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DiracDelta - Science & Engineering encyclopedia
Category: Sciences > Science & Engineering
Date & country: 01/12/2007, UK
Words: 4470
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VacancyAn unfilled lattice site in a crystal structure. See also: Crystal.
VacuumA given space filled with gas at pressures below atmospheric pressure. Various approximate ranges are: low vacuum, 101325 to 3000 Pamedium vacuum, 3000 to 0. 133 Pahigh vacuum, 0.133 to 1.333x10
-4 Pavery high vacuum, 1.333x10
-4 to 1.333224x10
-7 Paultrahigh vacuum, 1.333224x10
-7 Pa and belowHeat TransferIn…
Vacuum EngineThe working principle is similar to the hot air engine, except that this engine draws hot air into the cylinder.Heat is drawn into the cylinder, at BDC the inlet valve shuts the heat then cools down this in turn creates a vacuum, this vacuum plus air pressure then forces the piston back up the bore which gives the power stroke. A constant burning f…
ValenceThe charge on an ion based on the number of electrons transferred or shared within a specific structure.ExampleH
2O, oxygen has a valence of 2carbon in CH
4 has a valence of four.
Valence BandHighest filled or almost filled band in an atom. The number of electrons in this shell determines the conductivity of the atom. See also: Valence, Valence Electrons.
Valence BondIn the valence bond theory, a valence bond is a chemical bond formed by overlap of half-filled atomic orbitals on two different atoms.Valence Bond TheoryA theory that explains the shapes of molecules in terms of overlaps between half-filled atomic orbitals, or half filled 'hybridized' orbitals, these are a mixture of atomic orbitals.
Valence ElectronsThe electrons in the outermost shell of an atom. The valence electrons in this shell are on average farther from the nucleus than other electrons and are often directly involved in chemical reactions. See also: Atom, Core Electron, Electron, Valence Band.
ValveIn electronics, a valve is a device consisting of two or more metal plates enclosed in an evacuated glass bulb. One of the metal plates is heated, causing electrons to be emitted. If a positive charge is applied to the other plate, the electrons will move towards it and the valve will conduct electricity. Valves have largely been superseded by tran…
ValvesIn a combustion engine, an intake or exhaust valve usually consists of a disk with a stem, which is opened by a cam and closed by a spring.Also, it signifies the total intake and exhaust valves in an internal combustion engine. For example, a 24-valve V-6 would have 4 valves per cylinder 24/6=4. A 16-valve V-8 would have 2 valves per cylinder 16/8=…
ValvetrainThe collection of parts that make the valves operate in an internal combustion engine. The valvetrain includes the camshafts and all related drive components, the various parts that convert the camshafts rotary motion into reciprocating motion at the valves, and the valves and their associated parts.A valve train system in an automotive engine is d…
Van Allen BeltsRadiation belts of high-energy particles trapped in the earth's magnetosphere. They are doughnut-shaped regions containing mainly protons and electrons. They were discovered by the American scientist James Van Allen in 1958 when cosmic ray detectors on board the first US satellite Explorer I registered signals too high to record when they entered t…
van der Waals BondA secondary interatomic bond between adjacent molecular dipoles, which may be permanent or induced. See also: Bond Energy, Primary Bond.
van der Waals EquationAn equation for non-ideal gasses that accounts for intermolecularattraction and the volumes occupied by the gas molecules. Equation of state:Critical point:Reduced equation of state:wherep = PressureV
m = Molar volumeR = Molar gas constantT = Temperaturea, b = van der Waal's constantsT
c = Critical temperaturep
c = Cri…
van der Waals ForceA force acting between nonbonded atoms or molecules. Includes dipole-dipole, dipole-induced dipole, and London forces. See also: van der Waals Equation, van der Waals Radius.
van der Waals RadiusOne half the distance between two nonbonded atoms, when attractive and repulsive forces between the atoms are balanced. See also: van der Waals Equation, van der Waals Force.
VanadiumVanadium is a soft, shiny, silvery metal which can be found in many different minerals, including some crude oils. It has an abundance of 160 ppm in the earth`s crust and can be isolated after conversion to the pentoxide, V
2O
5, followed by direct reduction with aluminium. High purity metal can be obtained by the van Arkel proc…
VapourA gas whose temperature is below its critical temperature, so that it can be condensed to the liquid or solid state by increase of pressure alone. See also: Vapour Pressure.
Vapour PressureThe partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance. See also: Flash Point, Pressure, Vapour.
VaractorA p-n junction diode that is designed to act as a voltage controlled capacitance when it is operated under reverse bias.Used for tuning frequency dependent equipment. See also: Capacitor.
VariableA letter that is used to represent a numerical quantity whose value may or may not yet be known and may or may not be constant. It may not even have a specific value, but rather be allowed to range over a set of possible values.
VarianceThe square of the difference from the mean for each data value, summed and dividedby one less than the number of values. An expression of the so-called 'second moment'which describes the variability around the mean. The square root of the variance is thestandard deviation.for the data series: x
1, x
2, x
3,....,x
N…
Varnish
A resin in solution.Oil based varnishVarnishes where the resins are dissolved in linseed and/or other oils.Spirit based varnishConsist of resins dissolved in turpentine or alcohol.
Vector
A quantity with a magnitude and a direction. Vectors are added like arrows, end to end, and the sum (for two) is the vector from the tail of the first vector to the tip of the second.There are a number of different representations. Given the vector A it is normal to print it in bold or it can be expressed as follows: are unit vectorsIf x1
Vehicle Noise
The noise produced by a land based vehicle that is radiated to the surrounding environment (exterior noise) or that is observed by the occupants of the vehicle (interior noise).The basic breakdown of the noise sources and paths (as per the source-path-receiver model is):Interior Noise:Powertrain noiseAir intake noiseEngine radiated noiseExhaust noi…
Vein
A blood vessel that carries blood towards the heart. All veins, except the pulmonary vein contain deoxygenated blood. See also: Blood.
Velocity
Velocity is a vector quantity that specifies the time rate of change of displacement with respect to a reference frame. If the reference frame is not inertial, the velocity is often designated 'relative velocity'.Conversions1 mile per hour (mph)=1.609 kilometres per hour (kph)1 ms-1=3.280839 fts-11 in s-1=0.0254 ms<…
Velocity of Light in Vacuo
c = 2.997924580x108 ms-1ttle='Velocity of Light in Vacuo';consttxt='c';constval=2.997924580e8;constunt='ms-1';The speed of light is the same whether the observer measuring it is stationary or in constant motion. This was first proved experimentally by Michelson and Morley from 1881 onwards, in their failure to detec…
Velocity Pressure
The difference between dynamic pressure and static pressure.
Velocity Transducer
An electrical/mechanical transducer whose output is directly proportional to the velocity of the measured unit. A velocity transducer consists of a magnet suspended on a coil, surrounded by a conductive coil. Movement of the transducer induces movement in the suspended magnet. This movement inside the conductive coil generates an electrical current…
Vena Contracta
Point where the flow becomes parallel.Coefficient of velocity,Coefficient of contraction,
Venetian Red
Venetian Red is a permanent red pigment composed of ferric oxide and obtained by igniting ferrous sulphate.
Ventilation
The controlled exchange of air.
Venturi
A short constriction in a tube between two longer tapered portions that are usually of unequal length but terminate with the same diameter, so that there is a drop in pressure in a fluid flowing through the constriction which may be used to determine the rate of flow or used as a source of suction; also devices having this form and the effect invol…
Venus
The second planet from the sun in our solar system. Venus is one of the terrestrial planets and forms part of the inner solar system. It is only slightly smaller than the Earth.Venus has a very thick atmosphere. The atmosphere is so dense that Venus is perpetually covered by clouds. Hence, the surface is permanently shielded from direct view. The c…
Verdigris
Verdigris is a mixture of acetates of copper used in making some green pigments. It forms on the surface of copper and brass when they are exposed to damp, and is highly poisonous. See also: Copper.
Vernier
A vernier is an auxiliary scale, invented by Pierre Vernier, which enables the reading of the smallest divisions of a graduated scale. It is important in theodolites, sextants and the mountings for telescopes for accurate determination of angular positions.The scale is read by first taking note of where the zero mark on the vernier scale falls on t…
Vernier Caliper
The vernier caliper provides the three basic functions of inner, outer and depth gauge. In all cases the measurement is read from the same scale. The vernier caliper is first set over the object to be measured with the caliper shut so that it is firm but not tight.
Vernier Engine
A rocket engine of small thrust used primarily to obtain a fine adjustment in the velocity and trajectory of a rocket vehicle just after the thrust cutoff of the last sustainer engine, and used secondarily to add thrust to a booster or sustainer engine.
Versus
As a function of, as distance versus time.
Vertical
The direction of gravity at the point of observation.
Very High Frequency (VHF)
Radio frequencies that are between 30 to 300 MHz. See also: Frequency, Super High Frequency, Ultra High Frequency, Very Low Frequency.
Very Low Frequency (VLF)
Radio Frequencies from 3 kHz to 30 kHz. See also: Frequency, Very High Frequency.
Vestigial Sideband
The transmitted portion of the sideband which has been largely suppressed by a transducer having a gradual cutoff in the neighbourhood of the carrier frequency, the other sideband being transmitted without much suppression.
Vibrating String
When a string vibrates it radiates as a dipole and is very inefficient and so needs to be attached to a panel that will be excited by the vibrations in the string and act as a far more efficient radiator of sound.
Vibrating Wire
A sensor that monitors pressure by measuring the change in resonant frequency of a pre-tensioned wire in response to pressure or force changes. See also: Mode Shape, Node.
Vibration
An oscillation about an equilibrium position or shape.Vibration is generally interpreted as the cyclical (symmetrical or nonsymmetrical) fluctuations in the rate at which an object accelerates. In longitudinal vibration the direction of motion of the particles is the same as the direction of advance of the vibratory motion; in transverse vibration …
Vibration Isolation
Vibration problems are solved by considering the system as a number of springs and masses with damping. It is sometimes possible to reduce the problem to a single mass supported by a spring and a damper.If the vibration is produced by a motor inside a machine, it is usually desirable to ensure that the frequency of motor oscillations (the forcing f…
Vicker's Hardness
Determined by indenting the material with a diamond pyramid under a specific load and then measuring the size of the impression. See also: Hardening, Hardness, Knoop Hardness Test.
View Factor
The fraction of the total energy emitted by one surface that is directly incident on another surface.
Vigesimal
Related to intervals of 20.
Vignetting
Standard image.Severe vignetting.The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures of elements within an optical system.
Villari Effect
The change in magnetic properties of a ferromagnetic material in response to the presence of stress in the ferromagnetic material.
Vinculum
The horizontal bar in a fraction separating the numerator from the denominator. See also: Fraction.
Virgin Metal
Primary metal obtained directly from ores rather than recycled scrap. See also: Metal.
Virtual Image
The image formed when light only appears to come from the location of the image. It cannot be shown on a surface but is visible e.g. a mirror. See also: Real Image.
Virtual Mass
The actual mass of a body, plus its apparent additional mass. See also: Mass.
Virtual Particles
Particles that exist only for the brief moment allowed by Heisenberg's uncertainty principle. See also: Heisenberg Uncertainty Principle.
Virus
A small microbe. Viruses cause diseases such as the common cold, flu, polio and smallpox.
Viscoelasticity
A type of deformation exhibiting the mechanical characteristics of viscous flow and elastic deformation. See also: Viscosity.
Viscosity
That molecular property of a fluid which enables it to support tangential stresses for a finite time and thus to resist deformation; the ratio of shear stress divided by shearing strain. A measure of the internal friction within a fluid. The time required for a liquid to drain out of a capillary tube is directly proportional to its viscosity.The ta…
Viscous Damping
Viscous damping is the dissipation of energy that occurs when a particle in a vibrating system is resisted by a force proportional to the velocity of the particle particle and direction opposite to the direction of the particle.Viscous damping is used largely for system modeling since it is linear. See also: Damping.
Viscous Flow
The flow of fluid through a duct under conditions such that the mean free path is very small in comparison with the smallest dimension of a transverse section of the duct.This flow may be either laminar or turbulent.
Viscous Fluid
A fluid whose molecular viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid. See also: Viscosity.
Viscous Force
The force per unit volume or per unit mass arising from the action of tangential stresses in a moving viscous fluid. This force may then be introduced as a term in the equations of motion. See also: Stress.
Viscous Stresses
The components of the stress tensor when the pressure, i.e., the mean of the three normal stresses, has been subtracted out from each of the normal stresses. See also: Stress.
Viseoelastic Material
A material which can behave as an elastic solid on a short-time scale and as a viscous fluid on a long-time scale.
Visible Light
Visible light is electromagnetic radiation with a wavelength between 400 and 750 nm. ? x10-7mf x1014HzRed6.470 - 7.0004.634 - 4.284Orange5.850 - 6.4705.125 - 4.634Yellow5.750 - 5.8505.215 - 5.125Green4.912 - 5.7506.104 - 5.215Blue4.240 - 4.9127.115 - 6.104Violet4.000 - 4.2407.495 - 7.115
Vitamin
A substance that is critical for proper functioning of a living organism that the organism is unable to produce in sufficient quantities for itself. They were first named by Dr Casimir Funk in 1912.Vitamin A Vitamin A (carotene) occurs in certain fats and the fatty parts of some foods. Also found in carrots and tomatoes. It is used by the human bod…
Vitiated Air
Air from which oxygen has been removed, thus mainly nitrogen. See also: Air, Nitrogen, Oxygen.
Vitreous Enamel
Vitreous Enamel is an opaque or transparent glaze, generally coloured, which adheres to a suitable metallic surface when applied in a liquid state.
Vitrifying Tendency
Tendency of the crystalline phase of a ceramic to transform into an amorphous or glassy phase when subjected to aging or temperature cycling. See also: Ceramic.
Voice Coil
Coil attached to the diaphragm of a moving coil loudspeaker. The coil is moved through an air gap between magnetic pole pieces.
Volatile
A solid or liquid material that easily vaporizes. A material with a significant vapour pressure.
Volatile Memory
A type of memory that loses the numbers stored in it when electrical power is removed from it. Most RAM memories are also volatile memories.
Volatile Organic Compound
Organic chemicals and petrochemicals that emit vapours. In paints, VOC generally refers to the solvent portion of the paint which, when it evaporates, results in the formation of paint film on the substrate to which it was applied.
Volcanoes
Vents or fissures in the earth's crust through which gases, molten rock, or lava , and solid fragments are discharged.A caldera on Mount Etna.An old lava flow on Mount Etna. See also: Aa, Richter Scale, Seismic, Seismograph.
Volt
The volt is the SI unit of electromotive force. One volt produces a current of 1 amperethrough a resistance of 1 ohm. Electromotive force is the differenceof potential produced by an electrical source to drive a current through an externalelectrical circuit.Conversions1 volt=1.0x108 abvolts1 volt=1 J C-11 volt=0.003333 statvol…
Voltage
A potential difference causes current to flow in a circuit, no voltage no current. See also: Differential Voltage, Open Circuit Voltage, Volt, Voltmeter.
Voltage Divider
A precision resistor network that reduces the output voltage of a device. See also: Volt.
Voltage Drop
Voltage or difference in potential developed across a component due to current flow.
Voltage Gain
Ratio of amplifier output voltage to input voltage usually expressed in decibels. See also: Decibel.
Voltage Multiplier
Rectifier circuit using diodes and capacitors to produce a DC output voltage that is some multiple of the peak value of AC input voltage. This is a cost effective way of producing higher DC voltages. Typical examples are voltage doublers and voltage triplers.
Voltaic Pile
An early battery consisting of disks of dissimilar metals (usually zinc and copper) separated by moist paper or cloth soaked in an electrolyte solution. See also: Battery.
Voltampere (VA)
The product of rms voltage and rms current in an electronic circuit. It is the unit of apparent power in the International System of Units (SI). See also: Ampere, SI Units, Volt.
Voltmeter
A device for measuring voltage across a component in a circuit. See also: Volt, Voltage.
Volume
The amount of space an object takes up.The amount of space a container can hold.There are a number of derived units of volume in the British system of units and theSI system of units. Archimedes developed ways of measuring the volumes of solids in 220BC.Conversions1 stere (st)=1 m31 litre=0.001 m31 cubic yard=0.764555 m3<…
Volume Velocity
Within acoustics the volume velocity U of a source is the pressure p divided by the acoustic impedance Z. (U is not a vector and as such should not be described as a velocity.)The volume velocity may also be described as the surface area of the source multiplied by the normal surface velocity. This is particularly useful for airborne route tracking…
Volumetric Efficiency
Ratio of the volume discharged from a pump to the piston displacement of the pump. In diesel engines a term often used instead of the correct term charge efficiency.
Vortex
Any flow possessing vorticity.Specifically a flow with closed streamlines or the idealized case in which all vorticity is concentrated in a vortex filament.
Vortex Shedding
A phenomenon which can occur when a fluid flows past a flexible structure. In this situation, vortices are shed alternately to either side of the structure (assuming its long axis is perpendicular to the direction of flow) and an oscillating force results.If the frequency of this force is close to a natural frequency of the structure, large displac…
VOX
Voice operated Switch.
Vulcanization
A process of combining rubber with sulphur or other substances that causes the polymer chains to crosslink, making them stronger and more elastic.
Vulgar Fraction
A rational number expressed as a ratio rather than as a decimal fraction. See also: Fraction.
Wallis, Barnes (1887-1979)
British engineer and inventor famous for the R100, Wellington bomber and the bouncing bombs as used by the Dambusters in WWII.
Walnut
Cross grained wood used for ornamental work.
Warmth
A term that is used to describe a listening term in terms of frequency, generally considered the range from approximately 150Hz-400Hz. A system with the 'proper' warmth will sound natural within this range. See also: Acoustics, Dead Room, Live Room, Room Modes.
Warning Limits
Limits placed on a control chart for variables or attributes at 1 and 2 'sigma' to help determine how far points lie from the centerline. See also: Control Chart.
Warp
The yarns running lengthwise and parallel to the selvage in a woven fabric. See also: Weft.
Water
H2O dihydrogen monoxide. A colourless, tasteless liquid with some very peculiar properties that stem from the bent H-O-H structure of its molecules. Water is distinguished from other common terrestrial substances in existing in all three phases at atmospheric temperatures and pressures. The phase changes, are of great significance in man…
Water of Crystallization
In chemistry, water of crystallization is water present in the crystal of a hydrate. See also: Hydrate, Water.
