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

Pure Substance
A form of matter that has both definite composition and distinct properties.

Pure Tone
A tone with a sinusoidal waveform is called a pure tone because it is considered to be the simplest form of tone and sounds pure when played in isolation. See also: Sine, Tone, Wave.

Putrescine
Related to cadaverine and produced by the breakdown of amino acids in living and dead organisms. Symbol C₄H₁₂N₂ Also known as 1,4-diaminobutane, butanediamine, putrescin, putrescene

Pyrex Glass
A type of glass that is resistant to heat and chemical attack. Often used in cookware and in laboratory equipment.

Pyroelectricity
Piezoelectric materials are also pyroelectric. They produce electric charge as they undergo a temperature change. When their temperature is increased, a voltage develops having the same orientation as the polarization voltage. When their temperature is decreased, a voltage develops having an orientation opposite to the polarization voltage, creatin…

Pyrolyser
An instrument that breaks complex molecules into constituents by using heat. See also: Molecule.

Pyrometer
A device for measuring the high temperatures in a furnace. See also: Thermometer.

Pythagoras (581-497 B.C.)
Greek philosopher and mathematician; held that numbers were basic to matter; the Pythagorean Theorem is named for his geometric formulation; developed atomic theory; students of his philosophy emphasized geometrical form as a basic property of atoms; developed mathematical relationships which led to musical harmony.530BC Devised Pythagoras' theorem…

Pythagoras Theorem
The Square on the Hypotenuse is equal to the sum of the squares of the other two sides.

Pythagorean Triangle
A right triangle whose sides are integers. See also: Integer, Primitive Pythagorean Triangle, Pythagoras, Pythagoras Theorem, Triangle.

Pythagorean Triple
An ordered set of three positive integers (a,b,c) such that a²+b²=c². See also: Pythagoras, Pythagoras Theorem.

QED
Abbreviation for quod erat demonstrandum, used to denote the end of a proof.

Quad
Unit of energy referred to as “quadâ€? or “quadrillion.â€? It is equal to 1x10¹⁵ BTU or 1.825x10⁸ barrels of oil equivalent.ttle='Quad';xiunt='J';yiunt='Quad';mconv=1.05505585e+18;cconv=0.0; See also: Energy, Joule.

Quadratic Equation
An equation of the second order.the solution to this is

Quadridecagon
A 14 sided polygon. See also: Polygon.

Quadrilateral
A four sided polygon. See also: Kite, Polygon.

Quadrillion
A number equal to 10¹⁵. See also: Quintillion, Trillion.

Quality Factor
The quality factor defines the sharpness of resonance.It is defined as:whereQ = quality factor?₀ = resonance frequency [rads^-1]?₁ = is the frequency below resonance for which the average power has dropped to half that at resonance [rads^-1]?₂ = is the frequency above resonance for which the aver…

Quantization Error
When a continuous time signal is digitized, because there isn't an infinite number of discrete digital levels, the difference between the actual analog value and the digital representation of that value is defined as the quantization error.With a 3 bit ADC this gives 2³ = 8 levels.With a 5 bit ADC this gives 2⁵ = 32 levels. Wi…

Quantum
The smallest unit of a discrete property. For instance, the quantum of charge is the charge on the proton.

Quantum Charge Ratio
h/e = 4.135708x10^-15 JsC^-1ttle='Quantum Charge Ratio';consttxt='h/e';constval=4.135708e-15;constunt='JsC^-1'; See also: Physical Constants.

Quantum Mechanics
The rules for the behavior of particles at the atomic and subatomic levels. See also: Born, Max, Bose-Einstein Statistics, Casimir Effect, Partition Function, Schrodinger Equation.

Quantum Number
A number giving the value of a quantized quantity. For instance, a quantum number specifies the angular momentum of an electron in an atom. See also: Angular Momentum, Electron.

Quark
A constituent of hadrons. Quarks come in six flavors and three colors each. Three quarks make up the baryons, whereas a quark and an antiquark make up the mesons See also: Antiquark, Baryon, Bosons, Bottom, Charm, Down Quark, Gluon, Kaon, Strange, Strange Particle, Top.

Quart
A unit of volume. In the British imperial system of units 1 quart is equal to 2 pints.Conversions1 quart (UK)=1.136522x10^-3m³1 quart (US dry)=1.101221x10^-3m³1 quart (US liquid)=946.3529x10^-6m³ttle='Volume';xiunt='quart (UK)';yiunt='m³';mconv=879.877380288283;cconv=0.0; See…

Quarter
One of four equal parts. Note: a quarter is also an outdated British unit of weight 1 quarter = 28 pounds (lb). See also: Half, Pound.

Quarter Wave Tube
A resonator used most commonly on air intake systems to reduce resonance. Works over a narrow frequency range, i.e. used to attenuate a specific frequency.The quarter wave tube is open at the end that connects with the air inlet pipe and closed at the other end. The frequency at which the quarter wave tube attenuates is controlled by the length of …

Quartet
A group of four items. See also: Triple.

Quartic
A polynomial equation of degree 4. See also: Quintic.

Quartiles
Statistics which divide the observations in a numeric sample into 4 intervals, each containing 25% of the data. The lower, middle, and upper quartiles are computed by ordering the data from smallest to largest and then finding the values below which fall 25%, 50%, and 75% of the data.The middle quartile is usually called the median.Quartiles are sp…

Quartz
Photograph of a quartz molecular model, courtesy of Indigo® Instruments.Found naturally as faceted crystals. Its birefringent properties are used in the production of optical quarter-wave plates and polarizers. A mineral whose primary component is silicon dioxide (SiO₂). Its colour and other aspects of its appearance depended on the impu…

Quasi Static
Refers to forces or displacements which vary slowly with time. A force is considered to vary slowly if the frequency of variation is much lower than the lowest natural frequency of the system it is applied to.

Quaternary
The geological period covering about 2 million years ago to the present.

Quench Hardening
Hardening a ferrous alloy by austenitizing and then cooling rapidly enough so that some or allof the austenite transforms to martensite. The austenitizing temperature for hypoeutectoid steels is usually abovethe temperature at which transformation of ferrite to austenite is completed during heating and for hypereutectoid steels usually between Acl …

Quenching
Rapid cooling. When applicable, the following more specific terms should be used: direct quenching,jog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching and time quenching.The quenching media may be water, brine, oil, special solutions, salts or metals; and the intensity of the quench is determined by the tempera…

Quiescent Current
The standing current that flows in a circuit when the signal is not applied. The quiescent current is usually very low or lower than when processing a signal. See also: Current.

Quintal
A hundredweight in the metric system, 100kg.ttle='Quintal';xiunt='kg';yiunt='quintal';mconv=100.0;cconv=0.0; See also: Hundredweight, Mass, Short Hundredweight.

Quintic
A polynomial equation of degree 5. See also: Quartic.

Quintile
Statistics which divide the observations in a numeric sample into 5 intervals. See also: Quartiles.

Quintillion
A number equal to 10¹⁸. See also: Quadrillion, Sextillion.

Quotes
Famous quotes:'Science has 'explained' nothing; the more we know the more fantastic the world becomes and the profounder the surrounding darkness.'A Huxley'Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity.'Albert Einstein'Relativity teaches us the…

Quotient
The result when one number or quantity is divided by another. See also: Division.

RAC Horsepower Rating
An obsolete engine horsepower rating system that was used as a designation as to the power of an engine.The indicated brake horsepower per cylinder can be calculated if the mean effective pressure in the cylinder is known. This pressure is the mean of the varying pressures acting on the piston. Being the mean value, it is that notional unvarying pr…

Racemic Mixture
A mixture that has equal amounts of left- and right-handed enantiomers of a chiral molecule. See also: Chirality, Enantiomers, Stereoisomerism.

Racking
The distortion of a rectangular shape to a skewed parallelogram. See also: Rectangle.

Radar
A method, system, or technique of using beamed, reflected, and timed radio waves for detecting, locating, or tracking objects.The frequency bands used by radar (radar frequency bands) were first designated by letters for military secrecy. These are shown in the following table.Frequency BandFrequency Range x10⁹HzWavelength Range cmP-band…

Radial
A direction perpendicular to the centreline of a shaft.e.g. the spokes of a wheel.

Radial Engine
An engine with a number of cylinder arranged in a circle around the crankshaft centre line. A design often used for aircraft engines.1850hp Wright R-2600-29 Cyclone 14-cylinder radial. Note that this is effectively two 7 cylinder radials back to back with the rear bank of cylinders being rotated half the cylinder to cylinder spacing.A model 5 cylin…

Radial Force Variation
Radial Force Variation is a dynamic force generated at the wheel due to tyre non-uniformity. Radial Force Variation, which induces a continuous vertical force to the suspension varies with speed up to 50 km/h, but reaches a constant value, for the most type of tyres, afterwards.

Radial Vibration
Shaft dynamic motion or casing vibrationwhich is in a direction perpendicular to the shaft centreline. See also: Vibration.

Radian
SI Supplementary UnitThe 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.

Radiated Noise
The dissemination of sound energy as acoustic waves from a source (e.g. a vibrating panel, running engine etc.) The energy falls off as the inverse square of the distance from the service in the absence of absorption. See also: Coincidence, Engine Radiated Noise.

Radiation Absorbed Dose (Rad)
The rad is a unit used to measure absorbed radiation dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not't describe the biologic…

Radio Frequency Choke
A choke coil with a high impedance at radio frequencies. See also: Choke.

Radio Waves
Electromagnetic radiation in the region of the spectrum greater than 3Hz and smaller than about 3 x 10⁹Hz and wavelengths greater than about 100000km to 100mm. Above 3GHz the Super High Frequency and Extremely High Frequency bands are within the microwave spectrum. At one time radio waves were known as Hertzian Waves.NameFrequency [Hz]Wa…

Radioactive Decay
The spontaneous change of an atomic nucleus accompanied by the emission of alpha, beta particles or gamma radiation.Alpha DecayNuclear decay by emission of an alpha particle (⁴He nucleus).Beta DecayNuclear decay by emission of an electron or a positron. Positron decay is always accompanied by electron capture decay.Gamma RaysGamma ray em…

Radiosity Equation
Used in the construction of realistically lit scenes in computer graphics, because it can account for light falling on a scene from a light source and then being absorbed, reflected, and diffused throughout the elements of the scene. It must be solved at all wavelengths of input light, and thus results in the computationally intensive nature of com…

Radium
A radioactive element of the alkaline earth metals. White in colour and properties close to barium. Symbol Ra

Radius
In a circle, the radius is the distance from the centre of the circle to a point on the circle. See also: Circle, Diameter.

Radix Point
The generalization of decimal point to bases of numeration other than base 10.

Radon
A radioactive element and the heaviest of the noble gases. Symbol Rn

Rainflow Analysis
A method of representing a random signal by various numbers of sinusoids that would produce the same amount of fatigue damage. Rainflow and related counting methods are two-parameter methods. Typically they preserve information about the mean value as well as the amplitude of each cycle. There are several two-parameter methods. If the load history …

Ramjet
A jet engine with no mechanical compressor, consisting of specially shaped tubes or ducts open at both ends. The air necessary for combustion is shoved into the duct and compressed by the forward motion of the engine.It needs to be accelerated to high speed before it can become operative. See also: Gas Turbine, Rocket.

Ramsay, William (1852-1916)
English chemist; president of the Society of Chemical Industry.1894Shared discovery of Argon.1895 Independently discovered Helium on earth.1898 Discovered Krypton and Xenon1904 Received Nobel Prize for chemistry for discoveries of these rare, or 'noble' elements.

Random Access Memory
A data storage device for which the order of access to different locations does not affect the speed of access, except for bursts. Data is typically stored in RAM temporarily for use by the process or while the computer is operating. FPM, EDO, SDRAM, DDR, etc. are all types of RAM. See also: Memory, Semiconductor.

Random Copolymer
A polymer in which two different mer units are randomly distributed along the molecular chain. See also: Polymer.

Random Noise
A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy. See also: Noise.

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.

Random Sampling
A sampling method in which all elements in the population have an equal chance of being selected, and in which the value of one observation does not affect the outcome of other observations. Random samples have important properties which are necessary in many statistical tests.

Random Vibration
A vibration whose instantaneous magnitude is not specified for any given instant of time. The instantaneous magnitudes of a random vibration are specified only by probability distribution functions giving the probable fraction of the total time that the magnitude (or some sequence of magnitudes) lies within a specified range. Random vibration conta…

Range
The maximum and minimum allowable full-scale signal, input or output.The set of values taken on by a function. See also: Signals.

Rankine Temperature Scale
An absolute scale of temperature based on degrees Farenheit.ttle='Absolute Temperature';xiunt='Rankine';yiunt='Kelvin';mconv=1.8;cconv=0.0; See also: Celsius Temperature Scale, Fahrenheit, Kelvin.

Rankine Vortex
An idealized vortex in unbounded fluid with uniform vorticity inside a circular patch and zero vorticity outside. See also: Vortex.

Rapid Speech Transmission Index
Abbreviated as RASTI and expressed in a decimal range of 0.2 for 'bad' to 1.00 for 'Excellent'

Rarefaction
A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave. The opposite of compression. See also: Sound.

Rasp
A sort of blowing noise emitted often from the exhaust pipes of internal combustion engines, usually, it is dominated by a mixture of 4E, 6E and 8E engine orders on 4 cylinder engines (where E represents engine rotational frequency).

Rate of Reaction
The rate at which the concentration of a product increases or the concentration of a reactant decreases with time.

Ratio
The quotient of two numbers. See also: Quotient.

Ratio of Specific Heats
The ratio of molar heat capacity at constant pressure to molar heat capacity at constant volume.where? = ratio of specific heats [dimensionless]C_pm = molar heat capacity at constant pressure [Jmol^-1K^-1]C_vm = molar heat capacity at constant volume [Jmol^-1K^-1]

Rational B-Spline
equation that permits the modeling of free-form curves between two endpoints as a function of a set of intermediate points whose values influence the shape of the curve.

Rational number
Any number that can be expressed as the ratio of two integers. See also: Integer, Irrational Number, Real Number.

Rattle
A sound such as that made when shaking a tin full of coins. See also: Acoustics, Sound, Squeak.

Ray
At higher audio frequencies, sound may be considered to travel in straight lines, in a direction normal to the wave front.

Rayleigh Distribution
It is often necessary to determine the probability that the peaks in a time history will exceed certain values. This is done using a peak probability distribution which only considers the peaks in the time history and not values at any arbitrary time. For a narrow banded process which has a Gaussian probability distribution, the peaks follow a Rayl…

Rayleigh Waves
These are Also known as surface waves. Surface shear waves are polarised in the plane of the surface because that is the only axis of vibration available.One type of surface wave is the Rayleigh wave. This wave takes the form of an elliptical wave. The Rayleigh wave is a mixture of shear and longitudinal waves. It is sustained from reflections of…

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.

Reactance
The opposition to the flow of electricity posed by capacitors and inductors.

Reactants
Substances initially present in a chemical reaction. See also: First Order Reaction, Second Order Reaction, Zero Order Reaction.

Reaction
A reaction is a force exerted by a support on an object: sometimes called support reaction. Using this definition, a reaction is an external force. See also: Force.

Reaction Time
Human reaction time depends very much on the circumstances. E.g. the time that elapses between the driver`s perception/decision and the time the brakes are applied or the steering wheel turned is 0.75 of a second for the average person.The absolute shortest time it takes for a human to react to a sound is 0.110 of a second, based on this if a runne…

Reactive Absorber
A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as well as resistance. See also: Helmholtz Resonator, Quarter Wave Tube.

Reactive Sound Field
A sound field in which the particle velocity is 90° out of phase with the pressure. An ideal standing wave is an example of this type of field, where there is no net flow of energy and constitutes the imaginary part of a complex sound field.

Reactivity Index
The reactivity index in a given direction at a point is defined as the difference between the sound intensity level and the sound pressure level measured in the given direction at that point. In practise the reactivity index is normally negative. The reactivity index indicates an important character of the sound field as it is measured and is not a…

Real Axis
The x-axis of an Argand diagram. See also: Imaginary Axis, x axis.

Real Image
An image formed by the convergence of light. See also: Virtual Image.

Real Modes
In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time. See also: Modal Analysis, Mode.

Real Number
Any of the conventional numbers, including fractions and irrationals. See also: Fraction, Irrational Number, Rational Number.

Reaumur Temperature Scale
On the Reaumur temperature scale, abbreviated to R, the ice point is 0°R and thesteam point is 80°R.ttle='Reaumur';xiunt='°R';yiunt='°C';mconv=0.8;cconv=0.0; See also: Celsius Temperature Scale, Fahrenheit, Kelvin, Temperature.

Recency Effect
An increase in correct recall rate for the most recently presented items of a list compared with those presented earlier in the list.

Receptance
Displacement per unit force. See also: Accelerance, Apparent Mass, Mobility.