: In physics, thermal conductivity, λ, is the intrinsic property of a material which relates its ability to conduct heat. :It is the quantity of heat, ''Q'', transmitted through a unit thickness, ''L'', in a direction normal to a surface of unit area, ''A'', due to a unit temperature gradient, Δ''T'', under steady state conditions and when the heat transfer is dependent only on the temperature gradient. :: thermal conductivity = heat flow rate × distance / (area × temperature gradient) :: ''λ'' = ''Q'' × ''L'' / (''A'' × Δ''T'') (Halliday, 1997 p.470) ==Units== In the SI system of units, thermal conductivity is measured in watts per metre-kelvin, Wm-1K-1 or W/(m·K) where watt is the unit of power (physics) metre is the unit of distance kelvin is the unit of temperature Thermal conductivity should not be confused with ''thermal conductance'', which is explained below. In old literature you may find the following unit for thermal conductivity: cal/(mhC). This stands for ((cal/h)/m)/C. By multiplying this value by 0.001162 (=4.184/3600) it is converted to the SI unit (assuming the calories are the small or gram calories, not kilogram or food calories). ==Examples== In general thermal conductivity tracks electrical conductivity; metal being good thermal conductors. There are exceptions: the most outstanding is that of diamond which has a high thermal conductivity, between 1000 and 2600 Wm-1K-1, while the electrical conductivity is low. Thermal conductivity of other common materials:
Thermal conductivity changes with temperature. For most materials it decreases slightly as the temperature rises.
Since diamond has such a high thermal conductivity, natural blue diamond much higher still, one may test gemstone to determine if they are genuine diamonds using a thermal conductance tester, one of the instruments of gemology.
Diamonds of any size are notably cool to the touch because of their high thermal conductivity, perhaps the origin of the term "ice."
==Related terms==
The reciprocal of thermal conductivity is ''thermal resistivity'', measured in kelvin-metres per watt (KmW-1).
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 ''λ'', area ''A'' and thickness ''L'' this is ''λA/L'', measured in WK-1. This matches the relationship between electrical conductivity (Am-1V-1) and electrical conductance (AV-1).
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'' = ''λA''/''L'', measured in WK-1
** ''thermal resistance'' = ''L''/''λA'', measured in KW-1
*''heat transfer coefficient'' = ''λ''/''L'', measured in WK-1m-2
**''thermal insulance'' = ''L''/''λ'', measured in Km2W-1.
The heat transfer coefficient is also known as ''thermal admittance''. But thermal admittance may mean other things.
===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.
A third term, ''thermal transmittance'', incoporates 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.
The term ''K value'' is a synonym for thermal conductivity.
In summary, for a plate of thermal conductivity ''λ'', area ''A'' and thickness ''L'':
*''thermal conductance'' = ''λ''/''L'', measured in WK-1m-2
*''thermal resistance'' (''R value'', ''thermal resistivity'' in scientific terms) = ''L''/''λ'', measured in Km2W-1.
*''thermal transmittance'' (''U value'')= 1/(Addition(''L''/''λ'')) + convection + radiation, measured in WK-1m-2
==Molecular origins==
The thermal conductivity of a system is determined by how molecules comprising the system interact. There are no simple but correct expressions for the thermal conductivity. The simplest exact expression employs one of 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 [http://rsc.anu.edu.au/~evans/evansmorrissbook.htm simulation].
==See also==
*Heat conduction
*Heat transfer mechanisms under Heat
*Thermistor
*Thermocouple
==External links==
*http://physics.nist.gov/Pubs/SP811/appenB9.html
*http://gscassociates.com/wg8/edcs/text/unit.html
*http://www.iso.ch/iso/en/ittf/PubliclyAvailableStandards/ISO_IEC_18025_Ed1.html
*http://www.for.gov.bc.ca/hfp/pubs/silviculture_notes/sn16.pdf page 5
*http://www.npl.co.uk/thermal/faq_index.html#heat%20transfer%20property thermophysics FAQ5
*http://www.ornl.gov/roofs+walls/research/detailed_papers/rastra/dynamic.htm
==References==
Halliday, David; Resnick, Robert; & Walker, Jearl(1997). Fundamentals of Physics (5th ed.). John Wiley and Sons, INC., NY ISBN 0-471-10558-9.
Chemical properties
Thermodynamics
Physical quantity
In the section "First definition," the article states that "To physicists [1], thermal conductance is ..." and the footnote [1] points to the well-known and authoritative NIST Special Publication 811 which actually does not define or use the term "thermal conductance" anywhere. This link is not really good evidence or corroboration of the assertion that there is a standard definition of thermal conductance. (It is, however, a good place to find authoritative definitions of thermal conductivity, -insulance, -resistance, and -resistivity.) Perhaps it should be moved? Other semi-authoritative references like the CRC Handbook of Chemistry and Physics, 2002-203 edition don't define thermal conductance either. (Although they do have conductivity.) Does anyone have a more authoritative definition of thermal conductance as having the dimensions W / (m^2 K) (which can be more generally stated in terms of SI base units as: kg / (K s^3) ) Preferably from an authoritative standards body, and not some individual's webpage or questionable textbook. I have a feeling that "thermal conductance" is a not-well-standardized term, if it's proper at all. I also find the phrase "to physicists" very questionable, as if physicists don't or shouldn't follow the SI but rather have their own standards. (Last time I saw this in Wikipedia, it was a rather confused physicist arguing that "in physics," a pound was a measure of force, not mass.) This phrase should probably be removed. By the way, I'm interested in the right terminology because I'm building authoritative references in my programming language/calculating tool [http://futureboy.homeip.net/frinkdocs/ Frink]. : No, the NIST doesn't define ''thermal conductance'', but as I recall it does define ''electrical'' resistance and conductance as reciprocals of each other. I felt I was on pretty safe ground assuming thermal conductance was the reciprocal of thermal resistance. Thats also how it works in "building trade" units. : I've since added a few more web references that look fairly reliable: [http://gscassociates.com/wg8/edcs/text/unit.html] and for the future[http://www.iso.ch/iso/en/ittf/PubliclyAvailableStandards/ISO_IEC_18025_Ed1.html]. The first says that the terms I've used - including thermal conductance - are in ISO 31-4:1992, ''Quantities and units - Part 4: Heat''. Another new reference ([http://www.npl.co.uk/thermal/faq_index.html#heat%20transfer%20property], thermophysics FAQ 5) is much better for "builders" units. : I've replaced "To physicists" with "For general scientific use". I would have put "in SI units" or something similar, but I think maybe only some of these units are defined as part of the SI, while others are derived. After all, "bulider's" thermal resistance is loosely "in SI" if it's expressed as (K m^2)/W. And for SI we should really refer to the BIPM rather than the NIST. :User:Andy G 18:25, 10 Sep 2003 (UTC) == Thermal Conductivity definition ok? == I was wondering if someone could explain the definition of thermal conductivity at the top of the page: (thermal conductivy = heat flow rate / (distance x temperature gradient). It does not appear to agree w/ several other places I have seen the term defined as: thermal conductivity = (heat flow rate x distance)/ (Area x temperature gradient) I am not an expert in the field, and may be missing something - can anyone explain the discrepancy? : You are right, and this equation is wrong. This definition is also wordy and vague. I propose the following edit: : In physics, thermal conductivity, λ, is the Intrinsic property of a material which relates its ability to conduct heat. It is the quantity of heat, H, transmitted through a unit thickness, d, in a direction normal to a surface of unit area, A, due to a unit temperature gradient, 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 gradient) :: λ = H × d / A × ΔT : I also propose changing thickness, T, to thickness, d, in the equations later in the article. This choice of variables avoids confusion with temperature, T, and temperature gradient, λT. :If no one objects, please update the wiki. Comment posted 5/26/05 == Example values for common elements OK? == I notice that the Thermal conductivity for Silver and Copper is given as 406 and 385 W/(m*K) on this page, but is stated as 429 and 401 W/(m*K) on those elements' own pages. What is the ultimate authority on these numbers? --User:Drhex 11:21, 2005 Jun 10 (UTC)
See other meanings of words starting from letter:
Words begining with Thermal_conductivity:
Thermal_conductivity
Thermal_conductivity
Thermal conductivity
: In physics, thermal conductivity, λ, is the intrinsic property of a material which relates its ability to conduct heat. :It is the quantity of heat, ''Q'', transmitted through a unit thickness, ''L'', in a direction normal to a surface of unit area, ''A'', due to a unit temperature gradient, Δ''T'', under steady state conditions and when the heat transfer is dependent only on the temperature gradient. :: thermal conductivity = heat flow rate × distance / (area × temperature gradient) :: ''λ'' = ''Q'' × ''L'' / (''A'' × Δ''T'') (Halliday, 1997 p.470) ==Units== In the SI system of units, thermal conductivity is measured in watts per metre-kelvin, Wm-1K-1 or W/(m·K) where watt is the unit of power (physics) metre is the unit of distance kelvin is the unit of temperature Thermal conductivity should not be confused with ''thermal conductance'', which is explained below. In old literature you may find the following unit for thermal conductivity: cal/(mhC). This stands for ((cal/h)/m)/C. By multiplying this value by 0.001162 (=4.184/3600) it is converted to the SI unit (assuming the calories are the small or gram calories, not kilogram or food calories). ==Examples== In general thermal conductivity tracks electrical conductivity; metal being good thermal conductors. There are exceptions: the most outstanding is that of diamond which has a high thermal conductivity, between 1000 and 2600 Wm-1K-1, while the electrical conductivity is low. Thermal conductivity of other common materials:
| Material | Thermal conductivity Wm-1K-1 |
|---|---|
| Diamond | 1000-2600 |
| Silver | 406 |
| Copper | 385 |
| Gold | 320 |
| Aluminium | 205 |
| Brass | 109 |
| Platinum | 70 |
| Steel | 50.2 |
| Lead | 34.7 |
| Mercury (element) | 8.3 |
| Quartz | 8 |
| Ice | 1.6 |
| Glass | 0.8 |
| water (molecule) | 0.6 |
| Wood | 0.04-0.12 |
| Wool | 0.05 |
| Fiberglass | 0.04 |
| Expanded polystyrene ("beadboard") | 0.03 |
| Air (300 K, 100 kPa) | 0.026 |
| Silica aerogel | 0.017 |
| Polystyrene | 0.01 |
Thermal conductivity
In the section "First definition," the article states that "To physicists [1], thermal conductance is ..." and the footnote [1] points to the well-known and authoritative NIST Special Publication 811 which actually does not define or use the term "thermal conductance" anywhere. This link is not really good evidence or corroboration of the assertion that there is a standard definition of thermal conductance. (It is, however, a good place to find authoritative definitions of thermal conductivity, -insulance, -resistance, and -resistivity.) Perhaps it should be moved? Other semi-authoritative references like the CRC Handbook of Chemistry and Physics, 2002-203 edition don't define thermal conductance either. (Although they do have conductivity.) Does anyone have a more authoritative definition of thermal conductance as having the dimensions W / (m^2 K) (which can be more generally stated in terms of SI base units as: kg / (K s^3) ) Preferably from an authoritative standards body, and not some individual's webpage or questionable textbook. I have a feeling that "thermal conductance" is a not-well-standardized term, if it's proper at all. I also find the phrase "to physicists" very questionable, as if physicists don't or shouldn't follow the SI but rather have their own standards. (Last time I saw this in Wikipedia, it was a rather confused physicist arguing that "in physics," a pound was a measure of force, not mass.) This phrase should probably be removed. By the way, I'm interested in the right terminology because I'm building authoritative references in my programming language/calculating tool [http://futureboy.homeip.net/frinkdocs/ Frink]. : No, the NIST doesn't define ''thermal conductance'', but as I recall it does define ''electrical'' resistance and conductance as reciprocals of each other. I felt I was on pretty safe ground assuming thermal conductance was the reciprocal of thermal resistance. Thats also how it works in "building trade" units. : I've since added a few more web references that look fairly reliable: [http://gscassociates.com/wg8/edcs/text/unit.html] and for the future[http://www.iso.ch/iso/en/ittf/PubliclyAvailableStandards/ISO_IEC_18025_Ed1.html]. The first says that the terms I've used - including thermal conductance - are in ISO 31-4:1992, ''Quantities and units - Part 4: Heat''. Another new reference ([http://www.npl.co.uk/thermal/faq_index.html#heat%20transfer%20property], thermophysics FAQ 5) is much better for "builders" units. : I've replaced "To physicists" with "For general scientific use". I would have put "in SI units" or something similar, but I think maybe only some of these units are defined as part of the SI, while others are derived. After all, "bulider's" thermal resistance is loosely "in SI" if it's expressed as (K m^2)/W. And for SI we should really refer to the BIPM rather than the NIST. :User:Andy G 18:25, 10 Sep 2003 (UTC) == Thermal Conductivity definition ok? == I was wondering if someone could explain the definition of thermal conductivity at the top of the page: (thermal conductivy = heat flow rate / (distance x temperature gradient). It does not appear to agree w/ several other places I have seen the term defined as: thermal conductivity = (heat flow rate x distance)/ (Area x temperature gradient) I am not an expert in the field, and may be missing something - can anyone explain the discrepancy? : You are right, and this equation is wrong. This definition is also wordy and vague. I propose the following edit: : In physics, thermal conductivity, λ, is the Intrinsic property of a material which relates its ability to conduct heat. It is the quantity of heat, H, transmitted through a unit thickness, d, in a direction normal to a surface of unit area, A, due to a unit temperature gradient, 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 gradient) :: λ = H × d / A × ΔT : I also propose changing thickness, T, to thickness, d, in the equations later in the article. This choice of variables avoids confusion with temperature, T, and temperature gradient, λT. :If no one objects, please update the wiki. Comment posted 5/26/05 == Example values for common elements OK? == I notice that the Thermal conductivity for Silver and Copper is given as 406 and 385 W/(m*K) on this page, but is stated as 429 and 401 W/(m*K) on those elements' own pages. What is the ultimate authority on these numbers? --User:Drhex 11:21, 2005 Jun 10 (UTC)
See other meanings of words starting from letter:
T
TA | TB | TC | TD | TE | TF | TG | TH | TI | TJ | TK | TL | TŁ | TM | TN | TO | TP | TR | TS | TU | TW | TX | TY | TZ |Words begining with Thermal_conductivity:
Thermal_conductivity
Thermal_conductivity
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