Absorption (EM radiation)
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The process by which the energy of a photon is taken up by matter, typically the electrons of an atom. The electromagnetic energy is transformed to another form of energy, e.g. to heat.
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Absorption (physical)
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A physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different from Adsorption, since the molecules are taken up by the volume, not by surface.
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Absorption Spectroscopy
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Spectroscopic techniques that measure the absorption of radiation as a function of frequency or wavelength, due to its interaction with a sample.
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Activated Alumina
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A manufactured form of alumina (aluminium oxide) having a nanoporous structure.
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Activated Carbon
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(or activated charcoal, activated coal, carbo activatus) A form of carbon processed to be riddled with small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Due to its high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 square meters. An activation level sufficient for useful application may be attained solely from high surface area; however, further chemical treatment often enhances adsorption properties.
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Added Mass
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(or Virtual Mass) The inertia added to a system because an accelerating or decelerating body must move (or deflect) some volume of surrounding fluid as it moves through it.
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Adiabatic Cooling
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Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings. Adiabatic cooling does not have to involve a fluid.
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Adiabatic Heating
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Adiabatic heating occurs when the pressure of a gas is increased from work done on it by its surroundings, e.g. a piston. Diesel engines rely on adiabatic heating during their compression stroke to elevate the temperature sufficiently to ignite the fuel.
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Artificial Photosynthesis
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A chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel
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If the gas is carbon dioxide.
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Auger Effect
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When an electron is removed from a core level of an atom, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in a release of energy. Although sometimes this energy is released in the form of an emitted photon, in the case of the Auger Effect, the energy is instead transferred to another electron, which is ejected from the atom.
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Auger electron spectroscopy involves the emission of Auger electrons by bombarding a sample with X-rays or energetic electrons and measuring the intensity of Auger electrons as a function of the Auger electron energy. The resulting spectra can be used to determine the identity of the emitting atoms and some information about their environment.
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Basset Force
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The force due to the lagging boundary layer development with changing relative velocity (acceleration) of bodies moving through a fluid.
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Bioluminescence
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The production and emission of light by a living organism as the result of a chemical reaction
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Bubble
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A globule of one substance in another, usually gas in a liquid. Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.
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Bubbles can be observed optically - their presence or movement can indicate the presence of a gas or a moving gas stream.
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Capacitance
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The property of a device or material medium to store an electric charge as a result of an electric potential. The most common form of charge storage device is a two-plate capacitor.
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Capacitors with an exposed and porous dielectric can be used to measure humidity in air.
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Capillary Condensation
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The process by which multilayer adsorption from the vapour phase into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapour phase. The unique aspect of capillary condensation is that vapor condensation occurs below the saturation vapor pressure of the pure liquid. Can affect contact between solids, modifying macroscopic adhesion & friction properties.
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By detecting condensed gas within the capillary system.
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Chemiluminescence
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The emission of light (luminescence) with limited emission of heat as the result of a chemical reaction.
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Chemisorption
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A type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond, as opposed to the Van der Waals forces which cause physisorption
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Cherenkov Effect
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Electromagnetic radiation emitted when a charged particle (such as a electron) passes through an insulator at a speed greater than the speed of light in that medium.
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Chromatography
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A family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a 'mobile phase' through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated. Chromatography may be preparative or analytical.
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Coherent Light
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Two light waves are said to be coherent if they have a constant relative phase, which also implies that they have the same frequency.
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Conduction (electrical)
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The movement of electrically charged particles through a transmission medium (electrical conductor). The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is, by diffusion .
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Conduction (thermal)
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The spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature. Conduction acts to equalize temperature differences. It is also described as heat energy transferred from one material to another by direct contact.
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Corona Discharge
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An electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient (the strength of the electric field) exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or arcing.
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Dielectric Permittivity
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The measure of how much resistance is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. Permittivity is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material. Thus, permittivity relates to a material's ability to transmit (or 'permit') an electric field.
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The presence of a material will affect the amount of electric flux produced in response to an electric charge.
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Doppler Effect
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The change in frequency and wavelength of a wave for an observer moving relative to the source of the waves. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium.
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For example, a moving gaseous medium will result in a doppler shift in a sound wave propagating through it.
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Drag
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A drag (sometimes called fluid resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). The most familiar form of drag is made up of friction forces, which act parallel to the object's surface, plus pressure forces, which act in a direction perpendicular to the object's surface.
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By measuring the drag force on an object moving relative to the gas.
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Echo
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A reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.
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The speed of a returning echo will be affected by the presence of a gas.
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Electret
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An Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.
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Gas ions can be attracted by the charge of an electret. Electrets are used in radon gas detectors.
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Electric Field
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The space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field (that can also be equated to electric flux density). This electric field exerts a force on other electrically charged objects.
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Electric Glow Discharge
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A type of plasma formed by passing a current at 100 V to several kV through a gas at low pressure, usually argon or another noble gas. It is found in products such as fluorescent lights and plasma-screen televisions, and is used in plasma physics and analytical chemistry.
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Electric Spark
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A type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air. This causes an increase in the number of free electrons and ions in the air, temporarily causing the air to become an electrical conductor through dielectric breakdown.
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Without a gas to ionise, no conductive path can be established.
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Electrical Impedance Tomography
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A medical imaging technique in which an image of the conductivity or permittivity of part of the body is inferred from surface electrical measurements. Typically, conducting electrodes are attached to the skin of the subject and small alternating currents are applied to some or all of the electrodes. The resulting electrical potentials are measured, and the process may be repeated for numerous different configurations of applied current.
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Electrical Resistance
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The degree to which an object opposes an electric current flowing through it (usually measured in Ohms). A function of both its physical geometry and the resistivity of the material the object is made from.
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Electrical Resistivity Tomography
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Electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical measurements made at the surface, or by electrodes in one or more boreholes. It is closely related to the medical imaging technique electrical impedance tomography (EIT), and mathematically is the same inverse problem. In contrast to medical EIT however ERT is essentially a direct current method.
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Electrohydrodynamics
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Includes the following types of particle and fluid transport mechanisms:Electrophoresis, electrokinesis, dielectrophoresis, electro-osmosis, and electrorotation. In general, the phenomena relate to the direct conversion of electrical energy into kinetic energy, and vice versa.
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Electrolysis
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A method of separating chemically bonded elements and compounds by passing an electric current through them. Electrolysis involves the passage of an electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes.
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For a gaseous electrolyte. The passage of current, or presence of the products of electrolysis signals the presence of the gas.
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Electron Paramagnetic Resonance
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(EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion. The basic physical concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of spins of atomic nuclei
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Electrophoresis
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The motion of dispersed particles relative to a fluid under the influence of an electric field.
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Electrostatic Discharge
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The sudden and momentary electric current that flows between two objects at different electrical potentials
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Electrostatics
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The phenomena arising from stationary or slowly moving electric charges
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Enzyme
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Proteins that catalyze (i.e., increase the rates of) chemical reactions. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates sufficient for life. Enzymes differ from most other catalysts in that they are highly specific for their substrates
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Feedback
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A circular causal process whereby some proportion of a system's output is returned (fed back) to the input. This is often used to control the dynamic behavior of the system.
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Filter (physical)
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A device (usually a membrane or layer) that is designed to block certain objects or substances while letting others through. Filters are often used to remove harmful substances from air or water, for example to remove air pollution, to make water drinkable, to prepare coffee. In domestic food and drink preparation where bulk solids are removed from liquids, this is often called a strainer.
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For example, detecting the forces on a fliter located in a moving gas stream.
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Fin
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A flat surface that extends from an object, usually for the purpose of increasing surface area, increasing stiffness or for obtaining a hydrodynamic or aerodynamic interaction with an external relatively moving fluid.
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Through the presence of aerodynamic forces on the fin.
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Flow Separation
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For a solid object travelling through a fluid (or alternatively a stationary object exposed to a moving fluid) flow separation occurs when the boundary layer travels far enough against an adverse pressure gradient that the speed of the boundary layer falls almost to zero.The fluid flow becomes detached from the surface of the object, and instead takes the forms of eddies and vortices.
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Fluorescence
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Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range.
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By doping or otherwise attaching a flourescent component to the substance or object to be detected and then exposing it to appropriate radiation (e.g. security pen and UV light)
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Focusing
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The gathering of wavefronts of a wave (e.g. radiation) into a spherical or cylindical shape. Focusing of light is used in optics, however focusing can be applied to any radiation or wave.
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Through changes in refractive index.
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Fractionation
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A separation process in which a certain quantity of a mixture (solid, liquid, solute, suspension or isotope) is divided up in a number of smaller quantities (fractions) in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the individual components.
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Gettering
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A process for removal of impurities by reacting them with or attracting them to a suitable agent or part of the system (a 'getter'), with the result that their harmful effect is eliminated. Gettering originated in vaccuum tubes, where Ti was used to getter trace remnant gasses. Gettering is important today for the removal of unwanted trace elements (typically metals) from Silicon integrated circuits.
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Graphene
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A one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be visualized as an atomic-scale chicken wire made of carbon atoms and their bonds. The name comes from graphite + -ene; graphite itself consists of many graphene sheets stacked together.
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Molecules that land on graphene affect its electronic properties in a measurable way. Graphene has been suggested as the basis of detection sensors.
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Helmholtz Resonance
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The phenomenon of air resonance in a cavity, such as when one blows across the top of an empty bottle.
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Hot Chocolate Effect
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The pitch heard from tapping a cup of hot liquid rises after addition of a soluble powder. May be observed by stirring chocolate powder into a mug of hot milk, tapping bottom of mug with a spoon while milk is in motion. Pitch of the taps will increase progressively. Subsequent stirring decreases the pitch. Due to the effect of bubble density on the speed of sound in the liquid. The note heard is the frequency of standing wave with fixed wavelength of liquid column height.
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Entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.
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Hydrates
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In organic chemistry, a hydrate is a compound formed by the addition of water or its elements to a host molecule. In inorganic chemistry, hydrates contain water molecules that are either bound to a metal center or crystallized with the metal complex. Such hydrates are also said to contain 'water of crystallisation' or 'water of hydration'.
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Some hydrates change colour on hydration and can therefore be used to indicate the presence of water vapour. E.g. cobalt (II) chloride turns from red to blue on hydration.
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Hydrogel
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(also called aquagel) is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are highly absorbent (they can contain over 99.9% water) natural or synthetic polymers. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content.
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Hydrogels that are responsive to specific molecules, such as glucose or antigens, can be used as biosensors
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Image Processing
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Processing of images using mathematical operations by using any form of signal processing for which the input is an image, a series of images, or a video, such as a photograph or video frame The output of image processing may be either an image or a set of characteristics or parameters related to the image.
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Infrared Radiation
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Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer than that of visible light (400-700 nm), but shorter than that of terahertz radiation (3-300 µm) and microwaves (~30,000 µm). Infrared radiation spans roughly three orders of magnitude (750 nm and 1000 µm).
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Ionisation
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The physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.
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Isoelectric Focusing
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(also known as electrofocusing) A technique for separating different molecules by their electric charge differences. It is a type of zone electrophoresis, usually performed in a gel, that takes advantage of the fact that a molecule's charge changes with the pH of its surroundings.
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Kármán Vortex Street
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A repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies. It is responsible for such phenomena as the 'singing' of suspended telephone or power lines, the vibration of a car antenna at certain speeds.
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The nature of the vortcies will depend upon the viscosity of the fluid.
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Laser
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A device that emits light (electromagnetic radiation) through a process called stimulated emission.
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Light
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Light is electromagnetic radiation of a wavelength that is visible to the human eye (in a range from about 380 or 400 nanometres to about 760 or 780 nm)
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Luminescence
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The generation of light that usually occurs at low temperatures, and is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light generated by high temperatures.
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Magnetism
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One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
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Magnetohydrodynamic Effect
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A magnetic field can induce currents in a moving conductive fluid, which create forces on the fluid, and also change the magnetic field itself.
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Magnetotellurics
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An electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m by recording higher frequencies down to 10,000m or more with long-period soundings.
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Metastability
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A general scientific concept which describes states of delicate equilibrium. A system is in a metastable state when it is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction. It is analogous to being at the bottom of a small valley when there is a deeper valley close by
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The 'tipping' of a metastable system into a lower energy state in response to a small stimulus could be used to make a senstive detector.
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Microsphere
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Small spherical particles, with diameters in the micrometer range (typically 1µm to 1000µm (1mm)). They may be used to encapsulate and transport other substances and/or control their release.
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High sphericity of polyethylene microspheres, as well as availability of colored and fluorescent microspheres, makes them highly desirable for flow visualisation and fluid flow analysis, microscopy techniques, health sciences, process troubleshooting and numerous research applications.
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Moment of Inertia
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(or mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass). The inertia of a rotating body with respect to its rotation. The moment of inertia plays much the same role in rotational dynamics as mass does in linear dynamics,
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Nanopore
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A small pore in an electrically insulating membrane, that can be used as a single-molecule detector. A nanopore is a Coulter counter for much smaller particles. It can be a biological protein channel in a lipid bilayer or a pore in a solid-state membrane. The detection principle is based on monitoring the ionic current of an electrolyte solution passing through the nanopore as a voltage is applied across the membrane.
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Nanoporous Material
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Materials consisting of a regular organic or inorganic framework supporting a regular, porous structure. Pores are by definition roughly in the nanometre range.
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Nucleation
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The extremely localised budding of a distinct thermodynamic phase. Some examples of phases that may form via nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation. Most nucleation processes are physical, rather than chemical, but a few exceptions do exist (e.g. electrochemical nucleation).
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For example, through preferential nucleation of the gas on (or near) a detector.
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Ohmmeter
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An electrical instrument that measures electrical resistance.
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Indirectly by its electrical resistance
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Penning Effect
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The reduction of the ionization voltage of an inert gas due to the presence of a small quantity of another inert gas or other impurity.
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Permeation
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The penetration of a permeate (such as a liquid, gas, or vapour) through a solid. The permeate always migrates to the lower concentration in three steps: 1. Sorption (at the interface); 2. Diffusion (through the solid); 3. Desorption (the adsorbate leaves the solid as a gas).
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Photodissociation
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A chemical reaction in which a chemical compound is broken down by photons. Any photon with sufficient energy can affect the chemical bonds of a chemical compound. Since a photon's energy is inversely proportional to its wavelength, electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, x-rays and gamma rays are usually involved in such reactions.
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By detecting the products of photodissociation.
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Photoelectric Effect
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The emission of electrons from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light.
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By using the gas to block the radiation that is powering the photoelectric effect.
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Photography
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The process, activity and art of creating still pictures by recording radiation on a radiation-sensitive medium, such as a photographic film, or electronic image sensors.
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Photoionisation
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The physical process in which an incident photon ejects one or more electrons from an atom, ion or molecule. This is essentially the same process that occurs with the photoelectric effect with metals. In the case of a gas, the term photoionisation is more common.
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Photoluminescence
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A process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. One of many forms of luminescence (light emission) and is distinguished by photoexcitation (excitation by photons).
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Pitot Tube
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A pressure measurement instrument used to measure fluid flow velocity. Consists of a tube pointing directly into the fluid flow.
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Polarisation
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A property of waves that describes the orientation of their oscillations. For transverse waves such as many electromagnetic waves, it describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel. The oscillations may be oriented in a single direction (linear polarisation), or the oscillation direction may rotate as the wave travels (circular or elliptical polarisation).
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Pressure-sensitive Paint
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A method for measuring air pressure, usually in aerodynamic settings. It is paint which fluoresces in differing intensities depending on the external air pressure being applied to its surface.
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Pyrophoricity
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A pyrophoric substance ignites spontaneously in air at or below 55 degrees centigrade. The creation of sparks from metals is based on the pyrophoricity of small metal particles. The sparking mechanism in cigarette lighters is an example. Some liquids and gases are phyrophoric.
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In the case of a gas that supports combustion, such as air or oxygen.
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Radiation
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A process in which energetic particles or waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing.
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Radioactive Decay
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The process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation.
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Radioactive Tracing
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(or radioactive label) A chemical compound in which one or more atoms have been replaced by a radioisotope. By virtue of its radioactivity it can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products.
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Redox Reactions
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Redox (shorthand for oxidation-reduction) reactions describe all chemical reactions in which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of sugar(C6H12O6) in the human body through a series of complex electron transfer processes.
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Reduction
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A chemical reaction that involves the gain of electrons or a decrease in the oxidation state by a molecule, atom or ion.
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Reflection
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The change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.
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Refraction
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The change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when sound waves pass from one medium into another or when water waves move into water of a different depth.
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Scattering
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A general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass (includes deviation of reflected radiation from the angle predicted by the law of reflection).
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Scintillation
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A flash of light produced in a transparent material by an ionization event.
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Semipermeable Membrane
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A membrane that will allow certain molecules or ions to pass through it by diffusion (and occasionally specialized facilitated diffusion). The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute.
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Shadow
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An area where direct light (or other radiation) from a source cannot reach due to obstruction by an object. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light (or radiation).
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By observation of a cast shadow.
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Shadowgraph
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An optical method that reveals non-uniformities in transparent media like air, water, or glass. In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, these disturbances refract light rays, so they can cast shadows. The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.
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Shock Wave
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A type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or through a field such as the electromagnetic field. Characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Associated with rapid rise in pressure, temperature and density of the flow. A shock wave travels through most media at a higher speed than an ordinary wave.
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By detection of the shock wave produduced by a fast moving gas.
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SODAR
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(SOnic Detection And Ranging), or sodar, is a meteorological instrument also known as a wind profiler which measures the scattering of sound waves by atmospheric turbulence. SODAR systems are used to measure wind speed at various heights above the ground, and the thermodynamic structure of the lower layer of the atmosphere. Sodar systems are like radar systems except that sound waves rather than radio waves are used .
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Solar Energy
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Collection or use of energy from the Sun.
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By its shadow or by refraction of light by the gas.
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Sorption
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The action of both absorption and adsorption taking place simultaneously, i.e. the effect of gases or liquids being incorporated into a material of a different state and adhering to the surface of another molecule. Absorption is the incorporation of a substance in one state into another of a different state (e.g., liquids absorbed by solid or gas absorbed by liquid). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule.
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Sound
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A mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard.
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Supercritical Fluid
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Any substance at a temperature and pressure above its critical point. It can diffuse through solids like a gas, and dissolve materials like a liquid. Additionally, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties to be 'tuned'.
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Using Supercritical fluid chromatography.
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Surface Acoustic Wave
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(SAW) An acoustic wave traveling along the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate. This kind of wave is commonly used in devices called SAW devices in electronics circuits.
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SAWs have a longitudinal and a vertical shear component that can couple with any media in contact with the surface. This coupling strongly affects the amplitude and velocity of the wave, allowing SAW sensors to directly sense mass and mechanical properties.
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Terminal Velocity
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A free falling object achieves its terminal velocity when the downward force of gravity equals the upward force of drag. This causes the net force on the object to be zero, resulting in an acceleration of zero. Mathematically an object asymptotically approaches and can never reach its terminal velocity.
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Terminal vecolity depends on the presence of a surrounding medium - which may be a gas.
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Theremin
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An early electronic musical instrument controlled without contact from the player.
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Thermochromic Paint
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A paint based on colour-changing pigments. It involves the use of liquid crystals or leuco dye technology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures.
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In the case of a hot gas.
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Thermochromism
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The ability of substance to change color due to a change in temperature. Thermochromism is one of several types of chromism. The two basic approaches are based on liquid crystals and leuco dyes.
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In the case of a hot gas.
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Thermocouple
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A junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat into electricity.
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In the case of a hot gas.
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Thermography
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Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9000-14,000 nanometers or 9-14 µm) and produce images of that radiation, called thermograms.
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Time of Flight
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A variety of methods that measure the time that it takes for an object, particle or acoustic, electromagnetic or other wave to travel a distance through a medium. This measurement can be used for a time standard (such as an atomic fountain), as a way to measure velocity or path length through a given medium, or as a way to learn about the particle or medium (such as composition or flow rate).
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Turbine
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A rotary engine that extracts energy from a fluid flow. The simplest turbines have one moving part, a rotor assembly, which is a shaft with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they rotate and impart energy to the rotor. Early turbine examples are windmills and water wheels.
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Turbulence
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A fluid regime characterised by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time (flow that is not turbulent is called laminar flow).
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By observing resulting turbulence around an existing solid object.
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Vapour Cone
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(also referred to as shock collar, or shock egg) The creation of a visible condensation cloud caused by a sudden drop in air pressure. Occurs under the right atmospheric conditions - for example by an aircraft traveling at transonic speeds.
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Vapour Pressure
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The pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.
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Viscous Heating
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The work done by a fluid on adjacent layers due to the action of shear forces is transformed into heat.
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Zeolite
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Microporous, aluminosilicate minerals commonly used as commercial adsorbents. Widely used in industry for water purification, as catalysts, for the preparation of advanced materials and in nuclear reprocessing.
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