frequency spectrum from 30 to 3000 hertz.
Elliptical Orbit: An orbit which describes an ellipse or oval shape.
EMF: Abbreviation of Electro Motive Force.
Emission Spectrum: The collection of discrete wavelengths emitted by atoms that have been excited by heating or by electric currents.
Emissivity: The ratio of the radiant energy emitted by a any source to that emitted by a blackbody at the same temperature.
Emittance: The rate at which emission occurs.
Emitter: One terminal of a transistor. Compares generally to the cathode of a tube.
Emitter Coupled Logic: Where transistors are held in the turned-on state to increase the speed of the gate.
Empirical Law: A law strictly based on experiment, which may lack theoretical foundation.
Energy: Energy is defined as the capacity of a body for doing work.
Energy Balance: The difference between the total incoming and total outgoing energy.
Energy Band: A collection of closely spaced energy levels.
Energy Conservation: Practice of decreasing the quantity of energy used.
Energy Storage: Device or physical media that store some form of energy to perform some useful operation at a later time.
Epitaxial: Of or pertaining to a layer of single crystal material grown on a crystalline substrate, with the same crystal orientation in both regions.
Epsilon: A letter of the Greek alphabet.
Equations of Motion: There are four basic equations that describe the motion of a body moving with constant acceleration.
Equations of Rotational Motion: The rotational forms of the equations describing a body undergoing constant angular acceleration.
Equatorial Axis: Among the two mutually perpendicular axes of a telescope, the one that points at the celestial pole.
Equatorial Orbit: An orbit that occurs when the plane of a satellite coincides with the plane of the earth at the equator.
Equilibrium: In mechanics a body may be in one of three states of equilibrium: stable, unstable and neutral. In chemistry when the reactants and products are in a constant ratio. The forward reaction and the reverse reactions occur at the same rate when a system is in equilibrium.
Equilibrium Constant: Value that expresses how far the reaction proceeds before reaching equilibrium.
Equivalence Principle: Constant acceleration is completely equivalent to a uniform gravitational field.
Ergosphere: The region around a rotating black hole, between the event horizon and the static limit.
Erosion: The natural wearing away of rocks.
Errors: No measurement is exact, they all contain some degree of errors.
Eta: A letter of the Greek alphabet.
E-Transformer: A special form of differential transformer employing an E-shaped core.
E-Type Thermocouple: Chromel-constantan thermocouple with a temperature range of 0 to 800°C.
Euler Codes: Computer software that is a mathematical representation of the motion of a fluid whose behavior and properties are described at fixed points in a coordinate system.
Eulers Constant: γ = 0.5772156�..
Eutectic: An isothermal reversible reaction or an alloy having a structure or composition indicated by the eutectic point.
Eutectic Alloy: An alloy that changes directly from a solid to a liquid with no plastic or semiliquid state.
EUV: Extreme Ultraviolet, a portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
eV: Abbreviation of Electron Volt.
Evaporate: To convert a liquid into a gas.
Evaporation: The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor.
Evaporimeter: An instrument for measuring the rate at which water evaporates, also called an atmometer.
Event Horizon: The radius that a spherical mass must be compressed to in order to transform it into a black hole.
exa: Prefix representing a multiplication of 10¹⁸
Excitation: Excitation is an external force or motion (or other input) applied to a system that causes the system to respond in some way.
Excited State: Atom with an electron in a higher energy level than it normally occupies.
Exclusion Principle: No two electrons can have the same set of quantum numbers. This statement also applies to protons, neutrons, and other baryons.
Exosphere: This region is considered the very outer limits of the earth's atmosphere.
Explosion: The sudden production of a large quantity of gas, usually hot, from a much smaller amount of a gas, liquid, or solid.
Extinction: The attenuation of light as it passes through a medium.
Extinction Potential: The potential at which ionization of the gas within a gas-filled tube ceases and conduction stops.
Extragalactic: Outside the Milky Way.
Extraterrestrial: A term used to describe anything which does not originate on the Earth.
Extreme Ultraviolet: A portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Extremely High Frequency: The portion of the electromagnetic spectrum between 30 GHz and 300 GHz.
Extremely Low Frequency: That portion of the radio frequency spectrum from 30 to 3000 hertz.
Extrinsic Semiconductor: A semi-conducting material for hich the electrical behavior is determined by impurities.
f: Prefix representing a multiplication of 10^-15
Face Centred Cubic: Roughly 20% of the elements crystallize with face centred cubic structures.
Faculae: Bright granular structures on the surface of the Sun that are slightly hotter than the surrounding photosphere.
Faint Young Sun Paradox: Calculations suggest that at about the time of the formation of Earth, the Sun was roughly two-thirds the brightness that it is now. However, there is no geological evidence.
Famous Scientists & Engineers: Lists all the Famous Scientists & Engineers in the Encyclopaedia
Far Field: The distribution of sound energy at a very much greater distance from a sources than the linear dimensions of the source and in which the sound waves can be considered to be plane waves.
Fast Fourier Transform: An algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform.
Fathom: An old unit of length more normally used to measure the depth of water.
Fatigue: An important mode of failure of engineering components subjected to dynamic stresses.
Femto: Prefix representing a multiplication of 10^-15
Fermat′s Principle: The path taken by a ray of light between any two points in a system is always the path that takes the least time.
Fermi Constant: The Fermi constant shows the coupling between a nucleon and a lepton field.
Fermi Energy: The average energy per particle when adding particles to a distribution but without changing the entropy or the volume.
Fermi Enrico: He received the Nobel Prize for physics for the development of neutron-induced nuclear reactions in 1938.
Fermions: Fermions are particles that satisfy the Pauli exclusion principle and obey the Fermi-Dirac statistics of statistical mechanics.
Ferrite: A solid solution of one or more elements in body-centered cubic iron.
Ferrite Rod Aerial: A coil of wire wound on a ferrite material to increase the inductance of the coil. It's signal capturing capability.
Ferroelectric: A dielectric material that may exhibit polarization in the absence of an electric field.
FFT: An abbreviation of Fast Fourier Transform, an algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform.
Fibre: Any material that has been drawn into a cylinder with a length-to-diameter ratio greater than about ten.
Ficks Laws: Fick′s first law: The diffusion flux is proportional to the concentration gradient.
Field Effect Transistor: A transistor that makes use of the field established in a p-type channel semiconductor material to control the flow of current through the channel.
Field Excitation: The creation of a steady magnetic field within the field windings by the application of a dc voltage either from the generator itself or from an external source.
Finite Impulse Response Filter: A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
FIR Filter: A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
Firing Potential: The potential required to ionize the gas of a gas-filled tube.
Firmament: The celestial sphere and the collection of stars whose position is fixed on it.
First Class Lever: The fulcrum is situated between the load and the applied force.
First Ionization Energy: The energy needed to remove an electron from an isolated, neutral atom.
First Postulate of Special Relativity: The laws of physics are the same for all inertial reference systems.
Fission: The splitting of a heavy nucleus into two or more lighter nuclei.
Fizeau Method: One of the first truly relativistic experiments, intended to measure the speed of light.
Flavour: A characteristic that distinguishes different types of hadrons and leptons with different masses.
Flip Flop: An astable multivibrator.
Flow Rate: Volume per unit of time.
Fluid Dynamics: Lists all Fluid Dynamics topics in the Encyclopaedia
Fluid Friction: Friction due to the viscosity of fluids.
Fluids: Substances in which the binding forces are weaker than in solids.
Fluorescence: The property of a material whereby it emits visible light when it is illuminated by ultraviolet light.
Flux: The rate of flow of a physical quantitiy through a reference surface.
Flywheel Effect: The continuous interchange of electric energy between the capacitor and inductor of a parallel resonant circuit wherein the energy level is diminished only by circuit resistance and radiated energy.
FM: An abbreviation of Frequency Modulation where the information signal is used to vary the carrier signal frequency.
Focal Plane: A surface upon which the image of all points in the field of view of an optical instrument is created.
Focal Point: The location at which a mirror or a lens focuses rays parallel to the optic axis or from which such rays appear to diverge.
Focus: Point at which converging rays meet and at which a clearly defined image can be obtained.
Focusing Anode: An electrode of a CRT that is used to focus the electrons into a tight beam.
Force: A force is that which when acting on a body that is free to move accelerates the motion of the body.
Forced Vibration: The oscillation of a system under the action of a forcing function.
Foucault Currents: Also known as Eddy Currents.
Fourth State of Matter: Plasma are sometimes referred to as the fourth state of matter.
Free Charges: Those electrons that can be moved by an externally applied voltage.
Free Electron: Electron which is not attached to a nucleus.
Free Energy: A thermodynamic quantity that is a function of both the internal energy and entropy of a system.
Free Progressive Wave: Wave in a medium free from boundary effects.
Free Space Loss: The loss of energy of radio waves caused by the spreading of the wavefront as it travels from the transmitter.
Freeze: To change from a liquid to a solid by cooling.
Frequency: The rate of repetition of periodic motion measured in hertz (cycles per second).
Frequency Modulation: The information signal is used to vary the carrier signal frequency.
Frequency Spectrum: The entire range of frequencies contained in a pulse or signal.
Friction: The resistance to motion which is called into play when it is attempted to slide one surface over another, with which it is in contact.
Frisch, Otto: Advanced the theory that uranium, when bombarded by neutrons, breaks into smaller atoms.
Froude Number: A dimensionless number used in the study of fluid flow problems with models.
f-stop: The ratio of the focal length of the lens to diameter of the aperture.
Fuel: Any substance burned for heat or power.
Fulcrum: The support on which a lever turns.
Full Scale Deflection: The maximum value on the scale of an instrument.
Fully Ionized Plasma: A plasma in which all the atoms or molecules have been ionized.
Fundamental: In periodic forced vibration, the term fundamental refers to the lowest frequency component present in a harmonic train.
Fundamental Interaction: In the Standard Model the fundamental interactions are the strong, electromagnetic, weak, and gravitational interactions.
Furlong: A distance of one-eight of a mile.
Fusion: The combining of light nuclei to form a heavier nucleus. Also, the complete mixing of two materials along an edge.
g: Abbreviation often used for standard acceleration due to gravity, g = 9.80665 ms^-2
G: Prefix representing a multiplication of 10⁹
G: The constant of proportionality in Newton′s law of universal gravitation.
Galactic Corona: A huge spherical region that exists around our own and other spiral galaxies.
Galactic Disc: The plane in which the spiral arms of spiral galaxies or barred spiral galaxies exist.
Galaxy: A system of about 100 billion stars.
Galaxy Clusters: Groups of galaxies that may contain up to a few thousand galaxies.
Galilean Principle of Relativity: The laws of motion are the same in all inertial reference systems.
Galileo Galilei: Astronomer, mathematician and physicist he dwelt, not on the useless question, why do things happen? but, how do things happen?
Galvanic Cell: A cell in which chemical change is the source of electrical energy.
Galvanic Corrosion: Corrosion associated with the current of a galvanic cell consisting of two dissimilar conductors in an electrolyte or two similar conductors in dissimilar electrolytes.
Galvanic Couple: A pair of dissimilar conductors, commonly metals, in electrical contact.
Galvanic Current: The electric current that flows between metals or conductive nonmetals in a galvanic couple.
Galvanometer: An instrument for detecting and measuring a small electric current by movements of a magnetic needle or of a coil in a magnetic field.
Gamma: A letter of the Greek alphabet.
Gamma Rays: Gamma rays are electromagnetic waves or photons emitted from the nucleus (centre) of an atom.
Gas: Matter with no definite shape or volume.
Gas Constant: Molar Gas Constant, R = 8.31441 JK^-1mol^-1
Gate: One of the terminals of a Field Effect Transistor (FET).
Gauge Pressure: Zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.
Gauss: A unit of magnetic field strength, 10^-4 tesla.
Gay-Lussac, Joseph Louis: Developed the law of volumes concerning the combination of gases and discovered boron.
Geiger Counter: A radiation detector consisting of two electrodes with a low-pressure gas in between.
Geiger, Johannes: Helped to develop first successful counter of alpha particles.
General Theory of Relativity: An extension of the special theory of relativity to include the concept of gravity.
Geocentric Model: A model of the Universe with the Earth at its center.
Geocorona: The outermost layer of the Earth′s neutral atmosphere.
Geodetic Coordinates: A system of geographic position referencing. Angular measurements of latitude and longitude are projected onto a well-defined reference ellipsoid that approximates the earth′s irregular shape.
Geological Timescale: The ages of the history of planet earth from the Pleistocene to the Precambrian era.
Geometric Scattering: Acoustic scattering in which the wavelength of the sound used is much smaller than the size of object causing the scattering.
Geometrized Units: A system of units whereby certain fundamental constants are set to unity.
Geostationary: A satellite orbit traveling from west to east at speeds that allow it to remain fixed over a given place on the earth′s equator at approximately 22,300 miles in altitude. A geostationary satellite makes one revolution in 24 hours, synchronous with the earth′s rotation.
Getter: An alkali metal introduced into a vacuum tube during manufacture.
GeV: Abbreviation of Giga electron Volt.
Giant Molecular Cloud: Massive clouds of gas in interstellar space composed primarily of hydrogen molecules, though also containing other molecules observable by radio telescopes.
giga: Prefix representing a multiplication of 10⁹
Global Positioning System: An array of satellites, deployed and maintained by the U.S. Department of Defense, which can be monitored to triangulate an accurate position on the earth′s surface.
Globular Cluster: A spherically symmetric collection of stars which shared a common origin.
Glow Discharge: Discharge of electricity through a gas in an electron tube.
Gluon: An exchange particle responsible for the force between quarks.
GMT: Abbreviation of Greenwich Mean Time, local mean time at the Greenwich meridian.
Gnomon: The part of a sundial which casts the shadow, usually a rod or fin pointed at the celestial pole.
GPS: An abbreviation of Global Positioning System.
Grade: Grade is used to define an angle, grade of sandpaper, amongst others..
Graduation Mark: The marks that define the scale intervals on a measuring instrument are known as graduation marks.
Grain: An individual crystal in a poly-crystalline metal or alloy.
Grain Size: For metals, a measure of the areas or volumes of grains in a polycrystalline material, usually expressed as an average when the individual sizes are fairly uniform.
Gravimetric Energy Density: Weight based energy density.
Gravitational Collapse: When a massive body collapses under its own weight.
Gravitational Confinement: The containment of nuclei under the influence of a gravitational field.
Gravitational Constant: G = 6.67x10^-11 Nm²kg^-2
Gravitational Field: The space surrounding an object where each location is assigned a value equal to the gravitational force experienced by one unit of mass placed at that location.
Gravitational Interaction: The interaction of particles due to their mass/energy.
Gravitational Mass: The property of a particle that determines the strength of its gravitational interaction with other particles.
Gravitational Potential: The worrk done in bringing a unit mass from infinity to that point. The assumption being that the gravitational potential is zero at infinity.
Gravitational Potential Energy: The work done by the force of gravity when an object falls from a particular point in space to the location assigned the value of zero.
Gravitational Redshift: The decrease in the frequency of electromagnetic waves due to a gravitational field.
Gravitational Wave: Ripple in the structure of space-time which may occur individually or as continuous radiation.
Gravitationally Bound: Objects held in orbit about each other by their gravitational attraction.
Graviton: The carrier particle of the gravitational interactions; not yet directly observed.
Graviton Decoupling: An epoch, early in the life of the universe, in which a density was reached such that gravitons no longer constantly interacted with other particles.
Gravity: One of the forces of nature. It is an attractive force exerted between two or more particles all of which have mass.
Gravity Separation: Under the influence of gravity, separation of immiscible phases allows the denser phase to settle out.
Gravity Wave: A wave disturbance caused by the acceleration of masses.
Gray: This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material.
Greek Alphabet: The Greek alphabet from alpha to omega.
Greenhouse Effect: The 'greenhouse effect' is caused by certain gases in the atmosphere trapping heat radiated from the Earth.
Greenwich Mean Time: Local mean time at the Greenwich meridian.
Gregorian Calendar: This calendar modifies the Julian calendar for greater precision.
Ground Loop: An alternative path in which voltage can travel.
Ground State: The lowest energy state of a system allowed by quantum mechanics.
Ground Waves: Radio waves which travel near the surface of the earth.
Grounded Base: A transistor amplifier circuit comparable to the grounded-grid (signal ground) tube circuit.
Grounded Collector: A transistor circuit comparable to the cathode-follower tube circuit. (Also called emitter follower.)
Grounding: Establishing an electric connection to the Earth in order to neutralize an object.
Group Delay: The rate of change of phase shift with respect to frequency.
Grown Junction: A method of mixing P-type and N-type impurities into a single crystal while the crystal is being grown.
Gyro: Abbreviation for Gyroscope.
Gyroscope: A disc with a heavy rim mounted in such a way that its axis of rotation can adopt any position.
H: An abbreviation of henry, the SI unit of inductance.
h: Prefix representing a multiplication of 10²
Hadrons: The family of particles that participate in the strong interaction.
Half Life: The amount of time it takes for half an initial amount to disintegrate.
Half Reactions: The two parts of an oxidation-reduction reaction, one representing oxidation, the other reduction.
Hall Effect: The phenomenon whereby a force is brought to bear on a moving electron or hole by a magnetic field that is applied perpendicular to the direction of motion.
Hall Mobility: A measure of the flow of charged particles perpendicular to both a magnetic and an electric field.
Halo: A ring of light that appears around the Sun or Moon.
Haptic: Pertaining to the sense of touch, from the Greek word haptein, to grasp.
Hard X-Rays: Higher-energy part of the X-ray spectrum ranging from approximately 5 keV to 100 keV.
Hartree: The atomic unit of energy, named after the British physicist Douglas Hartree.
Hawking Radiation: A theory first proposed by British physicist Stephen Hawking, that due to a combination of properties of quantum mechanics and gravity, under certain conditions black holes can seem to emit radiation.
Hawking Temperature: The temperature inferred for a black hole based on the Hawking radiation detected from it.
h bar: Equal to Planck′s constant divided by 2 pi.
HCP: Abbreviation of Hexagonal Close Packed, a crystal structure found for some metals.
Heat Treatment: Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties.
hecto: Prefix representing a multiplication of 10².
Height: The vertical dimension of anything.
Heisenberg Uncertainty Principle: This principle states that it is not possible to know a particle's location and momentum precisely at any time.
Heisenberg Werner: Postulated the uncertainty principle in 1927 and received Nobel Prize for physics in 1932.
Heliocentric Model: A model of the Universe with the Sun at its centre.
Heliopause: The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance.
Heliosheath: A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter.
Helium Burning: When temperature in the core of a star reaches 100 million degrees, three colliding helium nuclei fuse to form a carbon nucleus.
Helix: A path formed as a point advances uniformly around a cylinder, as the thread on a screw or the flutes on a drill.
Helmholtz Hermann: German physicist, anatomist and physiologist.
henry: The SI unit of inductance.
Henry′s Law: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Hermetic: Sealing of an object so it is airtight.
Hertz: The SI unit of frequency indicating the number of cycles per second (symbol Hz).
Hertzian Waves: Now generally called radio waves.
Hertzsprung-Russell Diagram: The Hertzsprung-Russell diagram relates the brightness of a star to its temperature.
Heterodyning: The process of mixing the incoming signal with the local oscillator frequency. This produces the two fundamentals and the sum and difference frequencies.
Hexagonal Close Packed: A crystal structure found for some metals.
High Tension: A comparative term used in electronics to denote high voltages.
Hipparchus: A Greek astronomer who carried out his observations at Rhodes.
Hippocrates: He believed that "the four fluids or humours of the body (blood, phlegm, yellow bile and black bile) are the primary seats of disease."
Hohlraum: A cavity whose walls are in radiative equilibrium with the radiant energy within the cavity.
Hole: Particle associated with an empty electron level in an almost filled band.
Hole Flow: Current carriers (in transistors) which have a flow in a direction opposite to electron movement.
Holocene: The current geological time period that started about 10,000 years ago.
Hologram: A three-dimensional record of visual information.
Holography: The photographic process for producing three-dimensional images.
Horizontally Polarized: Waves radiated with their E field component parallel to the surface of the earth.
Horseshoe Magnet: A permanent magnet or electromagnet bent into the shape of a horseshoe or having a U-shape to bring the two poles near each other.
Hot Carrier: A carrier, which may be either a hole or an electron, that has relatively high energy with respect to the carriers normally found in majority-carrier devices.
Hot Spot: A region of high heat flow.
Hot Wire Meter Movement: A meter movement that uses the expansion of a heated wire to move the pointer of a meter; measures DC or AC.
Humidity: Measure of the amount of water present in the air at any given time.
Humidity at Saturation: The mass per unit volume of water vapour required to saturate the air.
Hundredweight: Abbreviated as cwt, this is an imperial unit of mass.
Hunds Rule: When electrons are put into orbitals having the same energy, degenerate orbitals, one electron is put into each orbital before putting a second electron into an orbital.
Huygens′ Principle: "Every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave".
Hydrogen Burning: Hydrogen burning is the fusion of four hydrogen nuclei into a single helium nucleus.
Hydrometer: An instrument used for measuring the specific gravity of a liquid.
Hydrosphere: All the water on the surface of a planet.
Hygrometer: A tool for measuring the humidity of the air.
Hypobaric: Pertaining to low atmospheric pressure.
Hypothesis: A hypothesis is a proposed answer to a problem, or an explanation that accounts for a set of facts and that can be tested by further experimentation and observation.
Hysteresis: If the input varies from an initial point to a final point and then retraces its value back to the initial point, then the two paths of the output should be identical. If there is hysteresis in the system then the output paths will not coincide.
Hysteresis Loss: The power loss in an iron-core transformer or other alternating-current device as a result of magnetic hysteresis.
Hz: An abbreviation of hertz. The SI unit of frequency.
Ice Point: The temperature at which pure ice can exist in equilibrium with water at standard atmospheric pressure.
Ideal Gas: The particles have no internal structure, are indestructible, do not interact with each other except when they collide, and all collisions are elastic.
Ideal Gas Constant: Also known as Molar Gas Constant.
Ideal Gas Law: Gases obey the gas laws at low pressures and at temperatures above those at which they liquefy.
IIR: Abbreviation of Infinite Impulse Response filter, a commonly used type of digital filter.
Illuminance: This is defined as the luminous flux arriving at a surface that is perpendicular to it per unit area.
Impact: A collision of a mass in motion with a second mass.
Impedance: Mechanical impedance is the ratio of a force-like quantity to a velocity like quantity when the arguments of the real (or imaginary) parts of the quantities increase linearly with time.
Impedance of Free Space: Z₀ = 376.7304 Ohm
Implosion: A violent inward collapse.
Impulse: The product of the force and the time during which it acts.
Impurity: A foreign atom in a crystal.
In Phase: Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."
in: An abbreviation of inches, a secondary unit in the British system of units, now replaced by the SI system and the metre.
Incandescence: The emission of light due to heat.
Incident Wave: The wave that strikes the surface of a medium.
Inclinometer: A gravity device that measures angular position in degrees.
Inclusion: Foreign particle present as an undesirable impurity in a material.
Indicator Diagram: A plot of pressure vs. volume.
Induced Charge: An electrostatic charge produced on an object by the electric field that surrounds a nearby object.
Induced Current: Current that flows as a result of an Induced EMF.
Induced EMF: Abbreviation of Induced Electromotive Force, a voltage induced in a conductor in a varying magnetic field.
Inelastic: A collision or interaction in which kinetic energy is not conserved.
Inert Pair: Valence electrons in an s orbital penetrate to the nucleus better than electrons in p orbitals, and as a result they′re more tightly bound to the nucleus and less able to participate in bond formation.
Inertance: Inertance is the ratio of an acceleration-like quantity to a force-like quantity when the arguments of the real (or imaginary) parts of quantities increase linearly with time.
Inertia: An object's resistance to a change in its velocity.
Inertial Confinement: Methods of producing nuclear fusion by compressing a pellet of fuel using a laser, particle beam, or other external driver.
Inertial Force: A fictitious force that arises in accelerating (noninertial) reference systems.
Inertial Mass: An object′s resistance to a change in its velocity.
Inertial Reference System: Any reference system in which the law of inertia (Newton's first law of motion) is valid.
Infra-Red Radiation: The region of the electromagnetic spectrum adjacent to that of visible light
inHg: Abbreviation for inches of mercury, the old imperial measure of pressure when measured using a mercury barometer.
Instantaneous Speed: The limiting value of the average speed as the time interval becomes infinitesimally small.
Insulator: A material that does not allow the passage of electric charge or is a poor conductor of thermal energy.
Interaction Energy: The total energy that is caused by an interaction between the objects being considered.
Interaction Space: The region in an electron tube where the electrons interact with an alternating electromagnetic field.
Interference: The combining of two or more signals results in an interaction called interference.
Intermetallic: A compound of two metals that has a distinct chemical formula. The bonds in intermetallic compounds are often partly ionic.
Intermolecular Forces: Forces between molecules.
Internal Force: Forces which hold an object together when external forces or other loads are applied.
International Phonetic Alphabet: A system of words identifying the letters of the alphabet and numbers. The system was reached through international agreement, and uses words chosen for their ease of pronunciation by people of all language backgrounds.
International System of Units: The international system of units is based on the seven basic units: Mass, Length, Time, Electric current, Temperature, Amount of substance and Luminous intensity.
Intramolecular Forces: Forces within molecules. Forces caused by the attraction and repulsion of charged particles.
Intrinsic Carrier Density: The density of electrons and holes in an intrinsic semiconductor.
Intrinsic Semiconductor: A semiconductor free of defects or impurities.
Invariant Point: A point on a binary phase diagram at which three phases are in equilibrium.
Ion: An atom, or group of atoms, which has either gained or lost electrons.
Ion Acoustic Wave: A longitudinal compression wave in the ion density of a plasma.
Ion Dipole Forces: Intermolecular force that exist between charged particles and partially charged molecules.
Ion Trap: A device that allows ions to be trapped for long periods of time, during which the ions can be interrogated and their state changes observed.
Ionic Bond: This describes the electrostatic attraction of two oppositely charged ions in a crystalline lattice.
Ionic Conductor: A material that conducts electricity with ions as charge carriers.
Ionic Current: Electrical current with ions as charge carriers.
Ionization: The process of producing ions.
Ionization Energy: Energy required to remove an electron from a specific atom.
Ionization Point: The potential required to ionize the gas of a gas-filled tube.
Ionize: To make an atom or molecule of an element lose an electron, as by X-ray bombardment, and thus be converted into a positive ion.
Ionizing Radiation: A particle or a photon with enough energy to knock off an electron from an atom.
Ionosphere: The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photo ionization.
Ionospheric Storms: Disturbances in the earth′s magnetic field that make communications practical only at lower frequencies.
Iota: A letter of the Greek alphabet.
Isobaric: Constant pressure.
Isochore: A contour line that corresponds to values measured at identical volumes.
Isochoric: Having constant volume.
Isolation: The prevention of unwanted interaction or leakage between components.
Isothermal: Constant temperature.
Isothermal Expansion: The expansion or compression of a gas at constant temperature.
Isotopes: Two atoms having the same number of protons but different numbers of neutrons are said to be isotopes of each other.
Isotropic: Having identical values of a property in all crystallographic directions
Isotropic Radiation: The radiation of energy equally in all directions.
J: An abbreviation of Joule, the SI unit of energy.
Jerk: A vector quantity that specifies time rate of change of acceleration.
Josephson Effects: Electrical effects observed when two superconducting materials are separated by a thin layer of insulating material.
Joules Laws: Describe the heat produced when a current flows through a resistance and the internal energy of an ideal gas.
J-Type Thermocouple: Iron-constantan thermocouple with a temperature range of 0 to 750°C.
Julian Calendar: Introduced in 46 BC by the Roman ruler Julius Ceasar, this calendar assumes a year of 365.25 days, and uses a cycle in which 3 "ordinary" years of 365 days are followed by a "leap year" with 366 days.
Julian Day: Instituted by J. Julius Scaliger in 1582. The Julian date is independent of various calendars and chronological events.
Junction Transistor: A bipolar transistor constructed from interacting PN junctions.
Jupiter: The largest planet in our solar system.
k: k is the abbreviation of kelvin, the unit of thermodynamic temperature.
k: Prefix representing a multiplication of 10³
Kaon: A meson containing a strange quark and an anti-up (or anti-down) quark, or an anti-strange quark and an up (or down) quark.
kappa: A letter of the Greek alphabet.
KE: Abbreviation of Kinetic Energy, the energy that a body possesses solely because it is moving.
Kelvin: The kelvin is the unit of thermodynamic temperature.
Kelvin Wedge: For waves in the wake of a boat (gravity waves). In this case the wedge semi-angle is independent of the speed of the body.
Keplers Laws: Kepler found three laws defining the orbit of planets.
Kerr Black Hole: A rotating black hole.
Kerr Effect: The ability of certain substances to differently refract light waves whose vibrations are in different directions when the substance is placed in an electric field.
keV: Abbreviation of kilo electron volts, one thousand electron volts.
kg: This is the abbreviation of kilogram, the SI unit of mass is the kilogram.
kgf: An abbreviation of kilogram-force, equivalent to the force exerted by 1 kilogram due to the local acceleration due to gravity.
kHz: Abbreviation of kilohertz, 1000Hz.
kilo: Prefix representing a multiplication of 10³
kilopond: Another name for kilogram force.
Kinetic Theory: The derivation of the bulk properties of fluids from the properties of their constituent molecules, their motions, and interactions.
Kirchoffs Law: The basic law for current flow and voltage sources.
Kirkwood Gaps: Gaps in the asteroid belt, caused by resonance effects from Jupiter.
kJ: An abbreviation of kilojoule, one thousand joules. The SI unit of energy is the joule.
Klystron: An evacuated electron tube used as an oscillator or amplifier at microwave frequencies.
Knot: A speed of 1 nautical mile per hour.
kp: Abbreviation of kilopond, another name for kilogram force.
kPa: Abbreviation of kilopascal.
kph: Abbreviation of kilometres per hour, a unit of velocity used by most land based vehicles.
K-Type Thermocouple: Chromel-Alumel thermocouple with a temperature range of -200 to 1200°C.
kV: One thousand volts.
kVA: 1000 VA
kW: One thousand watts.
l: The abbreviation of litre, a derived SI unit of volume.
Lag: The amount one wave is behind another in time.
lambda: A letter of the Greek alphabet.
Lambda: The lightest strange baryon, consisting of one up, one down, and one strange quark.
Lamberts Laws: The illuminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source.
Laminar Flow: Smooth flow in which no crossflow of fluid particles occur between adjacent streamlines, a flow conceived as made up of layers.
Laminated Core: A core built up from thin sheets of metal insulated from each other.
Landau Damping: The damping of a space charge wave by electrons which move at the phase velocity of the wave and gain energy transferred from the wave.
Langley: A unit of energy per unit area, equal to 1 gram-calorie per square centimeter, commonly employed in radiation theory.
Langmuir Probe: A small metallic conductor or pair of conductors inserted within a plasma in order to sample the plasma current.
Langmuir-Tonks Frequency: Also known as Plasma Frequency.
Lapse Rate: The rate at which temperature decreases with height in the atmosphere.
Laser: An acronym for light amplification by stimulated emission of radiation.
laser Cooling: A technique that uses laser beams to slow down the motion of atoms and cool them to temperatures a few millionths of a degree above absolute zero.
Laser Diode: A laser which uses a forward biased semiconductor junction as the active medium.
Latent Energy: The internal energy associated with the phase of a system.
Latitude: The angular distance of a point on the Earth from the equator.
Lattice: An orderly arrangement of atoms in a material.
Laue Pattern: The pattern produced on a photographic film when high-frequency electromagnetic waves are fired at a crystalline solid.
Law of Areas: Kepler′s second law states: Each planet moves in such a way that the imaginary line joining it to the sun sweeps out equal areas in equal times.
Law of Parsimony: The suggestion that the simpler a theory is, the better it is.
Lawson Criterion: A criterion specifying the product of density and confinement time required to satisfy some power balance condition in a fusion reactor.
lb: The abbreviation of pound in the British system of units.
lbf: Abbreviation of pound-force = 4.44822 N
lbf in^-2: Abbreviation of pounds force per square inch, an old British unit of pressure.
Lead: The opposite of Lag.
Leak Rate: The rate of gas leakage into or out of a system.
Leakage Current: The small amount of current that flows through the dielectric between the conductors of a transmission line.
Leakage Flux: Magnetic flux lines produced by the primary winding that do not link the turns of the secondary winding.
Leakage Resistance: The electrical resistance that opposes the flow of current through the dielectric of a capacitor.
Leclanch� Cell: A electrolytic cell also known as a dry cell that uses a moist paste rather than a liquid as an electrolyte.
Left Hand Rule: The procedure for finding electron-flow direction in a wire by grasping it so the fingers point in the direction of the magnetic lines of force.
Lemma: A proposition that is useful mainly for the proof of some other theorem.
Length: The SI unit of length is the metre.
Length Contraction: An observer in motion relative to an object measures the length of that object along the direction of motion to be contracted when compared to the length measured by an observer at rest relative to the object.
Lennard-Jones Potential: Most often used in modelling van der Waals interactions.
Lenticular Galaxy: An intermediate form of galaxy, between elliptical galaxies and spiral galaxies.
Lenzs Law: A basic law which states that an induced current set up by the relative motion of a conductor and a magnetic field always flows in such a direction as to form a magnetic field which opposes the motion.
Lepton: A family of elementary particles that includes the electron, muon, tau, and their associated neutrinos.
Lever: One of the basic tools that date from prehistoric times.
Leyden Jar: An early form of capacitor.
Libration: The apparant rocking motion of an orbiting body.
Lift: The lifting force on a flying object.
Light: Electromagnetic radiation that is visible to the unaided human eye.
Light Ray: A line that represents the path of light in a given direction.
Light Scattering: When light passes through a medium that is not perfectly homogeneous, the irregularities in the medium scatter some of the light in all directions.
Light Year: The light year is defined as the distance travelled by electromagnetic radiation in one tropical year.
Line Spectra: Spectra generated by excited substances. Consists of radiation with only specific wavelengths.
Line Vortex: Idealized vortex in which vorticity is zero everywhere except along a line in space where it is infinite.
Linear: Having an output that varies in direct proportion to the input.
Linear Absorption Coefficient: The fractional decrease in transmitted intensity per unit of absorber thickness.
Linear Phase Response: Any system which accurately preserves phase relationships between frequencies
Linear Region: A transistor operating mode where the collector current is proportional to the base current.
Linear System: A system is linear if for every element in the system, the response is proportional to the excitation.
Linear Thermal Expansion: Most substances expand when heated and contract when cooled.
Linke Turbidity Factor: A measure of atmospheric turbidity, equal to the ratio of total optical depth to the Rayleigh optical depth.
Liquefaction: The transformation of a gas into a liquid.
Liquid: Matter with a definite volume that takes the shape of its container.
Lissajou′s Figures: The pattern traced out when the displacements of two simple harmonic motions are traced in the x and y direction.
Litre: A derived SI unit of volume.
lm: An abbreviation of lumen, the derived SI unit of luminous flux.
Local Action: A continuation of current flow within a battery cell when there is no external load.
London Force: An intermolecular attractive force that arises from a cooperative oscillation of electron clouds on a collection of molecules at close range.
Lone Pair: Electrons that are not involved in bonding.
Longitude: The location of a point east or west of the prime meridian. Longitude is shown on a map or globe as north-south lines left and right of the prime meridian, which passes through Greenwich, England.
Longitudinal Wave: A wave in which the vibrations of the medium are parallel to the direction the wave is moving.
Lorentz Gas: A Plasma model in which the electrons are assumed not to interact with each other, but only with ions and where the ions are assumed to remain at rest.
Lorentz Transformation: A set of equations which are used, in relativity problems, to transform measurements from one frame of reference to another.
Lumen: The derived SI unit of luminous flux.
Lunar Eclipse: When the Moon enters the Earth′s shadow as the Earth moves between the Sun and the Moon.
Lunar Month: 1 lunar month = 2.551444x10⁶s
lux: An abbreviation of lumen m^-2, the metric unit of illuminance.
Luxon: A particle which travels solely at the speed of light in vacuum.
lx: An abbreviation of lux that itself is an abbreviation of lumen m^-2, the metric unit of illuminance.
ly: Abbreviation of Light Year.
Lyman Series: The series which describes the emission spectrum of hydrogen when electrons are jumping to the ground state.
m: This is the abbreviation of metre, the SI unit of length.
m: Prefix representing a multiplication of 10^-3
M: Prefix representing a multiplication of 10⁶
Mach Number: The ratio of the speed of a body or flow of a fluid to the local speed of sound in the fluid.
Mach Wedge: The wave front semi-angle for a wake generated by supersonic motion of a body in a non-dispersive medium.
Macroscopic: Anything big enough to be seen with the naked eye.
Macrosonics: The technology of sound at signal amplitudes so large that linear approximations are not valid.
Madelung Constant: A constant representing the sum of the mutual potential coulombic attractive energy of all the ions in a lattice in the equation for the energy of an ionic crystal.
Magnetic Quantum Number: Quantum number that labels different orbitals within a subshell. m can take on values from - to +.
Magnetohydrodynamics: Academic discipline which studies the dynamics of electrically-conducting fluids and their interactions with magnetic fields.
Magnetosphere: The outermost environment of Earth, dominated by the Earth′s magnetic field.
Magnetron Oscillator: An electron tube that provides a high power output.
Magnitude: The size of a vector quantity.
Majority Carriers: The term used in transistor design to indicate the primary current-movement process.
Malus′ Law: Malus′ law defines the transmitted intensity through 2 polarisers for a given angle between the transmission directions of said polarisers.
Manometer: A U-shaped glass tube, partly filled with a liquid, water or mercury, employed to measure pressure.
Mantle: The main bulk of the Earth, between the crust and the core, ranging from depths of about 40 to 3470 kilometers.
Mars: The fourth planet from the sun in our solar system.
Mass: The SI unit of mass is the kilogram.
Mass Charge Ratio: The ratio of the mass number of an element to the number of electronic charges gained or lost in ionization.
Mass Flow Rate: Defines the volumetric rate with which fluids flow, maintaining the equation of continuity.
Mass Number: The mass number (A) of an atom is the number of protons and neutrons it has in its nucleus.
Mass Spectrometer: An instrument for producing ions in a gas and determining their mass and hence composition.
Materials: Lists all Materials topics in the Encyclopaedia
Mathematical Model: A mathematical representation of a process or physical object.
Matter: Anything that has mass.
maxwell: The CGS unit of magnetic flux, equal to 1x10^-8weber.
Maxwell Equations: Describe the relationship between electric and magnetic fields at any point in space as a function of charge density and electric current at such a point.
Mean Free Path: For sound waves in an enclosure, it is the average distance travelled between successive reflections.
Measurement: The collection of quantitative data.
Mechanical Energy Storage: The storage of energy in a device such as a spring.
Mechanical Properties: The properties of a material that reveal its elastic and inelastic behaviour where force is applied.
Medium: The vehicle through which a wave travels from one point to the next.
Medium Altitude Orbit: An orbit from 2,000 to 12,000 miles above the earth.
mega: Prefix representing a multiplication of 10⁶
Megaparsec: One million parsecs.
Megohm: One million ohms.
Meissner Effect: The decrease of the magnetic flux within a superconducting metal when it is cooled below the transition temperature.
Melting Point: The temperature at which a solid substance changes to a liquid state.
Melting Range: The temperature range between solid and liquid.
MEMS: Abbreviation of Micro Electro Mechanical Systems.
Meniscus: The curved surface of a liquid in a container produced by the cohesive forces between the liquid molecules and the adhesive forces between the liquid and the container.
Mensuration: The measurement of lengths, areas and volumes.
Mercury: The planet nearest to the sun in our solar system.
Mercury Barometer: Consists of a closed tube filled with mercury inverted in a mercury reservoir.
Meson: A type of hadron with whole-number units of spin.
Metal: A metal is a substance that conducts heat and electricity, is shiny and reflects many colors of light, and can be hammered into sheets or drawn into wire.
Metallic Radius: The atomic radius from as established by measuring the distances between atoms in metallic crystals.
Metallurgy: The science and technology of metals and alloys.
Metastable: Nonequilibrium state that may persist for a very long time.
Meter: A device used to measure a specific quantity, such as current, voltage, or frequency.
Metonic Calendar: Based on the moon, counting each cycle of the phases of the Moon as one month.
MeV: One million electron volts.
Mho: The unit of conductance, transconductance, or admittance; it is the word ohm spelled backwards.
MHz: Abbreviation of Megahertz, 1000000Hz.
Mica: A group of silicates with similar chemical composition. Used as an electrical insulator.
micro: Prefix representing a multiplication of 10^-6
Micro Electro Mechanical Systems: A technology dealing with building mechanical structures on silicon wafers using integrated circuit processing techniques.
Microgravity: The condition of near weightlessness induced by free-fall or unpowered space flight. It also refers to the scientific discipline concerned with the evaluation of the processes in a near-zero gravity environment, particularly those of fluid physics, life, and material sciences.
Micrometeorology: The detailed study of the weather at a specific location.
Micron: A derived SI Unit of length equal to 10^-6m
Microscopic: Refers to objects that are too small for the unaided eye to see.
Microshock: Small discharges sometimes experienced when touching a metal object in an electric field, similar to touching a door knob after walking across a nylon carpet.
Microstructure: The structural features of an alloy that are subject to observation under a microscope.
Microwaves: Electromagnetic waves longer than infra-red but shorter than radio.
Mie Potential: Electrical potential
Milankovich Theory: Theory by which ice ages were caused by slow changes of the motion of the Earth in space.
Mile: A unit of distance most commonly used in Britain and the US.
Miles Per Hour: Traditional British unit of velocity used by land based vehicles.
Miller Indices: A set of three integers that designate crystallographic planes, as determined from reciprocals of fractional axial intercepts.
Miller-Bravis Indices: A set of four integers that designate crystallographic planes in hexagonal crystals.
milli: Prefix representing a multiplication of 10^-3
Millikan Oil Drop Experiment: A famous experiment designed to measure the electronic charge.
Millisecond: One thousandth of a second, abbreviated ms or msec.
Minority Carriers: The conduction band holes in n-type material and valence band electrons in p-type material.
Minute: The minute is defined as 60 seconds and as 1/60 of a degree.
Mirage: An optical effect that produces an image that looks like it has been reflected from the surface of a body of water.
Mismatch Loss: The loss of power delivered into a load as a result of the interconnection of devices having unequal impedances.
Mixed Dislocation: A dislocation that has both edge and screw components.
mm: An abreviation of millimetre. milli is the prefix representing a multiplication of 10^-3 and metre is the SI unit of length.
mmHg: When measuring pressure using a mercury barometer it is normally expressed in mmHg, Hg being the chemical symbol of mercury.
Moderator: A material used to slow down the neutrons in a nuclear reactor.
Moire Fringes: A pattern formed by the interference between two regular sets of divisions.
Molar Gas Constant: R = 8.31441 JK^-1mol^-1
Molar Volume: The volume occupied by one mole of a substance under specified conditions.
Mole: The SI unit of the Amount of Substance. It is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
Mole Fraction: The number of moles of a particular substance expressed as a fraction of the total number of moles.
Moments of Inertia: Principal axes for bodies of uniform density.
Monochromatic: Radiation that has a single wavelength.
Monocrystalline Solar Cell: More expenive to make as the crystals are grown under carefully controlled conditions.
Moon: Basic Moon data.
Mossbauer Spectrometer: An instrument that provides information on the bonding of an atom in a mineral by bombarding it with gamma rays.
Most Significant Bit (MSB): The bit within a digital word that represents the biggest possible single-bit coded value.
Motive Power: A natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover.
Motor Rule: Also known as Fleming′s Left Hand Rule, this is a way of determining the direction of a force on a current carrying conductor in a magnetic field.
Mpa: Abbreviation of megapascal.
Mpc: Abbreviation of Megaparsec, one million parsecs.
mph: Abbreviation of miles per hour, traditional British unit of velocity used by land based vehicles.
ms: Abbreviation of millisecond, one thousandth of a second.
ms^-1: Derived SI unit of velocity, metre per second.
MSB: Abbreviation of Most Significant Bit, the bit within a digital word that represents the biggest possible single-bit coded value.
mu: A letter of the Greek alphabet.
Mumetal: An alloy of high magnetic permeability containing up to 78% nickel with iron, copper and manganese.
Muon: A type of lepton; often called a heavy electron.
Mutual Conductance: The ratio of the current at the output port and the voltage at the input ports.
Mutual Flux: The total flux in the core of a transformer that is common to both the primary and secondary windings.
mV: An abbreviation of millivolt, a unit of electrical potential equal to one thousandth of a volt.
N: An abbreviation of Newton, the SI unit of force.
n: Prefix representing a multiplication of 10^-9
Nadir: The point on the celestial sphere directly below an observer, diametrically opposite the zenith.
nano: Prefix representing a multiplication of 10^-9. From the ancient Greek word "nanos" or "dwarf".
Natural Frequency: The frequency of free vibration of a system.
Natural Horizon: The line-of-sight horizon.
Nautical Mile: The average distance on the earth's surface subtended by one minute of latitude.
Navier Stokes Equation: The primary equation of computational fluid dynamics, relating pressure and external forces acting on a fluid to the response of the fluid flow.
Nd₂Fe₁₄B: Chemical formula for Neodymium Iron Boron.
Near Synchronous Orbit: An orbit in which the satellite rotates close to but not exactly at the same speed as the earth.
Nebula: A cloud of dust and gas in space which is visible to observers on the Earth because it either emits, reflects or absorbs starlight.
Neptune: The outermost gas giant in our solar system.
Neumann′s Law: The symmetry of the physical properties of a crystal must include the symmetry of the point group of the crystal.
Neutral Equilibrium: A body is in neutral equilibrium if it stays in the displaced position after if has been displaced slightly.
Neutral Particle Beam: A projected stream of neutral particles such as deuterium or heavy hydrogen at very high particle energies and low currents.
Neutrino: A neutral lepton; one exists for each of the charged leptons (electron, muon, and tau).
Neutron: A sub-atomic particle, within the nucleus of an atom, which has unit mass and zero charge.
Neutron Star: A star composed only of neutrons.
New General Catalogue: A catalogue of non-stellar celestial objects that was compiled by JLE Dreyer of Armagh Observatory and published in 1888.
Newton, Isaac: Newton was a mathematician and natural philosopher (physicist).
Newtonian Fluid: A fluid whose viscosity does not depend on gradients in flow speed.
Newton′s Law of Universal Gravitation: Every particle in the Universe attracts every other with a force which is proportional to the product of their masses and inversely proportional to the square of their seperation.
Newtons Laws of Motion: Sir Isaac Newton published Philosophiae Naturalis Principia Mathematica in 1687 containing the three basic laws of motion.
NGC: Abbreviation of New General Catalogue, a catalogue of non-stellar celestial objects that was compiled by JLE Dreyer of Armagh Observatory and published in 1888.
Nobel Prize: Awarded annually as per Alfred Nobel′s last will and testament.
Node: A point or line on a vibrating structure that remains stationary.
Noise: Noise is any undesired signal.
Noncrystalline: The solid state wherein there is no long-range atomic order. Sometimes used synonymously with the terms amorphous, glassy and vitreous.
Noninertial Reference System: Any reference system in which the law of inertia is not valid.
Nonluminous Bodies: Objects that either reflect or diffuse light that falls upon them.
Non-Newtonian Fluid: A fluid whose viscosity changes when the gradient in flow speed changes.
Nonpolar: Having a relatively even or symmetrical distribution of charge.
Nonstationary: A characteristic of a time series for which the distribution changes over time.
North Pole: That end of the axis of rotation of a celestial body at which, when viewed from above, the body appears to rotate in a counter-clockwise direction.
Northern Lights: Common name for the Aurora or Aurora Borealis, a faint visual phenomenon associated with geomagnetic activity.
N-Type Thermocouple: Nicrosil-Nisil thermocouple with a temperature range of -200 to 1200°C.
nu: A letter of the Greek alphabet.
Nuclear Binding Energy: Energy needed to break an atomic nucleus into separate protons and neutrons.
Nuclear Energy: Energy stored in the nucleas of an atom.
Nuclear Equations: Show the changes that take place in the nuclei.
Nuclear Fusion: The combining of light nuclei to form a heavier nucleus.
Nuclear Reaction: A reaction involving the nucleus of an atom that alters the energy, composition or structure of the atom.
Nuclear Reactor: An apparatus in which nuclear fission may be sustained in a self supporting chain reaction.
Nucleation: The initial stage in a phase transformation.
Nuclei: The plural of nucleus, the central part of an atom that contains the protons and neutrons.
Nucleon: Either a proton or a neutron.
Nucleus: The central part of an atom that contains the protons and neutrons.
Nuclide: An atom or ion with a specified mass number and atomic number.
Null: A low or minimum point on a graph. A minimum pressure region in a room.
Null Hypothesis: Proposes a general or default position, such as that there is no relationship between two measured phenomena, or that a potential treatment has no effect.
Observation: Observations refer to repeated values of a data variable. The rows of a column represent the observations.
Occams Razor: The suggestion that the simpler a theory is, the better it is.
Occultation: Interruption of the light from an object to an observer when a body in interposed.
Octahedral Position: The void space among closed-packed, hard sphere atoms or ions for which there are six nearest neighbours.
Oersted: The unit of magnetizing force representing the magnetic field intensity at a distance of 1 centimeter in air or vacuum from the unit magnetic pole.
Ohm: Defined as the resistance of a conductor through which a current of one ampere is flowing when the potential difference across it is one volt.
Ohms per Square: The resistance of any square area of thin film resistive material as measured between two parallel sides.
omega: A letter of the Greek alphabet.
omicron: A letter of the Greek alphabet.
Opacity: The condition of being non-transparent.
Opaque: Substances that do not transmit (pass) any light rays.
Opposition: The point at which a planet that is further away from the Sun than the Earth.
Orbit: The path of a body in space, generally under the influence of gravity.
Orbit Acquisition: Reception of the telemetry containing the information about the orbital parameters of the spacecraft.
Orbital: A wavefunction that describes what an electron with a given energy is doing inside an atom or molecule.
Orbital Angular Momentum Quantum Number: Also known as Angular Momentum Quantum Number.
Orbital Equation: Also known as Kepler′s first law; The orbit of each planet is an ellipse which has the sun at one of its foci.
Ordered System: A system with an arrangement belonging to a group with the smallest number (possibly one) of equivalent arrangements.
Ornithopter: A flying machine that is supposed to support itself in the air through the use of flapping wings.
Orthogonal: Mutually perpendicular.
Oscillation: Oscillation is the variation, usually with time, of the magnitude of a quantity with respect to a specified reference.
Oscilloscope: An instrument using a cathode-ray tube for visual display of electric signals.
Outcome Space: Another name for Sample Space.
oz: A secondary unit in the British system of units. 1 ounce (oz) = 28.34952 g
Ozone Layer: An atmospheric layer that contains a high proportion of ozone and acts as a filtering mechanism against incoming ultraviolet radiation.
p: Prefix representing a multiplication of 10^-12
P: Prefix representing a multiplication of 10¹⁵
P: The abbreviation of poise, the CGS derived unit of dynamic viscosity of a fluid.
Packing Density: The fractionof a volume filled by a given collection of solids.
Pair Annihilation: When a particle encounters its antiparticle, they annihilate each other, disappearing in a burst of photons.
Pair Production: The conversion of energy into matter in which a particle and its antiparticle are produced.
Paradox: A statement that appears to contradict itself, for example, suggesting a solution which is actually impossible.
Parallax: Change in the apparent position of objects when viewed from two widely separated positions.
Parsec: 1 Parsec = 3.2615 Light Years
Partial Pressures: The pressure exerted by a certain gas in a mixture.
Partial Vacuum: A volume that contains traces of gas at very low pressure.
Particle Accelerator: A device for accelerating charged particles to high velocities.
Particle Beam: A stream of particles guided into a defined direction by a particle accelerator.
Particulate: Composed of distinct particles.
Partition Function: A central construct in statistics and statistical mechanics, and it is a bridge between thermodynamics and quantum mechanics because it is formulated as a sum over the energies of states of a macroscopic system at a given temperature.
Parts Per Million: Used to describe the amount of a trace element or substance present in the main substance.
Pascal: The SI derived unit of pressure.
Pascal′s Law: When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied.
Paschen Series: The series which describes the emission spectrum of hydrogen when the electron is jumping to the third orbital.
Passive Satellite: A satellite that reflects radio signals back to earth.
Pauli Exclusion Principle: No two electrons can have the same set of quantum numbers.
pc: An abbreviation of Parsec, 1 Parsec = 3.2615 Light Years
pdl: An abbreviation of poundal, a derived unit of force in the British system of units.
Peck: An old unit of volume.
Peltier Effect: The change in temperature produced at a junction between two dissimilar metals or semiconductors when an electric current passes through the junction.
Pendulums: There are a number of different forms of pendulum. The main characteristic being that when the mass is displaced from it's position of rest it will oscillate at a fixed frequency.
Pennyweight: An old unit of mass equal to 24 grains.
Pentavalent Impurity: A type of impurity that contains five valence electrons and donates one electron to the doped material.
penumbra: The transition region between the darkest shadow and full brightness.
Perceived Colour: Also known as visual colour is the quality of light emission as conveyed by the human eye, combining the impressions of 3 types of light-sensitive cells which the eye contains.
Perfect Gas: A gas that obeys the ideal gas law.
Periapsis: The point of an orbit nearest to the center of attraction of an orbiting body.
Pericentre: The point on a spacecrafts orbit at which it is nearest to the centre of mass of the system.
Perigee: The point of a satellite's orbit closest to Earth.
Perihelic Opposition: When the Earth passes between a planet and the Sun when the planet is at its closest from the Sun.
Perihelion: The point of closest approach for any object orbiting the sun.
Period: The time taken for a full cycle of motion in a system executing harmonic motion.
Periodic: A term used to refer to a pattern of motion which repeats exactly after a given interval or period.
Permeability of Free Space: μ₀ = 4πx10^-7 Hm^-1
Permeance: The property which determines the magnitude of magnetic flux in a material.
Permittivity: The proportionality constant between the dielectric displacement D and the electric field E.
Permittivity of Free Space: ε₀ = 8.85419x10^-12 Fm^-1
Persistence: The length of time a phosphor dot glows on a CRT before disappearing.
Peta: Prefix representing a multiplication of 10¹⁵
pF: Abbreviation of picofarad, the farad is the SI unit of capacitance.
Pfund Series: The series which describes the emission spectrum of hydrogen when the electron is jumping to the fifth orbital.
Phase: Phase identifies the position at any instant which a periodic wave occupies in its cycle. A portion of a material system whose properties and composition are homogeneous and which is physically distinct from other parts of the system.
Phase Change: The change from gas to liquid to solid.
Phase Rule: This states that the maximum number of phases (P) that may coexist at equilibrium is two plus the number of components (C) in the mixture minus the number of degrees of freedom (F): P + F = C + 2.
Phase Shift: The time or angular difference between two signals.
Phase Velocity: The velocity with which planes of equal phase, crests or troughs, progress through the medium.
phi: A letter of the Greek alphabet.
Phlogiston: A hypothetical elastic fluid which was seen as a metalizing and combustible principle.
Photoelectric Effect: The ejection of electrons from metallic surfaces by illuminating light.
Photoelectron: An electron ejected from an atom or molecule that has absorbed a photon.
Photoluminescence: Luminescence caused by absorption of optical radiation.
Photons: Massless packet of energy, which behaves like both a wave and a particle.
Photosphere: The layer of the Sun from which all visible light reaches us.
Physical Constants: From Avogadro's number to the speed of light in vacuo.
Physics Books: Lists all Physics Books in the Encyclopaedia
Physics Calculations: Lists all Physics Calculations in the Encyclopaedia
Physics Conversions: Lists all Physics Conversions in the Encyclopaedia
Physics Weblinks: Lists all Physics Weblinks in the Encyclopaedia
pico: Prefix representing a multiplication of 10^-12
picosecond: A period of time equal to 10^-12 seconds.
Picture: Visual representation of something.
Piezoelectric: Any material which provides a conversion between mechanical and electrical energy.
Pint: A derived British unit of volume.
Pion: The least massive meson.
Pixel: Single picture element of a detection device.
Planck Radiation Law: A law which described blackbody radiation better than its predecessor, thus resolving the ultraviolet catastrophe.
Plancks Constant: h = 6.62620x10^-34Js
Plane of Polarization: The plane, with respect to the earth, in which the E field propagates.
Plane Wave: Wave in which the wavefronts are everywhere parallel planes normal to the direction of propagation.
Planet: A spherical ball of rock and/or gas that orbits a star.
Planetary Nebula: A bright cloud of dust and gas surrounding an old star, namely a red giant.
Plasma: The fourth state of matter in which one or more electrons have been stripped from the atoms forming an ion gas.
Plasma Discharge: Low-density, low-temperature plasma discharge such as in a fluorescent tube.
Plasma Frequency: The natural collective oscillation frequency of a charge species in a plasma, in the absence of a magnetic field.
Plasma Wave: A disturbance of a plasma away from equilibrium, involving oscillations of the plasma′s constituent particles and/or the electromagnetic field.
Plastic Deformation: Deformation that is permanent or nonrecoverable after release of the applied load.
Plato: A Greek philosopher, pupil of Socrates and Aristotle′s teacher.
Pluto: The outermost planet in our solar system and it′s double Charon.
Poiseuilles Equation: Gives the volume flow rate of an incompressible fluid in a round pipe.
Poisson Equation: A fundamental equation of mathematical physics, describing the spatial variation of a potential function for given source terms.
Poisson Spot: Poisson originally predicted the existence of the arago spot, and used the prediction to demonstrate how the wave theory of light must be in error to produce such a counterintuitive result.
Poissons Ratio: Ratio of strain in the lateral direction to strain in the axial direction.
Polariser: A polariser is a device which absorbs all the components of an electro-magnetic field in one direction leaving only the components perpendicular to that of polariser axis.
Polarity: The positive or negative direction of an electrical, acoustical or magnetic force.
Polarization: For an atom, the displacement of the centre of the negatively charged electron cloud relative to the positive nucleus, which is induced by an electric field.
Pole: An old English unit of length or the coldest regions on a planet.
Pole Piece: The shaped magnetic material upon which the stator windings of motors and generators are mounted or wound.
Pole Strength: The concept of an isolated magnetic pole (magnetic monopole) has been abandoned. However, it is used as a concept in magnetometry. Pole strength is defined as that which when placed in a unit induction experiences a force of 1 newton.
Polycrystalline Solar Cell: Cheap to make as the conditions required to produce the silicon wafers do not need to be so tightly controlled.
Positive Temperature Coefficient: The characteristic of a conductor in which the resistance increases as temperature increases.
Positron: The antiparticle of the electron.
Potential Barrier: The internal resistance of a transistor related to the internal potential existing between NP junctions.
Potential Difference: The difference of potential produced by an electrical source to drive a current through an external electrical circuit.
Potential Energy: The energy a body possesses due to it's position or the arrangement of it's parts.
Potentiometer: A variable resistor with three terminals-one on each end of the resistor and one "wiper" which contacts the resistor midpoint to indicate a variable position.
Pound: The unit of mass in the British system of units.
Pound Force: pound-force (lbf) = 4.44822 N
Poundal: A derived unit of force in the British system of units.
Power: Power is defined as the rate of doing work.
Power Spectral Density: A method of scaling the amplitude axis of spectra of random rather than deterministic signals.
ppm: Abbreviation for Parts Per Million.
Prandtl Number: A dimensionless group often used in heat exchange calculations.
Prebiotic: Related to the period before life appears on a planet.
Precession: A modern term, derived from the precession of the equinoxes and meaning a motion around a cone of the rotation axis of a spinning body.
Precession Vector: In a gyro, a vector representing the angular change of the spin axis when torque is applied.
Precipitation: The conversion of a dissolved substance into insoluble form by chemical or physical means.
Pressure: Defined as the force exerted per unit area.
Pressure Gradient: The change in pressure per unit of distance.
Primal Glow: Believed to be the cosmologically redshifted radiation released by the Big Bang itself.
Primary Reference Temperatures: A number of boiling and freezing points are used to define the thermodynamic temperature scale.
Prime Verical Circle: Passes through the east and west points of the horizon.
Principal Quantum Number: The number related to the amount of energy an electron has and therefore describing which shell the electron is in.
Principal Vertical Circle: Passes through the north and south points of the horizon and coincides with the celestial meridian.
Principle of Uniformity: The principle that the laws of physics here and now are not different from the laws of physics in previous or future epochs of time, or elsewhere in the Universe.
Projectile Motion: A type of motion that occurs near the surface of the Earth when the only force acting on the object is that of gravity.
Prompt Neutrons: Those neutrons released coincident with the fission process, as opposed to the neutrons subsequently released.
Prony: Unit of power.
Propagation: Waves traveling through a medium.
Propeller: An assembly of radially disposed blades with an airfoil shape that when rotated in air produce thrust.
Proper Time: The time interval measured by an observer in his own reference frame.
Proportional Limit: The point on a stress-strain curve at which the straight line proportionality between stress and strain ceases.
Propulsion: Process by which something can be moved by producing a reaction with a force of thrust.
Proton: The central nucleus of an atome is formed from protons and neutrons. The nucleus of a hydrogen atom has a single proton.
Proton-Proton Chain: In the Sun and other less massive stars, this chain is the primary source of heat and radiation.
Protoplanet: Any of the sun′s planets as it emerged or existed in the formative period of the solar system.
Protoplanetary Disc: The disc of dust surrounding a star out of which planets might form.
Protosun: The sun as it emerged in the formation of the solar system.
PSD: A method of scaling the amplitude axis of spectra of random rather than deterministic signals.
psi: A letter of the Greek alphabet.
Psychrometer: An instrument used to determine the dry and wet bulb temperature of air simultaneously.
Ptolemy: He believed that Earth was the centre of the universe and that everything orbited it.
Ptolemy′s System: The explanation given by ancient Greek astronomers to the motion of planets around the sky, described in a book by the Greek Ptolemy, around 150 AD.
p-type Semiconductor: A semiconductor for which the predominant charge carriers responsible for electrical conduction are holes.
Pulley: A disc with a groove around its circumference to locate a rope or cable and is used to change the direction of a force and/or multiply the force.
Pulsar: A stellar source, such as a rotating single star or pair of stars, emitting electromagnetic radiation which is characterised by rapid frequency and regularity.
PV Diagram: A plot of pressure vs. volume.
Pyranometer: An instrument which measures global radiation, the combined intensity of direct solar radiation and diffuse sky radiation.
Pyrgeometer: An instrument which measures the effective terrestrial radiation.
Pyrheliometer: An instrument which measures the intensity of direct solar radiation.
Pyroelectricity: Electric charge produced as a material undergoes a temperature change.
Pythagoras: Greek philosopher and mathematician; held that numbers were basic to matter.
Q: The quality factor defines the sharpness of resonance.
Q-band: A frequency band used in radar extending approximately from 36 to 46 kilomegacycles.
QED: Abbreviation for quod erat demonstrandum, used to denote the end of a proof.
Quality Factor: The quality factor defines the sharpness of resonance.
Quanta: The plural of quantum, the smallest unit of a discrete property.
Quantum: The smallest unit of a discrete property.
Quantum Charge Ratio: h/e = 4.135708x10^-15 JsC^-1
Quantum Mechanics: The rules for the behavior of particles at the atomic and subatomic levels.
Quantum Number: A number giving the value of a quantized quantity.
Quark: A constituent of hadrons. Quarks come in six flavors and three colors each.
Quark Theory of the Structure of Matter: The theory that all hadrons, such as the protons and neutrons in the nucleus, are made of quarks.
Quasi Static: Refers to forces or displacements which vary slowly with time.
Quiet Sun: The sun when it is free from unusual radio wave or thermal radiation such as that associated with sunspots.
Quintal: A hundredweight in the metric system, 100kg.
R: The constant that appears in the ideal gas equation.
Rad: The rad is a unit used to measure absorbed radiation dose.
Radar: A method, system, or technique of using beamed, reflected, and timed radio waves for detecting, locating, or tracking objects.
Radial Motion: Motion along a radius, or a component in such a direction.
Radian: The radian is the plane angle between two radii of a circle which cut off on the circumference an arc equal in length to the radius.
Radiancy: The rate of radiant-energy emission for a unit area of a source in all the radial directions of the overspreading hemisphere.
Radiant Intensity: Energy of radiation striking a unit area per unit time.
Radiation: This is the electromagnetic radiation emitted by a body as a function of its temperature.
Radiation Absorbed Dose: The rad is a unit used to measure absorbed radiation dose.
Radiation Belt: An envelope of charged particles trapped in the magnetic field of a spatial body.
Radiation Field: The electromagnetic field that radiates from an antenna and travels through space.
Radiation Intensity: Another name for Radiant Intensity.
Radiation Losses: The losses that occur when magnetic lines of force about a conductor are projected into space as radiation and are not returned to the conductor as the cycle alternates.
Radio Clock: A clock that automatically synchronizes to a signal received by radio.
Radio Frequency: Any frequency of electromagnetic energy capable of propagation into space.
Radio Horizon: The boundary beyond the natural horizon in which radio waves cannot be propagated over the earth′s surface.
Radioactive Decay: The spontaneous change of an atomic nucleus accompanied by the emission of alpha, beta particles or gamma radiation.
Radioactivity: Spontaneous emission of particles or high-energy electromagnetic radiation from the nuclei of unstable atoms.
Radiometer: Instrument for detecting and measuring radiant energy.
Radiometry: A branch of science which deals with the measurement of radiation.
Radiosity Equation: Used in the construction of realistically lit scenes in computer graphics.
Ramjet: A jet engine with no mechanical compressor, consisting of specially shaped tubes or ducts open at both ends.
Random Noise: A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy.
Random Process: A random process is a collection of time-history records that can be described by appropriate statistical parameters, such as averaged properties of these records at a number of fixed times.
Rankine Temperature Scale: An absolute scale of temperature based on degrees Farenheit.
Rankine-Hugoniot Shock Relations: These are solutions for a steady, normal shock in the frame moving with the shock front.
Rarefaction: A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
Rarefied Wave: A longitudinal wave that has been expanded or rarefied as it moves away from the source.
Rayl: The Rayl is the SI derived unit of specific acoustic impedance.
Rayleigh Distribution: An example is the variation of wave height in a sea where swell is the main component.
Rayleigh Scattering: Scattering of radiation in the course of its passage through a medium containing particles, the sizes of which are small compared with the wavelength of the radiation.
Rayleigh Waves: These are also known as surface waves.
Rayleigh-Jeans Law: A law describing blackbody radiation.
Rayleigh′s Method: A method used for calculating approximate natural frequencies for a vibrating system assuming a deflected shape and balancing kinetic and strain energies.
Reaction: A reaction is a force exerted by a support on an object: sometimes called support reaction.
Reactor: Vessel which contains nuclear or chemical reactions.
Reaumur Temperature Scale: On the Reaumur temperature scale, abbreviated to R, the ice point is 0�R and the steam point is 80�R.
Reciprocity: If in a space filled with air which is partly bounded by finitely extended fixed bodies and is partly unbounded, sound waves being excited at any point A, the resulting velocity-potential at a second point B is the same both in magnitude and phase, as it would have been at A, had B been the source of the sound.
Recombination Era: The time when matter and radiation first separated.
Recrystallization: The formation of a new set of strain-free grains within a previously cold-worked material; normally an annealing heat treatment is necessary.
Red Giant: An old star that has used up all the hydrogen in its nucleus and uses other elements as fuel to keep shining.
Red Supergiant: A supergiant with spectral type M. These are the largest stars in the universe.
Redshift: When the light an object emits is displaced toward the red end of the spectrum it is said to be redshifted.
Reference System: A collection of objects not moving relative to each other that can be used to describe the motion of other objects.
Reflected Wave: The portion of the wave that has been reflected.
Reflection: Reflection occurs when a wave is incident upon an opaque surface.
Refractory: A metal or ceramic that may be exposed to extremely high temperatures without deteriorating rapidly or without melting.
Regelation: The phenomenon of melting under pressure and freezing again when the pressure is reduced.
Relative Humidity: The ratio of the absolute humidity to the humidity at saturation.
Relativistic Kinetic Energy: The energy that a body possesses solely because it is moving.
Remanence: The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remanence will be less than the residual induction.
Renewable Energy: Energy obtained from sources that are essentially inexhaustible, unlike fossil fuels.
Repeatability: The consistency (or variation) of readings and results between consecutive sets of measurements.
Repulsive Force: The mechanical force tending to separate bodies having like electrical charges or like magnetic polarity.
Resistance: The electrical resistance of a conductor.
Resistivity: The resistance measured in ohms of a unit volume of a substance to the flow of electric current.
Resolution: The smallest change or amount a measurement system can detect.
Resonant Frequency: Any system has a resonance at some particular frequency.
Resonant Line: A transmission line that has standing waves of current and voltage.
Rest Energy: Rest energy is the energy an object has due to its mass.
Retrograde: The motion of a planet or other Solar System body in a clockwise direction.
Revolutions per Minute: Unit to describe the rate of rotation of a body.
Revolutions per Second: Unit to describe the rate of rotation of a body.
Reyn: British unit of dynamic viscosity.
Reynolds Number: This is a non-dimensional number (consistent units) which describes whether fluid flow is laminar, turbulent, or transitional between the two.
RF: Abbreviation of Radio Frequency.
RF Radiation Hazard: A health hazard caused by exposure to electromagnetic radiation or highenergy particles.
RGB: Abbreviation of Red, Green, Blue.
Roche Limit: The position around a massive body where the tidal forces due to the gravity of the primary equal or exceed the surface gravity of a given satellite.
Rocket: Commonly used term for a launch vehicle.
Rod: An old English unit of length.
Rotation: Motion of an object where the path of every point is a circle or circular arc.
Round-Off Error: The error accumulated during a calculation due to rounding intermediate results.
rpm: Abbreviation of revolutions per minute.
rps: Abbreviation of revolutions per second.
Rutherford, Ernest: The pioneers of subatomic physics.
Sagnac Effect: First discovered in 1913 by Georges Sagnac, it involves sending two beams of light in opposite directions around a closed path and then using a detector to examine the beams when they arrive back at the starting point.
Sample Space: The set of all possible outcomes of a given experiment.
Sand: A term generally given to rock with a grain size between 1/16mm and 2mm.
Satellite: An object in an orbit around a planet.
Satellite Eclipse: An eclipse where the rays of the sun do not reach the satellite.
Satellite Sun Conjunction: A period when the satellite and sun are close together and the noise from the sun prevents or hampers communications.
Saturated Liquid: Purely liquid, but at the saturation temperature any additional heat will cause some vapourisation.
Saturated Solution: A solution which does not dissolve any more solute.
Saturated Vapour: Vapour in equilibrium with its liquid.
Saturation: An overwhelming concentration.
Saturation Pressure: That pressure at which vapour and liquid, or vapour and solid, can coexist in stable equilibrium.
Saturation Temperature: The temperature at which no further moisture can be added to the air water vapour mixture, equal to the dew point temperature.
Saturn: The sixth planet from the sun in our solar system.
S-Band: A band of radio frequencies extending from 1550 to 5200 MHz.
Scale Effect: Any variation in the nature of the flow and in the force coefficients associated with a change in value of the Reynolds number.
Scanning Electron Microscope: A type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern.
Scattering: The irregular diffraction and reflection of waves in many directions.
Schlieren: An optical technique that detects density gradients occurring in a fluid flow.
Schrodinger Equation: The basic equation of quantum mechanics.
Schr�dinger, Erwin: Austrian physicist; developed atomic theory of wave mechanics.
Schuler Pendulum: A hypothetical pendulum with a period of 84 minutes.
Schuler Tuning: Adjusting a system performing the function of a pendulum so that is has a period of 84 minutes.
Schwarzschild Black Hole: A nonrotating, spherical black hole that has no electric charge.
Scientific Notation: Numbers entered as a number from one to ten multiplied by a power of ten.
Scintillation Counter: A scintillation counter consists of a material that emits light when radiation passes through it.
Scruple: An old unit of weight.
Second Class Lever: The load is situated between the fulcrum and the applied force.

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