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

Lemma
A proposition that is useful mainly for the proof of some other theorem. See also: Theorem.

Lempel-Ziv Welch Compression
Algorithm used by the Unix compress command to reduce the size of files, eg. for archival or transmission. The algorithm relies on repetition of byte sequences (strings) in its input. It maintains a table mapping input strings to their associated output codes. The table initially contains mappings for all possible strings of length one. Input is ta…

Length
The SI unit of LengthThe SI unit of length is the metre and is equal to 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition 2p₁₀-5d_s of a krypton 86 atom.Conversions1 foot=0.3048 m1 fathom=1.8288 m1 mile=1609 m1 nautical mile=1852 m1 cable=219.456 m1 inch=0.0254 mttle='Length';xiunt='m';yiunt='inc…

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.Example:From a spacecraft, travelling at close to the speed of light, the lengths of objects in the outside universe appear to contract. The …

Lennard-Jones Potential
whereV = potentiale, s = positive constantsp, q = positive integersr = radiusThis equation is most often used in modelling van der Waals interactions (p=6, q=12).

Lens
A lens is an optical component whos surface curvature and refractive index are designed such that the direction of light incident upon it is controlled.Formulae exist for thin lens approximations to determine focal lengths etc.For an object at distance d whos image is formed at d'

Lenticular Galaxy
An intermediate form of galaxy, between elliptical galaxies and spiral galaxies. They possess flattened forms and galactic discs but have no spiral arms. See also: Galaxy.

Lenz's 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 fundamental fermion that does not participate in strong interactions. The electrically-charged leptons are the electron, the muon, the tau, and their antiparticles. Electrically-neutral leptons are called neutrinos. See also: Electron, Muon, Neutrino, Tau.

Level
Mechanical EngineeringThe condition where an item is perpendicular to the force of gravity.Signal ProcessingIn common usage the level of a signal is its amplitude, but strictly speaking the term should be reserved for the amplitude expressed on a decibel scale relative to a reference value.

Lever
One of the basic tools that date from prehistoric times. Simply a beam (lever) is used to move a load with a fulcrum (pivot) and an applied force. The position of the three determines the mechanical advantage.First Class LeverWith this type of lever the fulcrum is situated between the load and the applied force.Example: Pliers.Second Class LeverWit…

Lever Rule
Mathematical expression whereby the relative phase amounts in a two-phase alloy at equilibrium may be computed. See also: Alloy.

Levorotatory
A chiral molecule which rotates plane-polarized light to the left. See also: Chirality.

Leyden Jar
An early form of capacitor. See also: Capacitor.

Libration
The apparant rocking motion of an orbiting body. The Moon exhibits longitudinal, latitudinal and diurnal libration. See also: Keplers Laws.

Lift
The lifting force on a flying object (in particular, a wing or an aircraft), due to its motion relative to the surrounding air. Lift is one of the four forces sensed by an airplane, the others being drag, thrust and weight.The lift generated by a wing is based on the principle that the pressure in a fluid decreases as its velocity increases (Bernou…

Lift Coefficient
A dimensionless value that allows the comparison of lift incurred by different sized and different shaped bodies. A positive lift coefficient represents lift, whilst a negative lift coefficient represents downforce. See also: Drag, Lift, Pressure Coefficient.

Lifting Body
An aircraft that uses the shape of its body to generate lift instead of using wings.

Light
Electromagnetic radiation that is visible to the unaided human eye.

Light Dependent Resistor (LDR)
A layer of Cadmium Sulphide material that changes resistance according to the amount of light falling on it. See also: Resistor.

Light Emitting Diode (LED)
A diode that emits light when current is passed through it.It has two leads: cathode (k) and anode. When forward-biased, free electrons combine with holes near thejunction. As they move from an area of higher energy to lower energy, they emit radiation.The light that is emitted is incoherent. Symbol :

Light Ray
A line that represents the path of light in a given direction. See also: Fermat's Principle, Young's Double-Slit Experiment.

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. Even the molecules of air are large enough to scatter light; in 1871, Lord Rayleigh showed that this accounts for the blue sky. See also: Tyndall Effect.

Light Year
The light year is defined as the distance travelled by electromagnetic radiation in one tropical year.Abbreviation ly1 light year = 9.46052973x10¹⁵mttle='Distance';xiunt='m';yiunt='light year';mconv=9.46052973e15;cconv=0.0; See also: Astronomical Unit.

Lignite
A type of coal with lowest carbon content (25% to 35%) and a low heat value called 'brown coal', used mainly for electric power generation.

Lignum Vitae
A very hard wood. Used for machinery bearings that are constantly under water, sheaves of pulley blocks and for purposes where great hardness and strength are required.

Limit
The number you approach as you plug values into a function, and the values get closer and closer to a given number. See also: Infinite, Infinitesimal.

Limit of Resolution
The limit to the performance of a lens imposed by the diffraction pattern resulting from the finite aperture of the optical system. See also: Resolution.

Limiting Reagent
The reactant that will be exhausted first.

Line
Line is one of the basic undefined terms in Euclidian geometry, but one intuitive definition is as follows: A straight set of points that continue on to infinity in both directions. Although a line is of infinite length, it has thickness and width of zero. See also: Plane, Point, Straight Line.

Line of Action
The line of action of a force is the infinite line defined by extending along the direction of the force from the point where the force acts. See also: External Force, Force.

Line Segment
A piece of a line which includes two definite endpoints and all points in between them.

Line Spectra
Spectra generated by excited substances. Consists of radiation with only specific wavelengths. See also: Continuous Spectrum.

Line Vortex
Idealized vortex in which vorticity is zero everywhere except along a line in space where it is infinite. See also: Vortex.

Linear Density
Mass per unit length.Conversions1 lb yard^-1=0.496055 kg m^-1ttle='Linear Density';xiunt='kg m^-1';yiunt='lb yard^-1';mconv=0.496055;cconv=0.0; See also: Density, Length, Mass.

Linear Equations
A first order differential equation is linear if it has the form

Linear Momentum
A vector quantity equal to the product of an object's mass and its velocity.wherep = momentum [kgms^-1]m = mass [kg]v = velocity [ms^-1]

Linear Phase Response
Any system which accurately preserves phase relationships between frequencies, i.e., that exhibits pure delay. See also: Group Delay.

Linear System
A system is linear if for every element in the system, the response is proportional to the excitation. This definition implies that the dynamic properties of each element in the system can be represented by a set of linear differential equations with constant coefficients, and that for the system as a whole superposition holds.

Linear Thermal Expansion
Most substances expand when heated and contract when cooled.whereL = length [m]L₀ = original length [m]a = linear expansion coefficient [K^-1]?T = temperature change [K]

Links
Our links to sites have now all moved to our Weblinks section where they are managed and ordered by subject.If you would like to swap links with us please email us at Editor@DiracDelta.co.uk with details and we will review your site and get back to you. See also: Weblinks.

Linseed Oil
Obtained from the seeds flax. Originally used in the production of paints.

Lip of a Drill
The sharp cutting edge on the end of a twist drill. See also: Drill Bits.

Lipowitz Alloy
A low melting point alloy used to form shields in radiotherapy.

Liquefaction
The transformation of a gas into a liquid. See also: Gas, Liquid.

Liquefied Natural Gas
Natural gas that has been refrigerated to cryonic temperatures where the gas condenses into a liquid. See also: Compressed Natural Gas, Gas.

Liquid
Matter with a definite volume that takes the shape of its container. See also: Fluids, Gas, Melting Range, Saturated Liquid, Solid.

Liquidus
In a constitution or equilibrium diagram, the locus of points representing the temperatures at which thevarious compositions in the system begin to freeze on cooling or to finish melting on heating.

LISP
LISt Processing. Invented by John McCarthy, one of the pioneers of artificial intelligence. The language is ideal for representing knowledge. Suitable for symbolic and logical programming. See also: Programming Languages.

Lissajou's Figures
The pattern traced out when the displacements of two simple harmonic motions are traced in the x and y direction. The displacements may be written as:wherex = displacement on x axisy = displacement on y axisA₁, A₂ = amplitudes?₁, ?₂ = angular frequencyf₁, f₂ = phase

Lithium
Lithium is a soft, silver metal, and is the first member of the alkali group of metals. It is the lightest of all metals and, whilst resembling sodium chemically, it is less active. As is the case for other alkali group elements, it is obtained by the electrolysis of the fused halides. Lithium reacts slowly in water and in air where it eventually f…

Lithium I Bromide
White crystalline solid. Symbol LiBr

Lithium I Chloride
White crystalline solid. Symbol LiCl

Lithium I Fluoride
White crystalline solid. Symbol LiF

Lithium I Hydride
White to grey crystalline solid. Symbol LiH

Lithium I Iodide
White crystalline solid. Symbol LiI

Lithium I Nitride
Purple or red crystalline solid. Symbol Li₃N

Lithium I Oxide
White crystalline solid. Symbol Li₂O

Lithium I Peroxide
White crystalline solid. Symbol Li₂O₂

Lithium Ion Battery
Lithium ions are added to a carbon electrode instead of using metallic lithium as the anode.

Lithium Polymer Battery
The battery has a lithium anode that is separated from the cathode by a thin polymer electrolyte. See also: Battery, Lithium, Lithium Ion Battery.

Lithium Superoxide
Yellow crystalline solid. Symbol LiO₂

Litre
The litre is a derived SI unit of volume.Conversions1 stere (st)=1 m³1 litre=0.001 m³1 cubic yard=0.764555 m³1 cubic inch=16.3871 cm³1 UK gallon=4.54609 litres1 US gallon=3.78541 litresttle='Volume';xiunt='pint';yiunt='litre';mconv=1.75975;cconv=0.0; See also: decilitre, SI Units, Volume.

Little Endian
A byte ordering system where the least-significant byte of a multiple bytenumber is placed in memory at the lowest address. See also: Big Endian, Byte.

Live Room
Room characterized by a relatively small amount of sound absorption. See also: Warmth.

lnduction Hardening
Quench hardening in which the heat is generated by electrical induction. See also: Eddy Currents, Hardenability, Hardening.

lnterrupted Quenching
Quenching in which the metal object being quenched is removed from the quenchingmedium while the object is at a temperature substantially higher than that of the quenching medium. See also: Direct Quenching, Quenching.

Load
ComputingTo transfer a computer program or data from a backup device to the computer memory.ElectricalA device or impedance that terminates the output of a device or transmission line. A load is typically a resistance, the magnitude of which is equal to the characteristic impedance of the transmission line.Mechanical EngineeringThe weight supported…

Local Stress Relieving
Stress relieving heat treatment of a specific portion of a structure. See also: Stress.

Localization Accuracy
Our ability to detect changes in localization or lateralization is not absolute. Experimentally, it ismeasured as the minimum audible angle, but is also called localization blur. The minimum audible angle can be determined for both horizontal and vertical directions. Spatially, our most acute sense of localization is directly in front (0° azimuth).…

Locus
The set of all points meeting some specified condition.

Logarithm
Allow large number ranges to be expressed over a smaller range of numbers, eg decibel scale. Also, multiplication and division may be expressed as simple addition and subtraction. Invented by Scottish mathematician John Napier in 1614.The power to which a base must be raised to yield a given number, usually abbreviated to;where b is the base. Commo…

Logic
The study of the formal laws of reasoning. See also: Theorem.

Logistic Equation
A classical starting point for the dynamics of quasiperiodic and chaotic systems. It describes the fluctuations in populations of animals as functions of their birth rate and the available resources. The addition of terms related to predation leads to predator-prey models, widely studied in computational biology.

Lombard Effect
As noise levels increase, talkers increase their voice levels to compensate and to maintain adequate conditions for verbal communication. See also: Background Noise.

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. See also: Earth, Latitude.

Longitudinal Wave
A wave in which the vibrations of the medium are parallel to the directionthe wave is moving.Sound waves propagating through air are a good example.In longitudinal waves, such as sound waves, wavelength is the distance from rarefaction to rarefaction, or compression to compression.

Lorentz Transformation
A set of equations which are used, in relativity problems, to transform measurements from one frame of reference to another moving with respect to each other along axis x.andwherev = velocity of (v', v', v') system along the x-axis [ms^-1]t = time [s]c = speed of light = 2.998x10⁸ [ms^-1]or withandthese can be re-writ…

Lossy Medium
A medium which absorbs or scatters radiation passing through it.

Loudness
A subjective term for the sensation of the magnitude of sound. Generally a 10dB increase in sound pressure level is perceived as a doubling of subjective loudness. At low frequencies (20Hz to 200Hz) an increase of as little as 5-6 dB is perceived as a two-fold increase in the subjective loudness. See also: Phon, Sound Pressure Level, Zwicker Loudne…

Loudspeaker
An electroacoustical transducer that changes electrical energy to acoustical energy. See also: Crossover Frequency, Subwoofer, Toe-In, Woofer.

Lowest Common Denominator
The smallest number that is exactly divisible by each denominator of a set of fractions. See also: Fraction.

lsothermal Annealing
Austenitizing a ferrous alloy and then cooling to and holding at a temperature at whichaustenite transforms to a relatively soft ferrite carbide aggregate. See also: Austempering.

Luder Bands
Elongated surface markings or depressions in sheet metal caused by discontinuous yielding.

Lumen
The derived SI unit of luminous flux, defined as the luminous flux emitted by a uniform point source of 1 candela [cd] emitting its luminous energy over a solid angle of 1 steradian [sr]; it thus has units of cd sr. The lumen is often abbreviated to lm. See also: Illuminance.

Lunar Month
1 lunar month = 2.551444x10⁶sttle='Lunar Month';xiunt='seconds';yiunt='lunar month';mconv=2.551444e6;cconv=0.0; See also: Time.

Lune
The portion of a sphere between two great semicircles having common endpoints, including the semicircles. See also: Sphere.

Lutetium
Lutetium is the hardest, densest and one of the rarest of the lanthanide group of elements. It is found in some of the less common minerals (e.g. gadolinite and xenotime) and is difficult to isolate. Applications of lutetium tend to be limited to research only. Symbol LuDiscovered1907 by G. Urbain in Paris, France, and, independently, by C. James a…

Lyman Series
The series which describes the emission spectrum of hydrogen when electrons are jumping to the ground state. All of the lines are in the ultraviolet. See also: Balmer Series, Hydrogen, Paschen Series.

Mach Number
The ratio of the speed of a body or flow of a fluid to the local speed of sound in the fluid.whereM = Mach NumberU = characteristic velocity, ms^-1c = speed of sound, ms^-1

Machine
Any mechanical or electrical device that performs a task for people.

Machine Tool
A power-driven machine designed to bore, cut, drill, or grind metal or other materials.

Machinery Vibration
The vibration of rotating machinery may infer certain defects. The spectrum of the vibration can also be used like a finger print.FrequencyPossible causes1 x operating speedImbalanceMisalignmentBent shaftLoosenessElectrical

Macromolecule
A huge molecule made up of thousands of atoms.

Macroscopic
Anything big enough to be seen with the naked eye. See also: Microscopic.

Macrostructure
The structure of metals as revealed by examination of the etched surface of a polished specimenat a magnification not exceeding ten diameters. See also: Microstructure.

Madelung Constant
A constant representing the sum of the mutual potential coulombic attractive energyof all the ions in a lattice in the equation for the energy of an ionic crystal.1.747564594633182190636212035544 .......ttle='Madelung Constant';consttxt='Madelung constant';constval=1.74756459463318219063621;constunt='';

Magic Square
A square array of n numbers such that sum of the n numbers in any row, column, or main diagonal is a constant (known as the magic sum).816357492

Magic Tour
If a chess piece visits each square of a chessboard in succession, this is called a tour of the chessboard. If the successive squares of a tour on an n x n chessboard are numbered from 1 to n², in order, the tour is called a magic tour if the resulting square is a magic square. See also: Knight's Tour, Magic Square.

Magnesium
Magnesium is a brilliant white metal, which is relatively soft. It is one of the more abundant elements, there being 23000 ppm in the earth`s crust. As a powder, magnesium is extremely reactive, but as a solid it oxidises slowly in air and reacts slowly in water. It does not occur naturally, but is found in combination with other elements in minera…