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

Gears
Wheels with teeth around the circumference that mesh to transfer rotation between the two at the ratio of the radius of the pitch circle. See also: Gear Design.

Geiger Counter
A radiation detector consisting of two electrodes with a low-pressure gas in between. A voltage is maintains such that if radiation passing through the counter ionizes the gas, an avalanche of electrons will occur. Geiger counters can count radiation but cannot distinguish either the energy or kind of radiation. See also: Radiation, Scintillation C…

Gene
Part of a chromosome found inside the cells of living organisms. Different genes control different characteristics, e.g. there are genes controlling eye colour and hair colour.

General Conversions
ac ftAbbreviation of acre foot, a volume one foot deep covering an area of one acre.ac inAbbreviation of acre inch, a volume one inch deep covering an area of one acre.Acre FootA volume one foot deep covering an area of one acre.Acre InchA volume one inch deep covering an area of one acre.BottleA unit of volume.BtuAbbreviation of British Thermal Un…

General Theory of Relativity
An extension of the special theory of relativity to include the concept of gravity. See also: Einstein Field Equation, Gravity, Special Theory of Relativity.

Genetic Algorithm
A type of optimization algorithm that uses Darwinian principles to evolve a solution to a mathematical model based on survival of the fittest.

Genetic Engineering
A man-made method of altering the genes to change the characteristics of an organism.Historical Notes1985 Genetic fingerprinting invented by Alec Jeffreys.1987 A crime suspect is convicted on the evidence of genetic fingerprinting in the UK.1988 A patent is granted to cover a genetically engineered mouse.1992 The first 'xenotransplant' from one typ…

Geocentric Model
A model of the Universe with the Earth at its center. See also: Earth, Heliocentric Model.

Geocorona
The outermost layer of the Earth´s neutral atmosphere, a huge cloud of hydrogen surrounding our planet. Its density diminishes with distance and it has been observed up to distances of 5-6 Earth radii.

Geodesic
The arc on a surface of shortest length joining two given points.

Geodesy
The study of the shape of the Earth, e.g. its deviations from an exact sphere. See also: Earth.

Geodetic Coordinates
A system of geographic position referencing. Angular measurements of latitude and longitude are projected onto a well-defined reference ellipsoid that approximates the earth's irregular shape.

Geological Timescale
The timescale of each Epoch, Period and Era are tabulated below:EpochfromtoHolocene10000present dayPleistocene1.64 million10000Pliocene5.2 million1.64 millionMiocene23.5 million5.2 millionOligocene35.5 million23.5 millionEocene56.5 million35.5 millionPalaeocene65 million56.5 millionPeriodfromtoQuaternary1.64 millionpresent dayTertiary65 million1.64…

Geomagnetic Field
The magnetic field observed in and around the Earth. The intensity of the magnetic field at the Earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole. See also: Earth, Gauss.

Geometric Mean
A statistic calculated by multiplying n data values together and taking the n-th root of the result. It is often used as a measure of central tendency for positively skewed distributions. The geometric mean may also be calculated by computing the arithmetic mean of the logarithms of the data values and taking the inverse logarithm of the result.

Geometric Optics
That branch of optics dealing with the tracing of ray paths through optical systems.

Geometric Scattering
Acoustic scattering in which the wavelength of the sound used is much smaller than the size of object causing the scattering. See also: Acoustics, Doppler Sonar.

Geometrized Units
A system of units whereby certain fundamental constants (G, c, k, and h) are set to unity. This makes calculations in certain theories, such as general relativity, much easier to deal with, since these constants appear frequently.As a result of converting to geometrized units, all quantities are expressed in terms of a unit of distance, traditional…

Geometry
The branch of mathematics that deals with the nature of space and the size, shape, and other properties of figures as well as the transformations that preserve these properties.

Geostationary
A satellite orbit traveling from west to east at speeds that allow it to remain fixed over a given place on the earth's equator at approximately 22,300 miles in altitude. A geostationary satellite makes one revolution in 24 hours, synchronous with the earth's rotation. See also: Earth.

German Silver
This describes a number of alloys that contain copper (25 to 50%), zinc (10 to 35%) and nickel (5 to 35%) within the limits given.

Germanium
Germanium is a silvery white brittle metalloid member of the carbon group of elements, its physical properties being similar to those of silicon, the element which precedes it in the group. Other elements within the carbon group are relatively common, but germanium is found only in trace amounts in some coals and as a minor component in some ores, …

Germination
The growth of a new plant from a seed.

Gestation Period
The length of gestation, the time from conception to birth. For humans this is 266 days.ExamplesMammalGestation period [days]Hamster16Mouse21Grey Squirrel44Dog62Pig113Sheep148Chimpanzee237Human266Dolphin276Horse337Whale365African Elephant640

Giant Molecular Cloud
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.

Gibbs Free Energy
A function of the system related to Suniv. It is used to determine the spontaneity of a reaction. It is represented by the symbol G. See also: Internal Energy.

Gibbsite
White crystalline mineral consisting of aluminum hydroxide; a constituent of bauxite and a source of alumina. See also: Aluminium, Aluminium Hydroxide.

Given
Information assumed to be true in a proof.

Glass
Wine glass.Transparent or translucent substance that is physically neither a solid or liquid. It is made by fusing certain types of sand (silica).

Glass Ceramic
A fine-grained crystalline material that was formed as a glass and subsequently devitrified (crystallized). See also: Ceramic.

Glass Cutter
Photograph courtesy of Draper.co.ukA handtool with a hardened wheel or diamond at the end that is used to score the surface of glass so that it may be broken along the scored line. See also: Glass.

Glass Transition Temperature
A property of non-crystalline solids which do not have a sharp melting point. It characterises the transition from true solid to viscous liquid in these materials. A noncrystalline ceramic or polymer transforms from a supercooled liquid into a rigid glass.

Glider
A highly efficient engineless aeroplane. Capable of flying for long periods in gently rising air currents. See also: Aircraft.

Global Positioning System
An array of satellites, deployed and maintained by the U.S. Department of Defense, which can be monitored to triangulate an accurate position on the earth's surface.

Globular Cluster
A spherically symmetric collection of stars which shared a common origin. The cluster may contain up to millions of stars spanning up to 50 parsecs.

Glucose
A sugar. It is made by plants during photosynthesis.

Glue
Cement or adhesive that is used to fix two or more parts together. See also: Abhesive, Adhesive, Cascamite, Cyanoacrylate, Epoxy Glue, Paper Glue.

Gluon
An exchange particle responsible for the force between quarks. There are eight gluons that differ only in their color quantum numbers. See also: Carrier Particle, Quark.

Glycerol
Glycerol is a small molecule with three alcohol groups. It is a basic building block of fats and oils. Thick sweet liquid used in the manufacture of explosives. Symbol C₃H₈O₃

Glycine
A naturally occurring aliphatic amino acid, found in large quantities in gelatin. Symbol NH₂CH₂COOH

Glycogen
The storage polysaccharide of animals. In mammals it is found in greatest concentrations in the liver.

Goggles
A device with coloured lenses which protect the eyes from harmful radiation during welding and cutting operations. See also: Cover Glass.

Gold
Gold is a soft metal with a characteristic yellow colour. It is the most malleable and ductile of any element. It is unaffected by air, water, alkalis and acids, with the exception of 'aqua regia', HNO₃/HCl. The fact that it is chemically unreactive means that it is often found in its natural state. It is a good thermal and electrical co…

Golden Mean
The proportion of the division of a line so that the smaller part is to the larger partwhat the larger part is to the whole. This may be applied to the lengths of the sides ofa rectangle. Classically this ratio has been used to define the size of objects as it isthought that this ratio yields objects that are pleasing to the eye. It is equal to the…

Googol
A number equal to 10¹⁰⁰. See also: Centillion, Decillion, Googolplex.

Googolplex
A number equal to 10^googol. Where a googol is 10¹⁰⁰. See also: Googol.

Governor
A mechanical or electronic device for controlling or limiting the rotational speed of an engine or shaft.

Grade
Grade is used to define an angle, grade of sandpaper, amongst others..grade1/100 of a right angle or p/200 radian.gradeRoughness of sandpaper or emery clothe.

Graduation Mark
The marks that define the scale intervals on a measuring instrument are known as graduation marks.

Grain
An individual crystal in a poly-crystalline metal or alloy.15.4324 grain (gr) = 1 g480 gr = 1 troy ounce

gram
The CGS unit of mass equal to 10^-3kg.Conversions1000 gram=1 kilogram (kg)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 Megagram=1000 kilogram (kg)ttle='Mass';xiunt='g';yiunt='ounces';mconv=28.34952;cconv=0.0; See also: CGS Units, Mass, Metri…

Graph Paper
Paper that is printed with fine lines making up a regular grid.

Graphical User Interface
Abbreviated to GUI and pronounced 'gooey'. It is the programming code defining the operation and graphics displayed on a computer monitor. It's the code that drives the 'look and feel' of a computer program. See also: Human Machine Interface.

Graphite
Photograph courtesy of MineraliteAn amorphous form of carbon, made of carbon atoms bound hexagonally in sheets. It is used as a thermal-insulating electrical-conductor, as a nuclear-reactor moderator and as a self-lubricant. Also known as Black Lead - natural graphite of the sort used in pencils.

Gravitational Collapse
When a massive body collapses under its own weight.Example:Interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.

Gravitational Confinement
The containment of nuclei under the influence of a gravitational field (such as occurs in stars) which can provide conditions in which nuclear fusion occurs.

Gravitational Constant
The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity.G = 6.67x10^-11 Nm²kg^-2ttle='Gravitational Constant';consttxt='G';constval=6.67e-11;constunt='Nm²kg^-2';

Gravitational Field
The space surrounding an object where each location is assigned a value equal to the gravitational force experienced by one unit of mass placed at that location.

Gravitational Interaction
The interaction of particles due to their mass/energy.

Gravitational Mass
The property of a particle that determines the strength of its gravitational interaction with other particles. See also: Inertial Mass, Mass.

Gravitational Potential Energy
The work done by the force of gravity when an object falls from a particularpoint in space to the location assigned the value of zero.The energy a body possesses based on it's height above a reference surface may be calculatedfrom:wherePE = potential energy [J]m = mass [kg]g = acceleration due to gravity [ms^-2]h = height above reference …

Gravitational Potential
The worrk done in bringing a unit mass from infinity to that point. The assumption being that the gravitational potential is zero at infinity.Gravitational potential,WhereU = gravitational potential [Jkg^-1]G = universal gravitational constant = 6.67x10^-11Nm²kg^-2m = mass of body [kg]r = distance from body …

Gravitational Redshift
The decrease in the frequency of electromagnetic waves due to a gravitational field. Einstein showed that, according to general relativity, time runs more slowly in a gravitational field.

Gravitational Wave
Ripple in the structure of space-time which may occur individually or as continuous radiation. According to Einstein's Theory of General Relativity, they are emitted when extremely massive objects experience sudden accelerations or changes of shape. They travel through space at the speed of light. Gravitational waves remain undetected.

Gravitationally Bound
Objects held in orbit about each other by their gravitational attraction.ExampleSatellites in orbit around the earth are gravitationally bound to Earth since they can't escape Earth's gravity. By contrast, the Voyager spacecraft, which explored the outer solar system, was launched with enough energy to escape Earth's gravity altogether, and hence i…

Graviton
The carrier particle of the gravitational interactions; not yet directly observed.Graviton DecouplingAn epoch, early in the life of the universe, in which a density was reached such that gravitons no longer constantly interacted with other particles. The gravitons became free to travel large distances, unhindered.

Gravity
One of the forces of nature. It is an attractive force exerted between two or more particles all of which have mass.Discovered by Isaac Newton around 1690, who stated the law of gravity:'Every mass in the Universe attracts every other with a force proportional to the product of their masses and inversely proportional to their distance of separation…

Gravity Wave
A wave disturbance caused by the acceleration of masses.

Gray
This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. The unit gray can be used for any type of radiation, but it does not't describe the biological effects of the different radiations. Absorbed dose…

Great Circle
A circle on the surface of a sphere whose centre is the centre of the sphere. See also: Circle, Small Circle, Sphere.

Greatest Common Factor
The greatest common factor of two numbers is the largest number that divides into both numbers evenly, with no remainder. For example, the greatest common factor of 60 and 84 is 12.This is Also known as the Greatest Common Divisor and the greatest common divisor of a and b is written as (a,b).Note that (0,0)=0

Greek Alphabet
alpha?abeta?ßgammaG?delta?depsilon?ezeta??eta??thetaT?iota??kappa??lambda??mu?µnu??xi??omicron??pi?prho??sigmaSstau?tupsilon??phiFfchi??psi??omegaO?

Greencap Capacitor
A type of polyester capacitor that the manufacturer dips in green paint to make it distinctive from all other capacitors. See also: Capacitance, Capacitor, Ceramic Capacitor, Electrolytic Capacitor.

Greenhouse Effect
The 'greenhouse effect' is caused by certain gases in the atmosphere trapping heat radiated from the Earth. These 'greenhouse' gases absorb outgoing infra-red radiation but do not significantly affect incoming visible radiation (i.e., they act like panes of glass in a greenhouse); the higher their concentration, the more radiation returned and so t…

Greenhouse Gases
The three most powerful long lived greenhouse gases in the atmosphere are: Carbon Dioxide (CO₂), Methane (CH₄) and Nitrous Oxide (N₂O). See also: Carbon Dioxide, Greenhouse Effect, Methane, Nitrous Oxide.

Greenockite
Mineral, main source of cadmium sulphide.

Greenwich Mean Time
Local mean time at the Greenwich meridian. See also: Time.

Gregorian Calendar
Introduced in 1582 by Pope Gregory the 13th, this calendar modifies the Julian calendar for greater precision, decreeing that century years such as 1900 are not leap years, except if the number of centuries is divisible by 4 (e.g. 2000). See also: Julian Calendar, Julian Day.

Grommets
The design of grommets is critical to the acoustic performance of any partition such as engine bay bulkhead.Design Guidelines Grommets should be of soft material (natural rubber recommended by a supplier though may not be suitable for high temperature areas) to ensure consistent fit. Maximise grommet thickness. High contact area to hose/harness, us…

Gross
One hundred and forty four pieces of something. See also: Dozen.

Gross Vehicle Weight (GVW)
The maximum loaded weight, including the vehicle itself, passengers, and cargo, for which a vehicle is designed and as specified by the manufacturer. Often used as a criterion of vehicle size for the purpose of legislation.

Ground Loop
An alternative path in which voltage can travel. Ground loops can produce measurement errors. See also: Grounding.

Ground Plane
The earth or negative rail of a circuit. A large or significant mass that presents the effect of earth (ground) to a signal.

Ground State
The lowest energy state of a system allowed by quantum mechanics.

Grounding
Establishing an electric connection to the Earth in order to neutralize an object. See also: Ground Loop.

Group
A vertical column in the periodic table. All elements in the same group have the same properties. e.g. Group 1 elements all react with water. See also: Periodic Table.

Group Delay
The rate of change of phase shift with respect to frequency. Mathematically, the first derivative of phase verses frequency. The rate of change is just a measure of the slope of the phase shift verses linear (not log) frequency plot. If this plot is a straight line, it is said to have a 'constant' (i.e., not changing) phase shift, or a 'linear phas…

Group Velocity
A group may be made up of a number of different frequencies, wavelengths and velocitiesthat are superposed. Such a group would disperse with time if the wave velocity of eachcomponent is different.wherev_g = group velocity [ms^-1]v_p = phase velocity [ms^-1]? = angular frequency [rads^-1]k = wavenum…

Gunmetal
This is a copper-tin alloy, similar to bronze, used where resistance to corrosion and/or wear requiredIt and it is easily cast. Composition: 88% copper, 8% tin, 4% zinc. The alloys now usually contain lead to improve machining characteristics and bearing properties. The American term for gunmetal is Red Brass.

Gyroscope
A disc with a heavy rim mounted in such a way that its axis of rotation can adopt any position. Once the disc is spinning, the rotation axis remains fixed with reference to fixed stars, which makes the gyroscope useful for determining movement away from a fixed course.Three gyroscopes rotating about perpendicular axes help to maintain the orientati…

Haas Effect
Also known as the precedence effect. Delayed sounds are integrated by the auditory system if they arrive at the ear within 20 to 40ms of the direct sound. The level of the delayed components contributes to the apparent level of the sound, and it is accompanied by a pleasant change in character. See also: Acoustics, Clarity, Direct Sound, Hearing.

Haber Process
The Haber process, named after Fritz Haber, is an industrial process in which ammonia is manufactured by direct combination of its elements, nitrogen and hydrogen. The reaction is carried out at 400 to 500°C and at 200 atmospheres. The two gases (nitrogen and hydrogen), in the proportions of 1:3 by volume, are passed over a catalyst of finely divid…

Habituation
The process by which an organism ceases to respond to some recurring or familiar stimulus. See also: Psychoacoustics.

Hacksaw
Photograph courtesy of Draper.co.ukA metal blade of hardened steel having small, close teeth on one edge, held under tension in a U-shaped frame.Photograph courtesy of Draper.co.ukThe junior hacksaw is a smaller version of the standard hacksaw and is ideal for cutting small delicate pieces of work where the weight alone of the standard hacksaw may …

Hadrons
The family of particles that participate in the strong interaction. Baryons (protons, neutrons, and hyperons) and mesons (particles with masses between those of electrons and protons) are the two subfamilies. Hadrons are composed of quarks. These can be either mesons which are quark doublets or baryons which are quark triplets. See also: Baryon, Ex…

Hafnium
Hafnium is a silvery coloured, ductile metal which is found in all minerals containing zirconium. The chemistries of the two metals are similar which makes them difficult to separate, and the properties of each are greatly affected by the presence of the other as an impurity. Both zirconium and hafnium are extracted as the pure metal by reducing th…

Half
One of two equal parts. See also: Quarter.

Half Duplex
Describes a circuit capable of transmitting in either direction, but only one direction at a time. See also: Full Duplex, Synchronous.

Half Life
The amount of time it takes for half an initial amount to disintegrate. Longer half lives typically correspond to lower radioactivity. Tritium, the heaviest isotope of hydrogen, decays with a relatively short half life of 12.3 years.

Half Reactions
The two parts of an oxidation-reduction reaction, one representing oxidation, the other reduction.