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


Oscillator
A regenerative circuit which has signal-generating characteristics.

Oscilloscope
An instrument using a cathode-ray tube for visual display of electric signals.

Osmiridium
Osmiridium (iridosmine) is a naturally occurring alloy of osmium and iridium. Used to make long-wearing tips for gold fountain-pen nibs.

Osmium
Osmium is a member of the platinum group of metals and is commonly found in conjunction with these elements. The mixed metals can be extracted from the ore with aqua regia, followed by treatment of the soluble and insoluble portions in various ways. Osmium is found in both portions and is removed as the volatile tetroxide, which can then be reduced…

Osmometry
Determination of the average molecular weight of a dissolved substance from measurements of osmotic pressure. See also: Osmosis.

Osmosis
Passage of solvent molecules from a dilute solution through a semipermeable membrane to a more concentrated solution. See also: Osmometry, Osmotic Pressure, Reverse Osmosis.

Osmotic Pressure
Pressure which must be applied to a solution to prevent water from flowing in via a semipermeable membrane. See also: Osmosis, Reverse Osmosis.

Otologist
A physician who specializes in the ear and its diseases. See also: Ear, Hearing.

Otto Cycle
An idealized reversible cycle of four operations occurring in a perfect four-stroke petrol engine. These are:constant volume temperature riseconstant pressure expansionconstant volume temperature fallconstant pressure volume decreaseThe cycle returns the gas to it's original state.

Ounce Force
A derived unit of force in the British system of units.ttle='Force';xiunt='N';yiunt='Ounce Force';mconv=0.27801390;cconv=0.0; See also: Force.

Ounces
A secondary unit in the British system of units, often abbreviated as oz. Now replaced by the SI system andthe kilogram.Conversions16 ounces (oz)=1 pound (lb)1 ounce (UK fluid)=28.34952 g1 ounce (avoirdupois)=28.34952x10-3kg1 ounce (US fluid)=29.57353 g1 kilogram (kg)=2.204623 pounds (lb)1 hundredweight (cwt)=112 pounds (lb)1 ton=2240 po…

Outlier
A data value which is unusual with respect to the group of data in which it is found. It may be a single isolated value far away from all the others, or a value which does not follow the general pattern of the rest. Most classical statistical techniques tend to be quite sensitive to outliers, so that it is important to be on the alert for them. Gra…

Overrun
Often used to describe the condition where rotating machinery is in a coast down condition i.e. the rotational speed is decreasing.

Overtone
A harmonic frequency related to the fundamental.Normally used in music to describe a higher frequency harmonic (not general used in wave mechanics). See also: Frequency, Harmonic.

Oxidation Number
A number assigned to each atom to help keep track of the electrons during a redox-reaction.

Oxidation Reaction
A reaction where a substance loses electrons. See also: Autoxidation.

Oxyacid
When one or more hydroxide (OH) groups are bonded to a central atom.

Oxygen
A colourless and odourless element that is essential for most life forms on earth. The name is derived from the French oxygène, which means 'acid generating'. Symbol O Also known as Vital air.Discovered1774 Priestley publishes his work on oxygen.AbundanceThe most abundant element on earth, making up about 1/5 of the earth's atmosphere and 8/9ths of…

Oxygen Cutting
A process of cutting ferrous metals by means of the chemical action of oxygen on elements in the base metal at elevated temperatures.

Ozone
Allotropic form of oxygen containing three atoms in the molecule. The stratosphere ozone layer, which is a concentration of ozone molecules located at 10 to 50 kilometres above sea level, is in a state of dynamic equilibrium. Oxygen molecules absorb ultraviolet (UV) light to form ozone which, in turn, decomposes back to oxygen. These processes abso…

Ozone Layer
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. The ozone is created by a series of processes beginning with the splitting up of the oxygen molecule to single oxygen atoms. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around …

Paint
A generally opaque coating including resin, a solvent and additives. See also: Lacquer, Paintbrush, Varnish, Volatile Organic Compound.

Paintbrush
Photograph courtesy of Draper.co.ukBrush used to apply paint. See also: Paint.

Pair Annihilation
When a particle encounters its antiparticle, they annihilate each other, disappearing in a burst of photons. See also: Antiparticle, Pair Production, Photons.

Pair Production
The conversion of energy into matter in which a particle and its antiparticle are produced. This usually refers to the production of a electron and a positron (antielectron). See also: Antiparticle, Pair Annihilation.

Palindrome
A positive integer whose digits read the same forward and backwards.e.g.11, 121, 2332

Palladium
Palladium is a member of the platinum group of metals. It is silvery white in colour, is malleable and ductile, and is one of the most reactive in the group. It has an abundance of 6x10-6 in the earth`s crust. Palladium has good corrosion resistance but is soluble in oxidising acids and fused alkalis. It readily absorbs hydrogen gas (up …

Paper
Made by pressing a pulp of rags, straw and wood into thin sheets. A a quire of paper is equal to 25 sheets and a ream of paper is equal to 500 sheets.

Paper Glue
Many different forms of paper glue, the most common being the solid stick form. See also: Glue, Paper.

Paraffin Oil
A term describing a whole series of saturated aliphatic hydrocarbons, Also known as alkane hydrocarbons. Symbol CnH2n+2

Paraffin Wax
An alkane obtained as a residue from the distillation of petroleum it is a waxy substance that is a mixture of alkanes with chains containing 18 to 36 carbon atoms.

Parallax
Change in the apparent position of objects when viewed from two widely separated positions.

Parallel
Two or more lines are said to be parallel if they are in the same plane, but never intersect. Two or more planes are parallel if the planes never intersect.For two straight lines defined as: andIf m1=m2 the lines are parallel.If m1*m2=-1 the two lines are perpendicular.

Parallel Transfer
A transfer of multiple bits from one register to another simultaneously. Parallel transfers require multiple bit paths.

Parallelogram
A quadrilateral in which opposite sides are parallel.

Paramagnetism
A relatively weak form of magnetism that results from the independent alignment of atomic dipoles (magnetic) with an applied magnetic field. Atoms with unpaired electrons are paramagnetic. Paramagnetic materials are weakly magnetized when brought into proximity to a magnet. See also: Curie Temperature.

Parent Isotope
An element that undergoes nuclear decay. See also: Daughter Isotope, Isotopes.

Pareto Distribution
A distribution used for random variables which are constrained to be greater or equal to 0. See also: Statistic.

Paris Bronze
A term given to brass containing 88% copper, 10% zinc and 2% lead used to produce trinkets and souvenirs.

Parsec (pc)
1 Parsec (pc) = 3.0856x1016m1 Parsec (pc) = 2.062648x105AU (Astronomical Units)1 Parsec (pc) = 3.2615 Light Years

Partial Pressures
The pressure exerted by a certain gas in a mixture. This was first published by John Dalton in 1801.Dalton's law of partial pressures states: The total pressure of a mixture of gases, which do not interact chemically, is equal to the sum of the partial pressures. The sum of the pressures that each gas would exert if it alone occupied the volume con…

Particle Accelerator
A device (i.e., machine) used to produce high-energy high-speed beams of charged particles, such as electrons , protons, or heavy ions, for research in high-energy and nuclear physics, synchrotron radiation research, medical therapies, and some industrial applications. See also: Acceleration, Centre European pour Rechearche Nucleaire, Cyclotron, El…

Particle Velocity
A fluctuating velocity superimposed by the presence of sound on the other velocities with the particles of the medium may have. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous particle velocity or peak particle velocity. In air, the other velocities are those due to thermal agitation and win…

Particulate
Composed of distinct particles. Smoke is particulate; pure gases are not.

Partition
A division between two volumes. See also: Acoustic Barriers, Flanking Sound Transmission, Sound Transmission Class, Sound Transmission Loss.

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. It is widely used in condensed-matter physics and is employed in theoretical studies of high-temperature supercondu…

Parts Per Million (ppm)
Used to describe the amount of a trace element or substance present in the main substance.Parts Per Million by Volume (ppmv)Used to describe the amount of a trace element or substance present in the main substance as measured by volume.Parts Per Million by Weight (ppmw)Used to describe the amount of a trace element or substance present in the main …

Pascal
The SI derived unit of pressure is the pascal or 1 newton per square metre.Conversions1 bar=1x105 Pa1 standard atmosphere (atm)=1.01325x105 Pa1 lbf in-2 (psi)=6894.76 Pa1 tonf in-2=15.4443x106 Pa1 torr=1 mmHg1 mmHg=133.322 Pa1 inHg=3386.39 Pa1 inH2O=249.089Pa1 mmHg=13.5951 mmH2<…

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. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. …

Pascal's Triangle
Each number is the sum of the two numbers above. The sum of each row is equal to a power of 2.1111211331146411510105116152015611721353521711828..................

Paschen Series
The series which describes the emission spectrum of hydrogen when the electron is jumping to the third orbital. All of the lines are in the infrared portion of the spectrum. See also: Balmer Series, Hydrogen, Lyman Series.

Passive Absorber
A sound absorber that dissipates sound energy as heat. See also: Absorbent Material, Absorption Coefficient.

Patent
A document granting an inventor sole rights to an invention. See also: Inventor.

Pathogen
Any microorganism or substance that causes disease.

Patina
A thin layer of corrosion products with a distinctive colouration that forms on a metal surface exposed to air and water. Patina usually refers to the greenish coating that forms on copper alloys over time. In most non-polluted environments it is basic copper carbonate but in industrial and urban areas it is mainly basic copper sulphate. See also: …

Pattern Recognition
A computational technique used to find patterns and develop classification schemes for data in very large data sets.

Peak
Extreme value of a varying quantity, measured from the zero or mean value.

Peak Hold
The peak signal level for each frequency component is held for display. See also: Averaging.

Peak Pick
A parameter estimation technique where the peak value of the imaginary part of the frequency response function is used to estimate the mode shape value at that point. The phase is given by its sense (positive or negative). This method is Also known as quad picking since the value is being picked off the imaginary or quadrature part of the frequen…

Peak to Peak Value
The over-all amplitude of a signal measured from its lowest (or most negative) peak to its most highest (or most positive) peak. See also: Amplitude, Peak Value, Trough.

Peak Value
The instantaneous maximum value of a waveform. See also: Amplitude, Crest Factor, Peak to Peak Value.

Pearlite
A lamellar aggregate of ferrite and cementite, often occurring in steel and cast iron. It results from the transformation of austenite of eutectoid compositions and consists of alternating layers of alpha-ferrite and cementite. See also: Cast Iron, Steel.

Peat
Partially carbonized vegetable material, usually found in bogs.

Peck
An old unit of volume.Conversions1 peck=0.25 bushel1 peck=8810 cm31 peck=0.00881 m31 peck=0.3111 ft31 peck=8.810 litres1 peck=537.605 in3ttle='Peck';xiunt='m3';yiunt='peck';mconv=8.81e-3;cconv=0.0; See also: Volume.

Peening
The mechanical working of metals by means of hammer blows. Peening tends to stretch the surface of the cold metal, thereby relieving contraction stresses.

Peierls Stress
The stress required to move a dislocation. See also: Stress.

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.function init(){function init(){ document.pndlm1.g1.value=9.81; document.pndlm2.g2.value=9.81;}init();}Simple PendulumAs used in the simplest mechanical clock designs.wher…

Penetrant Inspection
A method of nondestructive testing for determining the existence and extent ofdiscontinuities that are open to the surface in the part being inspected. The indications are made visible throughthe use of a dye or fluorescent chemical in the liquid employed as the inspection medium.

Pennyweight
An old unit of mass equal to 24 grains. Often it is abbreviated as dwt.Conversions1 pennyweight=24 grains1 pennyweight=0.05 troy ounce1 pennyweight=0.001555174 kg1 pennyweight=1.555174 gttle='Pennyweight';xiunt='kg';yiunt='pennyweight';mconv=0.001555174;cconv=0.0; See also: Grain, Mass.

Pentad
A period of 5 days. See also: Time.

Pentadecagon
A 15 sided polygon. See also: Polygon.

Pentagon
A polygon with five sides. Each internal angle is 108° and the sum of the internal angles is 540°. See also: Polygon, Regular Polygon.

Pentane
The fifth member of the paraffin series. Pentane exists in three isomeric forms. Symbol C5H12

Pentanol
A colourless liquid at room temperature with a choking aroma. Symbol C5H11OH

Penumbra
The transition region between the darkest shadow and full brightness. Only part of the light from the source reaches this region.

Peptide
A short polymer made by linking together amino acid molecules.

Per Diem
A traditional unit of frequency equal to once a day. See also: Frequency, Per Mensem.

Per Mensem
A traditional unit of frequency equal to once a month. See also: Frequency, Per Diem.

Perfect Cube
An integer is a perfect cube if it is of the form m3 where m is an integer. See also: Perfect Number, Perfect Square.

Perfect Number
A positive integer that is equal to the sum of its proper divisors. For example, 28 is perfect because 28=1+2+4+7+14. See also: Perfect Cube, Perfect Square.

Perfect Power
An integer is a perfect power if it is of the form mn where m and n are integers and n>1.

Perfect Square
An integer is a perfect square if it is of the form m2 where m is an integer. See also: Perfect Cube, Perfect Number.

Periapsis
The point of an orbit nearest to the center of attraction of an orbiting body. See also: Orbit, Pericentre, Perigee.

Pericentre
The point on a spacecrafts orbit at which it is nearest to the centre of mass of the system. See also: Periapsis, Perigee.

Perigee
The point of a satellite's orbit closest to Earth. See also: Earth, Periapsis, Pericentre, Perihelion.

Perihelic Opposition
When the Earth passes between a planet and the Sun when the planet is at its closest from the Sun. See also: Earth, Perihelion, Sun.

Perihelion
The point of closest approach for any object orbiting the Sun (Helios is Greek for Sun). See also: Aphelion, Perigee, Perihelic Opposition, Sun.

Period
The time taken for a full cycle of motion in a system executing harmonic motion. If the motion is at a single frequency f (i.e. sinusoidal) the period T is given by:whereT = period [s]f = frequency [Hz]? = angular velocity [rads-1]

Periodic
A term used to refer to a pattern of motion which repeats exactly after a given interval or period. This situation generally occurs when the input force is periodic as in a machine running at a steady speed. See also: Aperiodic.

Periodic Law
The properties of the elements are periodic functions of their atomic numbers. See also: Periodic Table.

Periodic Table
Devised by Dmitri Mendeleev in 1868. It clearly shows the relationship between all of the elements and allowed the existence of elements to be predicted before they were discovered. They are listed according to the number of protons in their atom. Rows from left to right are periods and columns are groups.HHeLiBeBCNOFNeNaMgAlSiPSClArKCaScTiVCrMnFeC…

Peritectic
An isothermal reversible reaction in metals where a liquid phase reacts with a solid phase to produce a single, different, solid phase upon cooling.

Perl
An interpreted language optimised for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. See also: Computer, Programming Languages.

Permeability
passage or diffusion of a gas, vapor, liquid, or solid through a material without physically or chemically affecting it.Term used to express various relationships between magnetic induction and magnetizing force; either absolute permeability or specific (relative) permeability. See also: Relative Magnetic Permeability.

Permeability of Free Space
µ0 = 4px10-7 Hm-1ttle='Permeability of Free Space';consttxt='m0';constval=12.5663706e-7;constunt='Hm-1'; See also: Physical Constants.

Permeance
The property which determines the magnitude of magnetic flux in a material; it is the reciprocal of reluctance. See also: Magnetic Flux, Reluctance.

Permittivity
The proportionality constant between the dielectric displacement D and the electric field E.

Permittivity of Free Space
e0 = 8.85419x10-12 Fm-1ttle='Permittivity of Free Space';consttxt='e0';constval=8.85419e-12;constunt='Fm-1'; See also: Physical Constants.

Permutations and Combinations
A permutation is a particular sequence of a given set of objects. A combination is the set itself without reference to order.The number of different permutations of n distinct objects taken r at a time isThe number of different combinations of n distinct objects taken r at a time isThe number of different permutations of n objects taken n at a time…