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DiracDelta - Science & Engineering encyclopedia Category: Sciences > Science & Engineering Date & country: 01/12/2007, UK Words: 4470

Aa
A Hawaiian term for lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior. See also: Volcanoes.

ab
A prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. See also: Abampere, Abcoulomb, Abfarad, Abhenry, Abmho, Abohm, Abvolt, abwatt, CGS Units.

Ab Initio
Latin meaning 'from first principles'.

Abampere
The CGS electromagnetic unit of current equal to 10 Amperes.ttle='Abampere';xiunt='abampere';yiunt='ampere';mconv=0.1;cconv=0.0;

Abaxial
Rays of light which do not coincide with the optical axis of a lens system. See also: Lens.

Abbe Number
The ratio of refractivity to dispersion in an optical medium.Specifically:wheren_d,F,C = index of refraction for the Fraunhofer d, F and C lines, respectively.

Abbe Refractometer
An instrument for measuring directly the refractive index of liquids, minerals and gemstones. See also: Abbe Number, Refractive Index.

Abcoulomb
The CGS unit of electric charge equal to 10 Coulomb.ttle='Abcoulomb';xiunt='abcoulomb';yiunt='coulomb';mconv=0.1;cconv=0.0;

Abel Flashpoint Apparatus
A petroleum-testing apparatus for determining the flash point. See also: Flash Point, Petroleum.

Abelian Group
A mathematical group of transformations with the property that the end result of a series of transformations does not depend on the order in which they are performed.

Abend
A procedure to halt a computer program prematurely, ABnormal END.

Aberration
A defect in a mirror or lens causing light rays from a single point to fail to focus at a single point in space. See also: Annual Aberration, Astigmatism, Chromatic Aberration, Diurnal Aberration, Spherical Aberration.

Abfarad
The CGS unit of capacitance equal to 1.0x10⁹Farad.ttle='Abfarad';xiunt='farad';yiunt='abfarad';mconv=1e9;cconv=0.0;

Abhenry
The CGS unit of inductance equal to 1.0x10^-9Henry.ttle='Abhenry';xiunt='henry';yiunt='abhenry';mconv=1e-9;cconv=0.0;

Abhesive
A substance which prevents two materials sticking together, e.g., teflon on frying pans. See also: Adhesion, Adhesive, Glue.

Ablate
To carry away. Specifically, to carry away heat generated by aerodynamic heating, from a vital part, by arranging for its absorption in a non-vital part, which may melt or vaporize, then fall away taking the heat with it. See also: Ablating Material.

Ablating Material
A material, especially a coating material, designed to provide thermal protection to a body in a fluid stream through loss of mass.Ablating materials are used on the surfaces of some re-entry vehicles to absorb heat by removal of mass, thus blocking the transfer of heat to the rest of the vehicle and maintaining temperatures within design limits. A…

Abmho
The CGS unit that is the reciprocal of the ohm equal to 1.0x10⁹Siemens.ttle='Abmho';xiunt='siemens';yiunt='abmho';mconv=1e9;cconv=0.0;

Abney Level
Hand-held instrument in which angles of steep sights are measured while simultaneously viewing a spirit-level bubble. See also: Level.

Abohm
The CGS unit of electrical resistance equal to 1.0x10^-9Ohms.ttle='Abohm';xiunt='ohm';yiunt='abohm';mconv=1e-9;cconv=0.0;

Abort
To terminate or stop a process prematurely.

Abraded
Pertaining to a surface that has been worn by surface rubbing. See also: Abrasive.

Abrasion
A process where hard particles are forced against and moved along a solid surface. See also: Abrasion Hardness, Abrasive, Abrasive Wear.

Abrasion Hardness
Resistance to abrasive wear, under specified conditions, of metal or mineral. See also: Abrasion, Hardness.

Abrasive
A very hard, brittle, heat-resistant substance that is used to grind the edges or rough surfaces of an object. Boron carbide, diamond, and corundum are abrasives.Natural Abrasivessandstone, emery, corundum, diamondsArtificial Abrasivessilicon carbide, aluminium oxide

Abrasive Cleaner
A cleanser with some hard grit used to remove the grime and oils from a surface. See also: Abrasive.

Abrasive Wear
A mechanism of wear due to the presence in one or both surfaces of hard particles. See also: Abrasion.

Abrasive Wheel
A grinding wheel composed of an abrasive grit and bonding agent. See also: Abrasive, Angle Grinder, Dressing.

Abscissa
The x coordinate on an (x, y) graph. The input of a function against which the output is plotted. y is the ordinate.The sign convention is that measurements to the right from the axis of ordinates are positive, measurements to the left negative. See also: Ordinate.

Absolute Accuracy
A measure of the uncertainty of an instrument reading compared to that of a primary standard traceable to a National standard.

Absolute Age
The geological age of a fossil, mineral, rock or event, generally given in years.

Absolute Ceiling
The maximum altitude above sea level at which a heavier-than-air craft can be maintained in level flight.

Absolute Humidity
The total mass of water vapour present in the air per unit volume, generally given in g/m³. See also: Dew Point, Humidity, Humidity at Saturation, Relative Humidity, Temperature.

Absolute Instrument
An instrument whose calibration can be determined by means of physical measurements on the instrument.

Absolute Pitch
The exact pitch value of a musical note (for example, middle C) as opposed to its position relative to other pitches. The ability to identify any musical note or to produce orally any named note is quite rare. See also: Pitch.

Absolute Pressure
Pressure measured from a starting point of 0 in perfect vacuum. See also: Pressure.

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. In the SI system, the kelvin scale is used to measure absolute temperature.

Absolute Unit
A unit defined in terms of fundamental quantities (such as length, mass, time, and electric charge). See also: SI Units.

Absolute Vacuum
A volume which contains no matter, also called perfect vacuum. An absolute vacuum is not obtainable. See also: Vacuum.

Absolute Value
The positive value for a real number, disregarding the sign.Examples: 3 =3 -4 =4 0 =0.

Absolute Zero
This is the lowest possible temperature for any substance -273.15°C or 0 Kelvin, 0 Rankine. Electrons stay in motion and bonds continue to vibrate even at absolute zero because of zero point energy. Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold so…

Absorb
To transform radiant energy into a different form, with a resultant rise in temperature. See also: Absorption.

Absorbed Dose
The amount of a chemical that enters the body of an exposed organism.

Absorbent Ducts
Practical problems:Settling of liner fill materialsThe fibres of absorbent materials have a tendancy to settle with time resulting in the properties of the absorbent chamber as a whole changing. This may not be for the worse.Temperature and fire retardationMaterials used for acoustic absorption typically have a low melting/flash point (~500°C) or d…

Absorbent Material
A material which absorbs energy from an incident sound wave and therefore can beused to reduce the amount of noise reflected from walls or roofs. There are three basicforms of sound absorbent materials:Porous materials (e.g. mineral wools, felt). Consisting of networksof interconnecting pores. In the narrow channels and cavities viscous loses turns…

Absorption
Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength. See also: Absorb, Absorption Coefficient, Extinction.

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. See also: Absorption Spectroscopy.

Absorption Coefficient
AcousticsThe fraction of sound energy that is absorbed at any surface. It has a value between 0 and 1 and varies with the frequency and angle of incidence of the sound.Measurement MethodsKundt's tubeUsing this method it is only possible to measure normal incidence absorption coefficient. A further development of this is the two microphone measureme…

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. See also: Absorption Spectrum.

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. See also: Absorption Band, Absorption Spectrum.

Absorption Spectrum
The collection of wavelengths missing from a continuous distribution of wavelengths. Caused by the absorption of certain wavelengths by the atoms or molecules in a gas.Absorption spectra are unique for each element and compound and they are often used as chemical fingerprints in analytical chemistry. The spectrum can represented by a plot of either…

Abundance Ratio
The ratio of the number of atoms of an isotope to the number of atoms of another isotope of the same element in a sample. See also: Isotopes.

Abundant Number
A positive integer that is smaller than the sum of its proper divisors. See also: Deficient Number, Divisor, Integer.

Abutment
A part which stops the motion of another part from proceeding any farther.A cement raised shoulder secured to the side of the road to prevent a vehicle from going over the edge.The contact made between opposing teeth of two gears.

Abutting Edge
The side or edge of a panel which joins another panel.

Abvolt
The CGS unit of electromotive force equal to 1.0x10^-9Volt.ttle='Abvolt';xiunt='volt';yiunt='abvolt';mconv=1e-9;cconv=0.0;

Abwatt
The CGS unit of power equal to 1.0x10^-7Watt.ttle='Abwatt';xiunt='watt';yiunt='abwatt';mconv=1e-7;cconv=0.0;

AC Brownout
The condition that exists when the ac line voltage drops below some specified value. See also: Alternating Current.

AC Coupling
Circuit that passes an AC signal while blocking a DC voltage.

AC Induction Motors
These are generally used for constant speed applications where a fixed frequency power source such as 60 Hz or 400 Hz is available. Typical applications are fans and pumps. Motor construction consists of windings on the stationary part of the motor and copper shorting bars on the iron laminations of the armature. The AC voltage applied to the windi…

AC Line
The set of conductors that route ac voltage from one point to another. See also: Alternating Current.

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. See also: Alternating Current.

Acacia
A durable wood used for palings and posts. A lot of the gums in common use are obtained from varieties of the Acacia.

Acausal
If a system is acausal it means the output begins before the input. See also: Causality Principle.

Accelerance
For a point excitation of a mechanical system this is the complex ratio of acceleration to applied force. A lumped mass, m has accelerance 1/m. It is commonly shown as a function of frequency in which acceleration is the output and force is the input. See also: Apparent Mass, Dynamic Stiffness, Receptance.

Accelerate
To increase the speed of an object. See also: Acceleration.

Acceleration
The change in velocity divided by the time it takes to make the change.An acceleration can result from a change in speed, a change in direction, or both.Negative acceleration is called deceleration.function init(){}There are four basic equations that describe the motion of a body moving with constant acceleration.

Accelerator
ChemistryA substance that makes vulcanization of rubber occur more quickly or at a lower temperature. A substance that makes crosslinking in a polymer occur more quickly or at a lower temperature.PhysicsA machine used to accelerate particles to high speeds and thus high energy compared to their rest-mass energy.

Accelerator Pedal
A pedal that when pressed is connected to the engine and demands more torque. Also known as gas pedal, or throttle pedal.

Accelerometer
An instrument for measuring acceleration, as of aircraft or guided missiles.The most common type is probably the piezoelectric accelerometer which is widely used in the measurement of machinery vibration. The output is usually produced by force applied to a piezoelectric crystal which generates a current proportional to the applied force. This curr…

Acceptance Test
An examination of a part or its assembly to determine if it meets a prescribed standard.

Acceptor
An atom which is likely to take on one or more electrons when placed in a crystal. Contrast with donor.A material that is intentionally added to a pure semiconductor material to increase the population of free holes in that semiconductor, resulting in a net positive charge. A semiconductor that has had acceptor material added to it is called 'p-typ…

Access
A way of reaching something that is usually hidden or covered.

Access Time
The average time interval between a storage peripheral (eg a disk drive or semiconductormemory) receiving a request to read or write a certain location and returning the value read or completing the write.

Accessible Emission Level
Magnitude of accessible laser or collateral radiation of a specific wavelength or emission duration at a particular point as measured by appropriate methods and devices.Also means radiation to which human access is possible in accordance with the definitions of the laser's hazard classification.

Acclimatization
The adjustments of a human body or other organism to a new environment; the bodily changes which tend to increase efficiency and reduce energy loss.

Accretion
The process by which the terrestrial planets grew, increasing their mass by gradually accumulating smaller bodies, called planetesimals. See also: Accretion Disc.

Accretion Disc
An accretion disc is a disc-shaped rotating mass formed by gravitational attraction. Such as around a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation. See also: Accretion.

Accumulator
ComputingA register within a microprocessor that is the source or destination of data for machine instructions.ElectronicsA device by which energy or power can be stored, such as the cylinder or tank for storing water for hydraulic elevators, the secondary or a storage battery used for storing the energy of electrical charges, etc.

Accuracy
Accuracy is the correctness of a single measurement. The accuracy of a measurement is assessed by comparing the measurement with the true or accepted value, based on evidence independent of the measurement. The closeness of an average to a true value is referred to as 'trueness'.Typically described in terms of a maximum percentage of deviation expe…

Acenaphthene
Found in petroleum residues. It is used as a dye intermediate, in manufacturing plastics, and as an insecticide and fungicide.

Acenaphthylene
A polycyclic aromatic hydrocarbon.

Acetaldehyde
Formed by the oxidation of ethanol this is a colourless fruity smelling liquid that with further oxidation forms acetic acid. Symbol C₂H₄O Also known as Ethanal

Acetamide
Colourless crystalline used industrially as a solvent. Symbol C₂H₅ON Also known as Ethanamide

Acetanilide
A white crystalline solid. Symbol C₈H₉ON Also known as Antifebrin

Acetic Acid
The acid contained in vinegar. This is a colourless corrosive liquid with a pungent smell. It is a simple organic acid that gives vinegar its characteristic odour and flavour. Glacial acetic acid is pure acetic acid. Symbol C₂H₄O₂ Also known as Ethanoic Acid

Acetic Anhydride
The anhydride of acetic acid, used in the manufacture of plastics. Symbol C₄H₆O₃

Acetone
Often used as a solvent. Used in acetylene cylinders to dissolve and stabilize acetylene under high pressure. Symbol C₃H₆O Also known as Dimethyl Ketone, Propanone

Acetonitrile
A colourless poisonous liquid, used as a solvent and in organic synthesis. Symbol C₂H₃N Also known as Methyl Cyanide

Acetophenone
Colourless sweet smelling liquid. Symbol C₈H₈O Also known as Acetylbenzene, Phenyl Methyl Ketone.

Acetyl Chloride
A colourless pungent liquid. Symbol C₂H₃OCl Also known as Ethanoyl Chloride

Acetyl-Salicyclic Acid
Commonly known as Aspirin, a white solid. Used in medicine as an antipyretic and analgesic. Symbol CH₃COOC₆H₄COOH Also known as Aspirin

Acetylene
Made by the action of water on calcium carbide. Used for welding as the flame temperature is roughly 3300°C when burnt with oxygen. Symbol C₂H₂ Also known as Ethyne

Acheson Process
The Acheson process is an industrial process for the manufacture of graphite by heating coke mixed with clay. The reaction involves the production of silicon carbide, which loses silicon at 4150°C to leave graphite. The process was patented in 1896 by the American inventor Edward Acheson.

Achromatic Lens
Lens for which all light colours have the same focal length made from two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration.

Acid
Compound that gives off H⁺ ions in solution.pH measures the acidity of a solution, the lower the pH, the more acid the solution. A pH of 7 corresponds to a neutral solution.pH0 hydrochloric acid, sulphuric acid1stomach acid 2battery acid 3vinegar lemon juice4tomato juice 5acid rain, black coffee human skin6 saliva, milk7distilled water S…

Acid Halide
Compounds containing a carbonyl group bound to a halogen atom. See also: Halogen.

Acid Rain
Rain that is weakly acidic due to pollution. The pollution can be from the burning of coal in power stations or from car exhausts. See also: Acid, Erosion, Weathering.

Acid-Base Indicator
A weak acid that has acid and base forms with sharply different colours. Changes in pH around the acid's pKa are indicated by colour changes.