Terminology


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Absolute temperature - temperature measured on the absolute scale where zero is the absolute zero. Also called the Kelvin temperature scale. Absolute temperature is measured in kelvins and now rarely degrees Rankine. Absolute zero is 0 K or 0 R (-273.15 C or -459.67 F).

Absolute zero - zero kelvin (0 K), temperature at which atomic motion stops. For some special systems and specific definitions of temperature, it is possible to obtain a negative temperature. A system with a negative temperature is not colder than absolute zero but rather it is, in a sense, hotter than infinite temperature.

Albedo - reflectivity of the sky or bodies observed in the sky or from space.  The albedo of the Moon is 12%, Earth (average)38%.                                              

Anisotropy - directional dependence of measured properties such as thermal conductivity; the condition of a material having different properties in different directions. Examples of an anisotropic material for heat conduction are laminated materials (composed of layers with different thermal conductivity), having different thermal conductivity perpendicular and parallel to the layers.

Black body - a body that absorbs all radiation falling on it and emits electromagnetic waves with spectrum given by the Planck's law. Further info at answers.com  

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Brownian motion - random motion of particles in a liquid or a gas due to thermal excitation. Robert Brown was the first to describe this phenomenon in 1827.

Carnot efficiency - Even an ideal, frictionless engine cannot convert anywhere near 100% of its input heat into work. The limiting factors are the temperature at which the heat enters the engine, Th, and the temperature of the environment into which the engine exhausts its waste heat, Tc. From Carnot's theorem, for any engine working between these two temperatures: (eta)th = 1 - Tc/Th. This limiting value is called the Carnot cycle efficiency because it is the efficiency of an unattainable, ideal, reversible engine cycle called the Carnot cycle. No heat engine, regardless of its construction, can exceed this efficiency.

Celsius temperature scale - a modern replacement of the Fahrenheit scale. The conversion is Tf = (9/5)*Tc+32 where Tc = temperature in degrees Celsius, Tf = temperature in degrees Fahrenheit.

Chemical potential - The chemical potential of a thermodynamic system is the amount by which the energy of the system would change if an additional particle is introduced, with the entropy and volume held fixed. If a system contains more than one species of particle, there is a separate chemical potential associated with each species, defined as the change in energy when the number of particles of that species is increased by one. The chemical potential is particularly important when studying systems of reacting particles.

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Conductance (thermal) the quantity of heat that passes in unit time through unit area of a material or system when its opposite faces differ in temperature by one degree.                                                            

Conductivity (thermal) the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area, when the heat transfer is dependent only on the temperature gradient. (See also apparent or effective thermal conductivity.)  Note: there are a number of qualifiers regarding this term due to the fact that heat is not transmitted by conduction processes alone.                                                                                        

Conservation of energy - energy is neither created nor destroyed, it simply changes its form. In thermal radiation terms this translates to: a + r + t = 1 where a = absorptivity, r = reflectivity, t = transmissivity.        

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Contact thermometry - putting a thermometer in contact with the measured object - see also Non-contact thermometry.                        

Cryogenic AFM - an atomic force microscope that can operate at very low temperatures of several kelvins.                                                     

Density (mass) - symbol r (Greek rho) - is a measure of mass per unit of volume. The higher an objects density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. Bulk density is a term for density of a collection of particles (eg grains of sand) in a container, as opposed to the material density that is the density of the individual particles (eg one grain of sand).  

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Diffusivity (thermal) of a substance is equal to the ratio of its thermal conductivity to its heat capacity. (See heat capacity.) 

Dye - a colouring substance which can be natural or synthetic. Dye molecules absorb and emit light within a specific range of wavelengths, thus they appear to have a certain colour.                                               

Emissivity - the emissivity of a surface is the ratio of the energy radiated from it to that from a blackbody at the same temperature, the same wavelength and under the same viewing conditions. The term "emissivity" has a lot of qualifiers, depending on wavelength and direction of emission or detection. For instance, "normal spectral emissivity" indicates the emissivity at a particular wavelength and in a direction normal to the surface, while "hemispherical total emissivity" indicates emissivity when radiation is emitted over all wavelengths into all of the space above a material surface. The term "emittance" is often used instead of emissivity. This is to distinguish the emissivity of a particular sample of material from the 'characteristic' value of the material when it has an optically smooth surface and is thick enough to be opaque. However, it is almost impossible to achieve the latter condition in practice and so this terminology is not universally adopted.                                                                                     

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Emittance or emissivity of a material is the ratio of the radiant heat flux emitted by the material to that emitted by a blackbody at the same temperature and under the same viewing conditions. (See radiant heat flux.) Note: there are four qualifiers that apply, namely, directional, hemispherical, spectral and total, depending on direction and wavelength range, respectively.   

Endothermic process - process that requires heat from its surroundings. 

Entropy - quantifies the degree of disorder in a closed system and the probability of a reaction taking place or not. In a closed system entropy increases until the equilibrium is reached. The thermodynamic definition of entropy reflects the fact that heat cannot be completely converted into work, as noted by Lord Kelvin: "There is no device that can transform heat withdrawn from a reservoir completely into work with no other effect".

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Eutectic - a mixture of substances having a lower melting point than any of its constituents.                                                                           

Exothermic process - one that gives up heat to its surroundings. Opposite of an endothermic process.

Fahrenheit - temperature scale, still commonly used in the United States, devised by Daniel Gabriel Fahrenheit (1686-1736), German physicist and maker of scientific instruments such as the alcohol thermometer (1709) and mercury thermometer (1714).                                                    

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First law of thermodynamics - states that heat is energy.                    

Fourier heat equation - a partial differential equation for temperature as a function of spatial coordinates and time.                                                      

Fuel cell - an electrochemical device that produces electricity and heat from a fuel (hydrogen and oxygen) without combustion. The process is an 'inverse electrolysis'. Hydrogen and oxygen are catalytically reassociated to water. Fuel cells could power electrical cars if the problem of handling and storage of hydrogen could be solved, possibly using nanotubes.               

Glass transition - a kinetically (rather than thermodynamically) conditioned temperature region below which on cooling the molecular mobility greatly decreases as a polymer changes from a rubbery to a glassy state.                                                                                            

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Grey body - a body that has emissivity less than 1 and independent of wavelength.                                                                                

Heat capacity - the quantity of heat required to change the temperature of a body by one degree. For a homogeneous body it is the product of mass and specific heat capacity; and for an inhomogeneous body it is the sum of the product of mass and specific heat capacity of all the individual homogeneous components.                                                             

Homogeneous material - is a material in which the relevant physical properties are uniform throughout the material.                                      

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Infrared camera - a system consisting of a lens transparent to the infrared radiation, a detector and associated electronics giving the user an image of energy in the infrared spectrum.                                                

Infrared region - a part of the electromagnetic adjacent to visible light.  Approximately from 0.7m to microwave.

Infrared thermal region - the portion of the infrared spectrum that the majority of heat energy is recorded from. This portion of the electromagnetic spectrum is defined as 3m to microwave. However, most of infrared imaging occurs between 3m and 14m.                                

Isotropic material - one in which the propagation of heat (or electric current or electromagnetic waves) does not depend on direction of  propagation.

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Joule - the unit of energy and name of the British scientist who first determined the work equivalent of heat.
                                                                                                 
Kelvin  - international temperature scale in which 0 kelvin represents absolute zero (the temperature at which no heat energy exists within an object).                                                                                                          

Kirchhoff's law (a = e, where a = absorptivity and e = emissivity).        

Le Chatelier principle - A system in stable chemical equilibrium submitted to the influence of an exterior force which tends to cause variation either in its temperature or its condensation (pressure, concentration, number of molecules in the unit of volume) in its totality or only in some or one of its parts can undergo only those interior modifications which, if they occur alone, would produce a change of temperature, or of condensation, of a sign contrary to that resulting from the exterior force. - Generalisation of van't Hoff's law. 

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Leidenfrost effect - the phenomenon whereby film boiling replaces the usual nucleated boiling above a certain temperature, thereby forming an insulating sheath of vapour around any sufficiently hot body immersed in a liquid. Only on cooling below a certain temperature (defined here as the Nukiyama temperature) will rapid heat exchange occur between water and hot, gas-free lava. Near the surface, this could produce phreatic explosions and extensive clouds of steam.

Maxwell distribution - gives the statistical distribution of speeds of  molecules in a gas that can yield the average and the most probable speeds. 

Melting point - the temperature at which a solid changes into a liquid. The melting point depends on pressure (Clausius-Clapeyron law).

Nano - a prefix meaning one billionth (1/1,000,000,000). 

Nanotechnology - is the creation of materials, devices, and systems on the nanoscale (up to 100 nm) through the manipulation of individual atoms and molecules. 

Nanotubes - tubelike molecules of carbon (diameter about 10,000 times smaller than a human hair) consisting of graphite sheets, being extremely strong materials with high thermal conductivity. Nanotubes can be single walled or multiwalled (composed of one or several sheets in a concentric formation). Nanotubes may have no 'temperature'.

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Non-contact thermometry - measuring temperature of a body without touching it (for example measuring the temperature of the sun by a pyrometer). 

Ohmic heating - is caused by passing electric current through a resistor (e.g. an electric heater).

Peltier cell - usually a cooling element utilising the Peltier effect, used to cool or refrigerate small objects. 

Peltier effect - a  change in temperature at the junction of two different metals or semiconductors produced when an electric current flows through them. The extent of the change depends on what the conductors are and the nature of change (rise or fall in temperature) depends on the direction of current flow. It is the reverse of the Seebeck effect. It is named after the French physicist Jean Charles Peltier (17851845) who discovered it in 1834.

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Phase diagram - a representation of the regions and boundaries of existence of various phases of a physico-chemical system depending on one or more thermodynamic quantities, such as temperature, pressure or concentration.

Planck's constant - h= 6.6260755 x 10-34 Js. 

Planck radiation law - determines radiation emitted at a particular wavelength and temperature. 

Planck's radiation law - was formulated in 1900 by Max Planck, a German physicist, to explain the spectral-energy distribution of radiation emitted by a blackbody. 

Platinum resistance thermometer - a thermometer with a sensor made of a thin layer of platinum based material whose resistance increases with temperature.  See also - resistance thermometer.

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Pyrometer - instrument for detecting temperature using infrared radiation. 

Quench rate - the speed at which a sprayed liquid or solid particle cools upon striking a cold surface. 

Radiant heat flux - the rate of radiant energy emitted from unit area of a surface in all directions into the hemisphere of space above the surface.

Rankine - W J M Rankine (1820-1872), a Scottish engineer, created his scale, which was merely the Kelvin scale using the Fahrenheit degree instead of the Celsius.

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Raumur -  now an obsolete temperature scale created by R A F de Raumur (1683-1757) a French scientist. He used the freezing point of water as his zero mark, and put the boiling point at 80 degrees.

Reflectance - is the ratio of reflected power to incident power, expressed in decibels or percentage.

Reflectivity  - The terms reflectivity and reflectance are often used interchangeably, although there is a difference between the two terms. Reflectance is a measurement of the amount of light that is reflected by a coating at a given film thickness. Reflecti­vity is the maximum amount of light that can be reflected by the coating; no further increase in coating film thickness or smoothness will increase the value.

Resistance thermometer - has a sensing element the resistivity of which changes with temperature. The sensing element can be a conductor (RTD) or a semiconductor (thermistor). 

Scanning thermal microscope - an atomic force microscope with an integrated sensor to detect local variations in the thermal conductivity. 

Second law of thermodynamics - states that not all heat energy can be recovered from a body (Lord Kelvin's formulation). An alternative formulation (E. Clausius) is that heat can flow from a colder body to a warmer body only at the expenditure of extra non-thermal energy (as in a refrigerator).

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Seebeck effect - generation of a voltage in a circuit containing two different metals, or semiconductors, by keeping the junctions between them at different temperatures. Discovered by the German physicist Thomas Seebeck (17701831), it is also called the thermoelectric effect, and is the basis of the thermocouple. It is the opposite of the Peltier effect (in which current flow causes a temperature difference between the junctions of different metals).

Specific heat capacity - of a material is the quantity of heat required to change the temperature of unit mass of the material by one degree. Note: can also be an average quantity rather than a point value, when applied to a specific temperature range.

Steady state - in heat transfer, the condition in which the temperature at any given point in a material or material system is independent of time, to a given precision for a specified time period. 

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Stefan-Boltzmann equation (determines radiation emitted over all wavelengths at a particular temperature)

Thermal conductivity - is the property of a material describing its ability to conduct heat. Its unit of measurement is Wm-1K-1 because thermal conductivity is numerically equal to thermal flux density (Wm-2) through an isothermal surface caused by a unit temperature gradient (Km-1).

Thermal diffusivity -  is a combination of properties (thermal conductivity, heat capacity and density) that indicates how rapidly heat is conducted in a material. Its unit of measurement is m2s-1.

Thermal efficiency -  is a dimensionless performance measure of a thermal device such as an internal combustion engine. Heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%).

Thermistor - temperature dependent resistor. 

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Thermocouple -  consists of two dissimilar conductor wires, an electromotive force is generated at the junction between the two conductors when heated (Seebeck's effect).

Thermometer - an instrument for measuring temperature of a body or a medium.  A thermometer consists of a sensor (temperature sensing element) and a readout  instrument i.e. a display device and connecting leads. The principle is that some property of the sensor changes with the temperature. The range of techniques is extensive: the expansion of a liquid, the change of gas pressure, the change of electrical quantity (resistance, voltage), the radiation change, etc. The temperature measurement may be done by contact with the medium or non-contact.

Third law of thermodynamics - postulates that it is impossible to cool a body to absolute zero by any finite process. Although one can approach absolute zero as closely as one desires, one cannot reach it. Walter Nernst formulated this law also known as the Nernst heat theorem.

Transference (thermal) - the total steady-state heat flow from (or to) a body to (or from) its surroundings by conduction, convection and radiation and is expressed as the time rate of heat flow per unit area of the body surface per unit temperature difference between the body and its surroundings.

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Triple point - the temperature and pressure at which three phases (gas, liquid, and solid) of a substance may coexist in thermodynamic equilibrium.

Triple point of water (TPW)The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 C) and a pressure of 611.73 pascals (ca. 6 millibars). At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature. For further info see Wikipedia.

Universal gas constant - R = 8.3144 J K-1 mol-1. The universal gas constant is the constant R appearing in the ideal gas law pV=nRT, where p is pressure, V is volume, n is number of moles of gas and T is absolute temperature. The universal gas constant is defined in terms of Boltzmann's constant k as R=kNA where NA is Avogadro's number. 

U-value - a quantity in building construction quantifying the insulating properties of walls, windows and other structures. The reciprocal of the U-value is the thermal resistance or R-Value. The lower the U-value number, the greater the heat transfer resistance (insulating) characteristics of the material or assembly of materials. Low U-values (down to 0.1 Wm-2C-1) are achievable using special insulating materials and techniques.

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van't Hoff's law - All equilibria between two different states of matter (systems) are displaced by a lowering of temperature toward that of the two systems whose formation develops the heat.

Vapour pressure - evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping. At this point the vapor is saturated and the pressure of that vapour is the saturated vapor pressure. Since the molecular kinetic energy increases with temperature, more molecules can escape the surface and the saturated vapor pressure increases. If the liquid is open to the air then the vapor pressure is a partial pressure contributing to the total pressure along with the other gases in the air. The temperature at which the vapour pressure is equal to the atmospheric pressure is the boiling point.

Wiedemann-Franz Law - states that the ratio of the thermal conductivity to the electrical conductivity of a metal is proportional to the temperature. This is because heat and electrical transport both involve free electrons in the metal. The thermal conductivity increases with the average particle velocity since that increases the transport of energy. However, the electrical conductivity decreases with increasing electron velocity  because the collisions hinder the electrons from transport of charge. This means that the ratio of thermal to electrical conductivity depends upon the average velocity squared, which is proportional to the kinetic temperature.

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Wien's displacement equation (Lmax T = Constant; determines the wavelength Lmax of peak radiation intensity of a blackbody at temperature T).

Zeroth law of thermodynamics - states that if two systems are at the same time in thermal equilibrium with a third system, they are in thermal equilibrium with each other. 

ZETA reactor - an experiment that earned British scientists fame for contributing to fundamental ideas about plasma (very hot gases). ZETA (Zero Energy Thermal Apparatus) was one of the first large-scale fusion devices built at Harwell to try and produce conditions under which thermal fusion reactions might take place. The first successful experiments were carried out in August 1957.