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


Polyhedral
In the shape of a solid formed by plane faces, as in a prism. See also: Dodecahedron, Hexahedron, Octahedron, Prism.

Polymer
A large molecule formed by the linking together of many smaller molecules known as monomers. See also: Alternating Copolymer, Atactic, Branched Polymer, Condensation Polymerization, Copolymer, Crosslinked Polymer, Crystallite, Degree of Polymerization, Homopolymer, Isotactic, Mer, Monomer, Network Polymer, Polymerization, Random Copolymer, Thermopl…

Polymerization
The process by which monomers are joined together to give a polymer. See also: Degree of Polymerization, Monomer, Polymer.

Polymethyl Methacrylate Resin
A transparent plastic. Also known as Perspex, Lucite

Polymorphism
Different crystal structures at different temperatures or pressures for a single compound.

Polynomial
A function that is a sum of power functions, with positive integer exponents, multiplied by constants.e.g.

Polystyrene
Expanded polystyrene was the first foamed polymer. It is the white rigid plastic used for thermal insulation and in flotation devices.

Polytetrafluoroethylene
Photograph courtesy of Draper.co.ukCommonly known as Teflon, a thermoplastic polymer used for non-stick coatings and bearings.

Polyvinyl Acetate
A colorless, thermoplastic, water soluble, resinous high polymer derived from the polymerization of vinyl acetate with a catalyst. Abbreviated as PVA.

Polyvinyl Chloride
Plastic used in many different forms. Commonly abbreviated as PVC.

Pop
An undesirable short duration microphone output, often caused by explosive exhalation of air during the formation of certain vocal sounds. See also: Microphone.

Poplar
A light wood, but not very strong and or very good wear resistance.

Poppet Valve
A valve opened by the action of a cam and closed by a spring. See also: Camshaft, Internal Combustion Engine, Rocker Arm, Sleeve Valve Engine.

Population
The total number of unique values. When some values are more probable than others, the effective population approaches the number of the more probable values. The unknown universe of values which we can only estimate by sampling. See also: Sample, Target Population.

Porcelain
Made from china clay, a clay consisting mainly of kaolinite.

Porosity
The presence of gas pockets within a solid material.

Positive
Symbol : +A quantity or value that is greater than zero.

Positron
The antiparticle of the electron. Discovered by Anderson in 1931. See also: Antiparticle, Blackett, Patrick Maynard Stuart, Electron, Electron-Positron Annihilation.

Potassium
Potassium is a soft, silvery coloured metal, which like other members of the alkali group of metals, is extremely reactive. As with other members of the group, it can be prepared by electrolysis of the fused halide. The main source being potassium chloride. It does not occur naturally. Potassium has limited use as the pure metal, although it can be…

Potassium Bromide
A white crystalline salt used in photography. It was used in medicine as a sedative. Symbol KBr

Potassium Carbonate
A white very soluble deliquescent salt. Made from the ashes of burnt wood, hence the old name 'Wood Ash'. Symbol K2CO3 Also known as Pearl ash, Potash, Wood ash

Potassium Chloride
White crystalline solid. Symbol KCl

Potassium Fluoride
White crystalline solid. Symbol KF

Potassium Hydroxide
A white deliquescent solid that dissolves in water to give an alkaline solution. Symbol KOH

Potassium Iodide
White crystalline solid. Symbol KI

Potassium Manganate VII
Deep purple, crystalline, soluble salt. Dissolves in water to produce a strong oxidizing agent. Used as a disinfectant. Symbol KMnO4

Potassium Nitrate
The salt of potassium and nitric acid. Used in pyrotechnics, explosives, the manufacture of glass and as a fertilizer. Symbol KNO3

Potassium Oxide
Yellowish white yo grey crystalline solid. Symbol K2O

Potassium Peroxide
Yellow or orange solid. Symbol K2O2

Potassium Sulphide
White, yellow, red or brown. Symbol K2S

Potassium Superoxide
Yellow crystalline solid. Symbol KO2

Potential Energy
The energy a body possesses due to it's position or the arrangement of it's parts. See also: Gravitational Potential Energy, Kinetic Energy.

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. See also: Resistance.

Potin
An ancient alloy of copper, zinc, lead and tin that was used to make coins.

Potting Compound
An insulating material for encapsulating one or more circuit elements.

Pound
This is the standard unit of mass in the British system of units. It has since beenreplaced by the SI system and the kg.Conversions16 ounces (oz)=1 pound (lb)1 stone=14 pounds (lb)1 quarter=28 pounds (lb)1 hundredweight (cwt)=112 pounds (lb)1 ton=2240 pounds (lb)20 hundredweight (cwt)=1 ton1 pound (lb)=0.453592 kgttle='Mass';xiunt='kg';yiunt='lb';m…

Pound Force
A derived unit of force in the British system of units.ttle='Force';xiunt='N';yiunt='Pound Force';mconv=4.44822;cconv=0.0; See also: Force.

Poundal
A derived unit of force in the British system of units.ttle='Force';xiunt='N';yiunt='Poundal';mconv=0.138255;cconv=0.0; See also: Force.

Pounds per Square Inch
Often abbreviated as psi. This is a British imperial unit of pressure. 1.0 psi = 6894.757 Pattle='Pressure';xiunt='Pa';yiunt='psi';mconv=6894.757;cconv=0.0;There are a number of other abbreviations appended to psi:psia = pounds-force per square inch absolute = gauge pressure plus local atmospheric pressure.psid = psi difference = difference between…

Power
MathematicsThe number of times a number or expression is multiplied by itself, for which the symbol is an index or exponent. For example, is the fourth power of x.Anything raised to the power of 0 is equal to 1, including 0 to the power of 0.Other notation used to denote x raised to the power of n, xn = x^n = x**n

Power Density
Characteristic parameter of a battery/electrical power source indicating its electrical power per unit weight or volume.Specific Power or Gravimetric Power DensityWeight based power density [watt/kg].Power Density or Volumetric Power DensityVolume based power density [watt/litre or watt/m3].Note: the terminology is not strictly defined.

Power Factor
The cosine of the phase angle between voltage and current.

Power of a Lens
The power of a lens is defined as the reciprocal of its focal length, in metres. It has units of DIOPTRE.

Power Spectral Density
Abbreviated as PSD, this is a method of scaling the amplitude axis of spectra of random rather than deterministic signals. Because a random signal has energy spread out over a frequency band rather than having energy concentrated at specific frequencies, it is not meaningful to speak of its RMS value at any specific frequency. It only makes sense t…

Power Stroke
The working stroke of a piston.As the piston approaches top dead centre the fuel-air mix is ignited.The fuel-air mix ignites and expands, as it expands it forces the piston down.As the piston is forced down this force is applied through the conrod to the crank and is transformed into a rotation of the crank.The combustion and expansion is complete …

Practical Number
A positive integer m such that every natural number n not exceeding m is a sum of distinct divisors of m. See also: Integer.

Prandtl Number
A dimensionless group often used in heat exchange calculations, relating the ratio of a fluid's capacity to diffuse momentum to its capacity to diffuse heat. whereP = Prandtl NumberCp = specific heat capacity [Jkg-1K-1]? = dynamic viscosity [kgm-1s-1]? = thermal conductivity [Wm-1K
Praseodymium
Praesodymium is a soft, white metal and a member of the lanthanide group of elements. It closely resembles neodymium and, along with other elements of the same group, is found in the same minerals. It reacts slowly with oxygen but rapidly with water. As a pure metal, its uses are limited; however, it is used as an alloying constituent for alloys us…

Prebiotic
Related to the period before life appears on a planet. See also: 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. See also: Gyroscope.

Precipitate
A solid that appears in the form of fine particles in what was previously a clear solution because of a chemical reaction. See also: Solute.

Precipitation Heat Treatment
Artificial aging in which a constituent precipitates from a supersaturated solidsolution.

Prepreg
A continuous fibre reinforcement pre-impregnated with a polymer resin which is then partially cured.

Presbycusis
Gradual and biologically normal loss of acute hearing with advancing age. See also: Threshold of Hearing, Threshold Shift.

Pressure
Pressure is defined as the force exerted per unit area. The SI unit of pressure is the pascal or newton per square metre.Pressure is a similar idea to stress, the force intensity at a point, except that pressure means something acting on the surface of an object rather than within the material of the object. When discussing the pressure within a fl…

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. See also: Drag Coefficient, Lift Coefficient.

Pressure Welding
Any welding process or method in which pressure is used to complete the weld. See also: Welding.

Pressure Zone
As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.

Prick Punch
A solid punch with a sharp point, used to mark centres or other locations on metal. See also: Hammer.

Primary Bond
Interatomic bonds that are relatively strong and for which bonding energies are relatively large. See also: Bond Energy, van der Waals Bond.

Primary Cell
A cell or battery which is not intended to be recharged and is discarded when the cell or battery has delivered all its electrical energy. See also: Battery, Secondary Cell.

Prime Numbers
A prime number has only two different factors. They are 1 and the number itself.13 is a prime number because the only factors of 13 are 1 and 13. Note that 1 is not a prime number.The following table lists all odd numbers from 3 to 991 and if the number is not a prime the lowest factor is given. 3 Prime

Primitive Pythagorean Triangle
A right triangle whose sides are relatively prime integers. See also: Pythagorean Triangle, Relatively Prime, Triangle.

Principal Component Analysis
Constructing new features which are the principal components of a data set. The principal components are random variables of maximal variance constructed from linear combinations of the input features. Equivalently, they are the projections onto the principal component axes, which are lines that minimize the average squared distance to each point i…

Principal Curve
A nonlinear principal component axis. Principal curves are smooth curves that minimize the average squared orthogonal distance to each point in a data set. Fitting a principal curve is a maximum-likelihood technique for nonlinear regression in the presence of Gaussian noise on both x and y. Principal points are individual points that minimize the a…

Principal Quantum Number
The number related to the amount of energy an electron has and therefore describing which shell the electron is in.

Prism
A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted. See also: Polyhedral.

Privacy Index
A measure for rating the speech privacy performance of an architectural space (or lack of speech intelligibility).Privacy Index is calculated from the Articulation Index (AI) according to the following: PI = (1 - AI) * 100%.A privacy level of PI > 95% represents confidential speech privacy, speech sounds can be heard but not understood.A PI between…

Probability
A number between 0 and 1 which represents how likely an event is to occur. Events with probability equal to 0 never occur. Events with probability equal to 1 always occur.In data analysis, probability is normally defined in terms of the relative frequency of occurrence of an event which can be repeated many times. For example, if you repeatedly sam…

Program Counter
A microprocessor register that holds the memory address of the next instruction to be fetched. See also: Instruction, Register.

Programming Languages
AdaAda is a large, complex, block-structured language aimed primarily at embedded applications. It has facilities for real-time response, concurrency, hardware access and reliable run-time error handling.AlgolA high-level programming language developed in the 1950s for scientific computations.Assembler LanguagesA symbolic representation of the mach…

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. See also: Equations of Motion.

Promethium
A radioactive element of the rare earth series. Symbol Pm

Pronic Number
A number of the form n(n+1).

Propane
A member of the alkane series, a colourless, odourless gas at room temperature. In liquid form it is often referred to as Liquid Petroleum Gas (LPG). It is extracted from natural gas or refinery gas streams. Symbol C3H8

Propanol
Used as an inexpensive alcohol. Symbol CH3CH2CH2OH Also known as Propyl alcohol

Propeller
An assembly of radially disposed blades with an airfoil shape that when rotated in air produce thrust.All propeller blade airfoils have a certain angle of attack range, in which the flow around the airfoil remains attached to airfoil's surface. While this is the case, the efficiency of the axial fan is satisfactory, but, when the angle of attack of…

Propene
A colourless highly flammable gas. Symbol C3H6 Also known as Propylene

Proper Fractions
A fraction whose numerator is of lower absolute value than it's denominator. Also known as a common or simple fraction. See also: Denominator, Fraction, Numerator.

Proper Time
The time interval measured by an observer in his own reference frame. See also: Time, Time Dilation.

Proportional Limit
The point on a stress-strain curve at which the straight line proportionality between stress and strain ceases. See also: Strain, Stress.

Proportionality
A relationship in which a quantity is related to the second quantity. See also: Inverse Proportionality, Mathematical Symbol s.

Propulsion
Process by which something can be moved by producing a reaction with a force of thrust. See also: Force, Propeller, Thrust, Thrusters.

Protactinium
A radioactive element. Symbol Pa

Protein
A very large, naturally occurring polyamide formed from a selection of the 20 or so naturally occurring amino acids.

Protium
The lightest element of hydrogen of unit mass and the most prevalent in the universe.

Protocol
The set of conventions that govern the treatment and formatting of data in anelectronic communications system.

Proton
The central nucleus of an atome is formed from protons and neutrons.The nucleus of a hydrogen atom has a single proton. A member of the baryonand hadron families of elementary particles.Click on an item to paste into clipboard or use clipboard symbol at end to clipboard all values proton mass 1.67262158e-27 kgClip proton-muon mass ratio 8.88024408 …

Proton-Proton Chain
In the Sun and other less massive stars, this chain is the primary source of heat and radiation. The proton-proton chain converts hydrogen into helium releasing energy in the form of particles and gamma rays. Hydrogen is converted into helium in a chain of reactions. The first reaction takes an average of 1 billion years to occur while the others a…

Protoplanetary Disc
The disc of dust surrounding a star out of which planets might form. See also: Planet, Star.

Psychoacoustics
The study of the interaction of the auditory system and acoustics. See also: Acoustics, Auralization, Binaural, Ear, Habituation, Head Related Transfer Function, Sensation Level.

Psychrometer
An instrument used to determine the dry and wet bulb temperature of air simultaneously.

Ptolemy (90-168)
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. It regarded Earth as the center of the universe and assumed the motion of planets was a superposition of circular motions. See also: Ptolemy.

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. See also: Lever, Telodynamic Transmission.

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. See also: Star.

Pulse Code Modulation
Coding where the input signal is represented by a given number of fixed-width samples per second. Often used for the coding employed in the telephone network. See also: Adaptive Differential Pulse Code Modulation.

Pulse Range
Difference in decibels between the peak level of an impulsive signal and the root-mean-square level of a continuous noise. See also: Impulse.

Pumice
A light porous stone of mixed silicates.

Pure Procedure Code
Program code that does not modify itself as it runs. See also: Code.