 FACTOID # 9: The bookmobile capital of America is Kentucky.

 Home Encyclopedia Statistics States A-Z Flags Maps FAQ About

 WHAT'S NEW

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

(* = Graphable)

Encyclopedia > Thermal conductivity

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in Fourier's Law for heat conduction. Hookes law accurately models the physical properties of common mechanical springs for small changes in length. ... A magnet levitating above a high-temperature superconductor demonstrates the Meissner effect. ... This article or section does not cite any references or sources. ... For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences. ... Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences. ...

It is defined as the quantity of heat, ΔQ, transmitted during time Δt through a thickness L, in a direction normal to a surface of area A, due to a temperature difference ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.

thermal conductivity = heat flow rate × distance / (area × temperature difference) $k=frac{Delta Q}{Delta t}timesfrac{L}{AtimesDelta T}$
Alternately, it can be thought of as a flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length) $k=frac{Delta Q}{Atimes{} Delta t}timesfrac{L}{Delta T}$

Typical units are SI: W/(m*·K) and English units: Btu·ft/(h·ft²·°F). To convert between the two, use the relation 1 Btu·ft/(h·ft²·°F) = 1.730735 W/(m·K). [Perry's Chemical Engineers' Handbook, 7th Edition, Table 1-4] flux in science and mathematics. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... The Imperial units are an irregularly standardized system of units that have been used in the United Kingdom and its former colonies, including the Commonwealth countries. ...

Thermal conductivity depends on many properties of a material, notably its structure and temperature. For instance, pure crystalline substances exhibit very different thermal conductivities along different crystal axes, due to differences in phonon coupling along a given crystal axis. Sapphire is a notable example of variable thermal conductivity based on orientation and temperature, for which the CRC Handbook reports a thermal conductivity of 2.6 W/(m·K) perpendicular to the c-axis at 373 K, but 6000 W/(m·K) at 36 degrees from the c-axis and 35 K (possible typo?). Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... For other uses, see Sapphire (disambiguation). ...

Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection. Examples of these include expanded and extruded polystyrene (popularly referred to as "styrofoam") and silica aerogel. Natural, biological insulators such as fur and feathers achieve similar effects by dramatically inhibiting convection of air or water near an animal's skin. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... Polystyrene (IPA: ) is a polymer made from the monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. ... A 2. ... Closeup on a single white feather A feather is one of the epidermal growths that forms the distinctive outer covering, or plumage, on a bird. ...

Thermal conductivity is important in building insulation and related fields. However, materials used in such trades are rarely subjected to chemical purity standards. Several construction materials' k values are listed below. These should be considered approximate due to the uncertainties related to material definitions. Common insulation applications inside an apartment building in Mississauga, Ontario, Canada. ...

The following table is meant as a small sample of data to illustrate the thermal conductivity of various types of substances. For more complete listings of measured k-values, see the references.

List of thermal conductivities

This is a list of approximate values of thermal conductivity, k, for some common materials. Please consult the list of thermal conductivities for more accurate values, references and detailed information. In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. ... In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat. ...

Material Thermal conductivity
W/(m·K)
Air 0.025
Wood 0.04 – 0.4
Alcohol or oil 0.15
Soil 0.15
Rubber 0.16
Epoxy (unfilled) 0.19
Epoxy (silica-filled) 0.30
Water (liquid) 0.6
Thermal grease 0.7 – 3
Glass 1.1
Ice 2
Sandstone 2.4
Stainless steel 15
Aluminium 237
Gold 318
Copper 390
Silver 429
Diamond 900 – 2320

Measurement

For good conductors of heat, Searle's bar method can be used . For poor conductors of heat, Lees' disc method can be used . An alternative traditional method using real thermometers is described at . A brief review of new methods measuring thermal conductivity, thermal diffusivity and specific heat within a single measurement is available at  A thermal conductance tester, one of the instruments of gemology, determines if gems are genuine diamonds using diamond's uniquely high thermal conductivity. In heat transfer analysis, thermal diffusivity (symbol: ) is the ratio of thermal conductivity to heat capacity. ... Gemology (gemmology outside the United States) is the science, art and profession of identifying and evaluating gemstones. ... For other uses, see Gemstone (disambiguation). ... This article is about the mineral. ...

Standard Measurement Techniques

• IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements" 
• IEEE Standard 98-2002, "Standard for the Preparation of Test Procedures for the Thermal Evaluation of Solid Electrical Insulating Materials", ISBN 0-7381-3277-2 
• ASTM Standard D5470-06, "Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials" 
• ASTM Standard E1225-04, "Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique" 
• ASTM Standard D5930-01, "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique" 
• ASTM Standard D2717-95, "Standard Test Method for Thermal Conductivity of Liquids" 

Related terms

The reciprocal of thermal conductivity is thermal resistivity, measured in kelvin-metres per watt (K·m·W−1). For other uses, see Kelvin (disambiguation). ... This article is about the unit of length. ... For other uses, see Watt (disambiguation). ...

When dealing with a known amount of material, its thermal conductance and the reciprocal property, thermal resistance, can be described. Unfortunately there are differing definitions for these terms.

First definition (general)

For general scientific use, thermal conductance is the quantity of heat that passes in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity k, area A and thickness L this is kA/L, measured in W·K−1. This matches the relationship between electrical conductivity (A·m−1·V−1) and electrical conductance (A·V−1). Electrical conductivity or specific conductivity is a measure of a materials ability to conduct an electric current. ... Electrical conductance is the reciprocal of electrical resistance. ...

There is also a measure known as heat transfer coefficient: the quantity of heat that passes in unit time through unit area of a plate of particular thickness when its opposite faces differ in temperature by one degree. The reciprocal is thermal insulance. In summary:

• thermal conductance = kA/L, measured in W·K−1
• thermal resistance = L/kA, measured in K·W−1
• heat transfer coefficient = k/L, measured in W·K−1·m−2
• thermal insulance = L/k, measured in K·m²·W−1.

The heat transfer coefficient is also known as thermal admittance

Second definition (buildings)

When dealing with buildings, thermal resistance or R-value means what is described above as thermal insulance, and thermal conductance means the reciprocal. For materials in series, these thermal resistances (unlike conductances) can simply be added to give a thermal resistance for the whole. R-value is a term predominantly used in the building industry to rate the insulative properties of construction materials and building assemblies. ...

A third term, thermal transmittance, incorporates the thermal conductance of a structure along with heat transfer due to convection and radiation. It is measured in the same units as thermal conductance and is sometimes known as the composite thermal conductance. The term U-value is another synonym. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... â€œRadiant heatâ€ redirects here. ...

In summary, for a plate of thermal conductivity k (the k value ), area A and thickness L:

• thermal conductance = k/L, measured in W·K−1·m−2;
• thermal resistance (R value) = L/k, measured in K·m²·W−1;
• thermal transmittance (U value) = 1/(Σ(L/k)) + convection + radiation, measured in W·K−1·m−2.

3 + 2 = 5 with apples, a popular choice in textbooks This article is about addition in mathematics. ... Convection in the most general terms refers to the internal movement of currents within fluids (i. ... â€œRadiant heatâ€ redirects here. ...

Textile industry

In textiles, a tog value may be quoted as a measure of thermal resistance in place of a measure in SI units. The tog is a measure of thermal resistance, commonly used in the textile industry, and often seen quoted on, for example, duvets. ...

Origins

The thermal conductivity of a system is determined by how atoms comprising the system interact. There are no simple, correct expressions for thermal conductivity. There are two different approaches for calculating the thermal conductivity of a system. The first approach employs the Green-Kubo relations. Although this expression is exact*, in order to calculate the thermal conductivity of a dense fluid or solid using this relation requires the use of molecular dynamics computer simulation. Green-Kubo relations give exact mathematical expression for transport coefficients in terms of integrals of time correlation functions. ...

• The term exact is applied to mean that the equations are solvable.

The second approach is based upon the relaxation time approach. Due to the anharmonicity within the crystal potential, the phonons in the system are known to scatter. There are three main mechanisms for scattering: Normal modes of vibration progression through a crystal. ...

• Boundary scattering, a phonon hitting the boundary of a system;
• Mass defect scattering, a phonon hitting an impurity within the system and scattering;
• Phonon-phonon scattering, a phonon breaking into two lower energy phonons or a phonon colliding with another phonon and merging into one higher energy phonon.

Further information can be found in the publication "The Physics of Phonons" by G P Srivastava.

Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and hence acts to even out temperature differences. ... For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... A thermal bridge is created when materials that are poor insulators come in contact, allowing heat to flow through the path created. ... In physics, thermal contact conductance is the study of heat conduction between solid bodies in contact. ... In heat transfer analysis, thermal diffusivity (symbol: ) is the ratio of thermal conductivity to heat capacity. ... Thermal resistance is the temperature difference across a structure when a unit of heat energy flows through it in unit time. ... NTC thermistor, bead type, insulated wires Thermistor symbol A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. ... In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. ... R-value is a term predominantly used in the building industry to rate the insulative properties of construction materials and building assemblies. ... Results from FactBites:

 Thermal conductivity - Facts, Information, and Encyclopedia Reference article (614 words) In physics, thermal conductivity, λ, is the intensive property of a material which relates its ability to conduct heat. Thermal conductivity is the quantity of heat, Q, transmitted through a thickness L, in a direction normal to a surface of area A, due to a temperature gradient ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient. A third term, thermal transmittance, incoporates the thermal conductance of a structure along with heat transfer due to convection and radiation.
More results at FactBites »

Share your thoughts, questions and commentary here
Press Releases | Feeds | Contact