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


Rechargeable Battery
A battery in which the chemical reaction system providing the electrical current is easily 'chemically' reversible. After discharging, it can be recharged by applying an electrical current to its terminals. Some batteries can be recharged hundreds to thousands times. See also: Battery, Power Density.

Reciprocal
A number that is made into a fraction. The reciprocal of 2 is 1/2.

Reciprocity
If in a space filled with air which is partly bounded by finitely extended fixed bodies and is partly unbounded, sound waves being excited at any point A, the resulting velocity-potential at a second point B is the same both in magnitude and phase, as it would have been at A, had B been the source of the sound. See also: Sound.

Recombination Era
The time when matter and radiation first separated. The Universe became `transparent'; the cosmic microwave background was emitted. The 'decoupling' between matter and radiation happened because the Universe was cool enough already to allow protons to capture one electron and form a neutral atom of hydrogen: the electrons, charged particles that in…

Recrystallization Annealing
Annealing cold worked metal to produce a new grain structure without phasechange. See also: Recrystallization.

Recrystallization Temperature
The approximate minimum temperature at which complete recrystallization of acold worked metal occurs within a specified time. See also: Recrystallization.

Recrystallization
The formation of a new set of strain-free grains within a previously cold-worked material; normally an annealing heat treatment is necessary. See also: Recrystallization Annealing, Recrystallization Temperature.

Rectangle
A quadrilateral with 4 right angles.

Rectangular Window
Also known as Dirichlet window. An amplitude weighting used to truncate continuous time signals to fit within the length of the DFT window. The rectangular window has a value of 1 over the length of the window and 0 outside.The fourier transform of a rectangular window is the sinc function:On a log scale the response is:

Rectifier
A device that passes current in only the forward direction. See also: Bridge Rectifier, Diode, Half Wave Rectifier.

Recursive Filter
A filter in which there is no need to store past measurements for the purpose of calculating current estimates. Such a filter makes use of a recursive algorithm for its operation. See also: Filter, Kalman Filter.

Recycling
The re-use of material.

Red Giant
An old star that has used up all the hydrogen in its nucleus and uses other elements as fuel to keep shining. The Sun will become a red giant in the future. These stars can be 25 times as big as the Sun, and hundreds of times brighter.

Red Supergiant
A supergiant with spectral type M. These are the largest stars in the universe. See also: Star, Supergiant.

Redox Reaction
A reaction in which one or more electrons are transferred.

Redshift
When the light an object emits is displaced toward the red end of the spectrum it is said to be redshifted. In general, photons of light that are emitted at a source at one energy and detected by an observer at a lower energy are redshifted. Often, the redshift of an object can be measured by examining atomic absorption or emission lines in its spe…

Reduced Instruction Set Computer
Abbreviated as RISC, this is a computer architecture that has reduced chip complexity by using simpler processing instructions. It is faster than its more complex counterparts, thanks to its simplicity, and is also designed and built more economically. See also: Computer.

Reduction Reaction
A reaction in which a substance gains at least one electron. Reduction is the opposite of oxidation.The oxidation number decreases, becomes more negative.

Reference System
A collection of objects not moving relative to each other that can be used to describe the motion of other objects. See also: Inertial Reference System, Noninertial Reference System.

Reflected Sound
That portion of a sound wave which bounces off a surface and reverses direction.If the sound takes more than 90 milliseconds to travel via the relected path compared to the direct path it will be perceived as an echo if it is not substantially attenuated.

Reflecting Telescope
A type of telescope using a mirror as the objective. See also: Refracting Telescope, Telescope.

Reflection
If waves are incident upon a reflecting surface (e.g. mirror, harbour wall, etc.) they will obey the law of reflection which states:angle of incidence = angle of reflection, where the angles are measured from the surface normal.Specular ReflectionWhen the energy is scattered in only one direction, where the angle of incidence equals the angle of re…

Reflection Nebula
A type of nebula that shines by reflected light. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in colour. The plural of nebula is neb…

Reflection Phase Grating
A diffuser of sound energy usingthe principle of the diffraction grating. See also: Diffusers.

Reflex Angle
An angle between 180° and 360°. See also: Angles.

Refracting Telescope
A type of telescope using a lens as the objective. See also: Reflecting Telescope, Telescope.

Refraction
The bending of light that occurs at the interface between two transparent media. It occurs when the speed of light changes. See also: Diffraction, Diffuse Reflection, Refractive Index.

Refractive Index
This is the ratio of the velocity of light in free space to that in the material.Click on an item to paste into clipboard or use clipboard symbol at end to clipboard all values Acetaldehyde 1.3311 at 589.3nm 101.3kPa 0°CClip Acetic Acid 1.3718 at 589.3nm 101.3kPa 0°CClip Acetic Anhydride 1.3904 at 589.3nm 101.3kPa 0°CClip Acetone 1.362 at 589.3nm 1…

Refractory
A metal or ceramic that may be exposed to extremely high temperatures without deteriorating rapidly or without melting. See also: Ceramic, Metal.

Refrigerator
A heat engine running backward.

Regenerative Braking
The absorption and re-use of energy dissipated whilst braking a vehicle.When the brakes are applied on a vehicle the energy is dissipated as heat. If electric motors are fitted to the drive system then some of this energy may be turned back into electrical energy that is then used to re-accelerate the vehicle.There are many alternative ways that ha…

Regenerative Receiver
An amplitude modulated (AM) radio receiver in which positive feedback is used in order to increase the sensitivity and selectivity of the receiver.

Register
An electronic device that holds a binary number. See also: Index Register, Pointer, Program Counter.

Regular Polygon
A polygon in which all of the angles and all of the sides are equal. See also: Decagon, Pentagon.

Relative Density
The ratio of the density of the substance to that of water at 4°C

Relative Frequency
A count of the number of occurrences of a data value in a sample, or the number of values falling within a fixed range, expressed as a proportion of the total number of observations. Data can be summarized in this manner by tabulating all the data values into distinct categories and then counting the number of times each category appears in the fre…

Relative Humidity
The ratio of the absolute humidity to the humidity at saturation. See also: Absolute Humidity, Dew Point, Humidity, Humidity at Saturation, Temperature.

Relative Magnetic Permeability
The ratio of the magnetic permeability of some medium to that of a vacuum. See also: Permeability.

Relatively Prime
If the greatest common divisor of a and b is 1 then a and b are known as relatively prime. See also: Greatest Common Factor, Prime Numbers, Primitive Pythagorean Triangle.

Relativistic Kinetic Energy
The energy that a body possesses solely because it is moving.If the particle (body) is travelling at a significant proportion of the speed of lightthe relativistic kinetic energy should be used, Twherem0 = rest mass of the body, [kg]c = speed of light, [ms-1]? = Lorentz factorv = velocity of particle (body), [ms-1]

Relay
Electromechanical device containing a coil and set of contacts. The contacts close when the coil is activated.

Relief Valve
A valve held closed by a spring and forced open when the pressure in the system rises above the desired limit. See also: Pressure.

Reluctance
The opposition offered by a material to magnetic flux. See also: Magnetic Flux, Permeance.

Remainder
The amount left over when one quantity cannot be exactly divided by another. See also: Division.

Renewable Energy
Energy obtained from sources that are essentially inexhaustible, unlike fossil fuels. See also: Energy, Solar Cell, Wind Power.

Repeatability
The consistency (or variation) of readings and results between consecutive sets of measurements. It has nothing to do with accuracy.

Residual Intensity
This is the sound intensity level measured when the same signal is fed to both channels of a sound intensity measuring system, or it is exposed to a purely reactive sound field. See also: Sound Intensity.

Resistance
The electrical resistance of a conductor is defined by:where I is the current flowing through the conductor and V is the potential difference across the conductor.The power disipated by a resistor is given by Joule's law:whereW = power disipated [Watt]I = current [Ampere]R = resistance [Ohm]V = potential difference across resistor [Volt]

Resistor
Passive component with a known resistance. The value of resistance is usually shown by a set of coloured bands on the body of the component.The resistor shown is coded Orange, Green, Blue with a silver tolerance band.Using the following table the resistance may be calculated from the first three bands, Orange = 3, Green = 5 and the multiplier is Bl…

Resolution
The smallest change or amount a measurement system can detect. Ability to discriminate fine detail in an image, a spectrum or data. The angular resolution of a telescope is the smallest angle between two point objects that produces distinct images. In a spectrum, the resolution determines how well closely spaced features in the wavelength spectrum …

Resonance
The resonant frequency of a system is defined as the frequency for which the response of the system is a maximum. If the excitation frequency is either increased or decreased the amplitude of response will decrease. See also: Anti Resonance, Quality Factor, Resonant Frequency.

Resonant Frequency
Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate. A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency. While…

Resonator
An acoustic element which is used to attenuate the sound at narrow band frequencies in ducts and tubes. A simple resonator comprises a cavity enclosing a mass of air, with a narrow opening to the outside. The air mass effectively acts as a 'spring' at the resonant frequency of the cavity and under those conditions absorb appreciable sound energy ex…

Response
The response of a device or system is the motion (or other output) resulting from an excitation under specified conditions. In mechanical systems, the output is usually displacement, velocity or acceleration and the input is usually a force.

Rest Energy
Rest energy is the energy an object has due to its mass.whereE = rest energy [J]m = mass [kg]c = speed of light [ms-1]

Rest Mass
The rest mass of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word 'mass', they always mean the 'rest mass' of the object in question. See also: Mass, Rest Energy.

Retrograde
The motion of a planet or other Solar System body in a clockwise direction. Most Solar System bodies orbit or rotate about their axes in an anti-clockwise direction when looked at from the north pole of the body or the Sun.

Retroreflectors
Three flat mirrors at right angles to each other that reflect light back to its source.

Reverberant Sound Field
A sound field made of reflected sounds in which the time average of the mean square sound pressure is everywhere the same and the flow of energy in all directions is equally probable. This requires an enclosed space with essentially no acoustic absorption, e.g., a reverberation chamber. Sometimes reverberation is confused with an echo but strictly …

Reverberation Room
Room having a long reverberation time, specially designed to make the sound field therein as diffuse as possible. See also: Reverberant Sound Field, Reverberation Time.

Reverberation Time
The persistence of sound in an enclosure after a sound source has stopped. Reverberation time is the time in seconds required for sound pressure at a specific frequency to decay 60 dB after a sound source is stopped.For the ISO standard the reverberation time is measured over a -5 to -35dB sound pressure level below the constant sound source level …

Reverse Osmosis
Solvent molecules flow spontaneously from a dilute solution through a semipermeable membrane to a more concentrated solution (osmosis). In reverse osmosis, pressure is applied to the more concentrated solution to force the flow of solvent to go from more concentrated to more dilute solution. Reverse osmosis is used to produce fresh water from sea w…

Revolution
A rotation through a full circle, or 360 degrees. See also: Circle, Plane Angle.

Revolutions per Minute
Unit to describe the rate of rotation of a body.Conversions1 revolution per minute=1 rev min-11 rpm=1 revolution per minute1 rpm=0.1047 rad s-11 rev s-1=60 rpmttle='Angular Velocity';xiunt='rad s-1';yiunt='rpm';mconv=0.1047;cconv=0.0;ExamplesSecond hand on clock1 rpmGramophone record331/3<…

Revolutions per Second
Unit to describe the rate of rotation of a body.Conversions1 revolution per second=1 rev s-11 rps=1 rev s-11 rev s-1=6.283 rad s-11 rev s-1=60 rpmttle='Angular Velocity';xiunt='rad s-1';yiunt='rev s-1';mconv=6.283;cconv=0.0; See also: Angular Velocity, Revolutions per Minu…

Reyn
British unit of dynamic viscosity.Conversions1 Reyn=1 lbf s in-21 Reyn=6895 Nsm-21 Reyn=6.895x106 cP1 Reyn=144 slugs ft-1s-1ttle='Dynamic Viscosity';xiunt='Nsm-2';yiunt='Reyn';mconv=6895;cconv=0.0; See also: Poise, Viscosity.

Reynolds Number
This is a non-dimensional number (consistent units) which describes whether fluid flow is laminar, turbulent, or transitional between the two. It is significant in the design of a model of any system in which the effect of viscosity is important in controlling the velocities or the flow pattern of a fluid.whereRe = Reynolds Number? = den…

Rhenium
Rhenium was named after 'Rhenus', the Latin name for the Rhine. It is a rare element (abundance 4 x 10-4ppm in the earth`s crust) and does not occur in quantity in any ore. It is found in ores which contain molybdenum from which it can be readily recovered. The metal is obtained by hydrogen reduction of the potassium perrhenate salt, obt…

Rheology
Study of flow characteristics.

Rheostat
A device to regulate the flow of electric current by transforming part or all of it into heat. See also: Resistance.

Rhodium
Rhodium does not appear naturally, tending to be found with other platinum group metals. It is a hard, lustrous, silvery coloured metal which is stable in air. Rhodium is inert to all acids but is attacked by fused alkalis. The metal has high thermal and electrical conductivities and is alloyed with platinum to form the positive wire of a Pt/Rh - P…

Rhombus
A parallelogram with four equal sides.

Ribbon Cable
Any cable constructed as a ribbon with parallel elements.

Richter Scale
A measure of the magnitude of an earthquake at its epicentre. Each point increase on the Richter scale is a 30 times increase in the magnitude.MagnitudeRelative amount of energy released112313960430000592000062900000078900000008280000000009850000000000 See also: Mercalli Scale, Plate Techtonics, Seismic, Seismograph.

Ride Quality
Ride quality is measured in terms of the level of isolation from road inputs the suspension transfers to the vehicle without compromising vehicle control. How well a suspension isolates a trailer and its cargo is measured by the level of forces that act on the cargo and trailer in normal operation. Minimising these forces results in less cargo dama…

Right Angle
The angle formed when two straight lines intersect and the 4 angles at theircrossing are all equal.right angle = p/2 radians = 90°

Right Triangle
A triangle that contains a right angle. See also: Hypotenuse, Pythagoras Theorem, Triangle.

Rigid
An idealized concept meaning something which does not deform under loading. In fact, all objects deform under loading, but in modelling it can be useful to idealize very stiff objects as rigid.

Rigid Body Modes
These modes exist for all objects and are the modes of vibration where the object itself remains rigid and are the rotation or translational modes of the object on its mounting system.A powertrain subsystem would be expected to develop six rigid body modes on its mounts. Ideally, all six of these modes would occur below the suspension modes, below …

Ringing
Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked. See also: Frequency, Natural Frequency, Oscillation.

Rise Time
The time required for the output of a transducer to rise from 10% to 90% of its final value, as it responds to a step change in the measurand. See also: Transducer.

Rivets
A permanent fixing used to join plates. The unheaded end is forged or flattened.

Road Noise
Generated by the interaction of the tyre with the road.The noise reaches the driver's ear via two fundamentally different routes:Structureborne via the suspension components to the vehicle cabin and thenairborne to the observer.Airborne from the tyre/road interface through thepanels of the body or through holes in the body structure to the observer…

Roche Limit
The position around a massive body where the tidal forces due to the gravity of the primary equal or exceed the surface gravity of a given satellite. Inside the Roche limit, such a satellite will be disrupted by tides. See also: Decaying Orbit.

Rocker Arm
A lever that transmits the action of the cam, usually by means of a push rod, to the stem of the intake or exhaust valve, sometimes also to the starting-air valve and fuel valve.The rocker arm highlighted on a Bentley BR.1 Rotary engine. See also: Camshaft, Internal Combustion Engine, Poppet Valve.

Rocket
Commonly used term for a launch vehicle. One burning liquid or solid fuel and carrying Its own oxidizer, enabling combustion to continue outside of the earth's atmosphere.Historical Notes1232 Rockets invented in China to defend city of Kaifeng against Mongol invaders.1500 Chinese scientist Wan Hu ties 47 gunpowder rockets to a chair in an effort to…

Rockwell Hardness
Hardness determined from size of indentation using a diamond or steel ball and given load.There are three scales used:A scale diamond cone with 60 kilogram load.B scale 1/16' steel ball with 100 kilogram load.C scale 120° diamond cone with 150 kilogram load.

Rod
An old English unit of length. Also known as Pole.Conversions1 rod=5.0292 m1 rod=0.25 chain1 rod=25 links1 rod=16.5 feet1 rod=5.5 yards1 rod=198 inchesttle='Rod';xiunt='m';yiunt='rod';mconv=5.0292;cconv=0.0; See also: Chain, Furlong, Length, Yard.

Roentgen
The roentgen is a unit used to measure radiation exposure. This canonly be used to describe an amount of gamma and X-rays, and only in air. Oneroentgen is equal to depositing in dry air enough energy to cause 2.58x10-4 coulombs kg-1.It is a measure of the ionizations of the molecules in a mass of air.The main advantage of this…

Roentgen Equivalent Man (rem)
The rem is a unit used to derive a radiation quantity called equivalent dose. This relates to the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem. To dete…

Roll
The oscillation of a rigid body (e.g. sprung mass) which consists primarily of angular displacement about a central longitudinal axis with respect to its static position.

Roll Off
A measure of the attenuation of a filter with frequency, e.g. a 40 Hz analogue filter will give some attenuation to signals slightly below 40 Hz and pass a small proportion of higher frequency signals. See also: Filter.

Rolled Thread
A thread formed by plastically deforming a blank rather than by cutting. This increases both fatigue strength and thread shear strength. See also: Thread.

Roller Support
In two dimensions, a roller support restrains one translation degree of freedom. See also: Pin Connection.

Roman Numerals
Roman NumeralDecimal Equivalent1510501005001000500010000500001000005000001000000ExamplesI = 1II = 2III = 3IV = 4V = 5VI = 6VII = 7VIII = 8IX = 9X = 10XI = 11XII = 12XIII = 13XIV = 14XV = 15XVI = 16XVII = 17XVIII = 18XIX = 19XX = 20XLIX = 49XCIX = 99

Room Absorption
Sum of Sabine absorption due to objects and surfaces in a room, and due to dissipation of energy in the medium within the room. See also: Absorption Coefficient, Room Acoustics, Sabin, Sound Absorption.

Room Acoustics
The general acoustic requirements of a room depend on the use. The basic characteristics are:Background noise levelFrequency responseReverberation timeSound break-in from external sourcesThe basic parameters that can be controlled when designing a room are:Room sizeRoom shape (cavity acoustics)Wall transmission loss (acoustic barriers) and in addit…

Room Modes
Frequencies at which sound waves in a room resonate (in the form of standing waves), based on the room dimensions. The acoustic modes will 'colour' the sound, ie enhance certain frequencies and dull others.Therefore, the standard modal approach for designing a room with good acoustics is to create as many different resonances as possible, and to sp…

Root
A root of a polynomial function, f, is just a solution to the equation f(x) = 0, that is an input that yields an output of 0.The Nth root of a number, is that which when multiplied by itself N times produces the desired number. The square root or 2nd root of 4 is 2 (2 x 2 = 4). The cube (3rd) root of 27 is 9 (3 x 3 x 3 =9). See also: Cub…

Root Mean Square (rms)
This is the square root of sum of squares of individual observations divided by the total number of observations made.The r.m.s. value of a function, x(t), over a time interval between t1 and t2 is specified by the following formula:Because energy varies as the square of amplitude, rms tracks the amount of energy in a complex …