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Thermal conductivity

In physics, thermal conductivity, k, is the property of a material describing its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin-metre (W·K−1·m−1, i.e. W/(K·m). Multiplied by a temperature difference (in kelvins, K) and an area (in square metres, m2), and divided by a thickness (in metres, m), the thermal conductivity predicts the rate of energy loss (in watts, W) through a piece of material. In the window building industry "thermal conductivity" is expressed as the [ U-Factor] measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.15 to 1.25 and are measured in Btu per hour - square foot - degree Fahrenheit (ie. Btu/h·ft²·°F). The lower the U-factor, the better the window insulates.

The reciprocal of thermal conductivity is thermal resistivity.


There are a number of ways to measure thermal conductivity. Each of these is suitable for a limited range of materials, depending on the thermal properties and the medium temperature. There is a distinction between steady-state and transient techniques.

In general, steady-state techniques are useful when the temperature of the material does not change with time. This makes the signal analysis straightforward (steady state implies constant signals). The disadvantage is that a well-engineered experimental setup is usually needed. The Divided Bar (various types) is the most common device used for consolidated rock samples.

The transient techniques perform a measurement during the process of heating up. Their advantage is quicker measurements. Transient methods are usually carried out by needle probes.


  • IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements", ISBN 0-7381-0794-8. See also soil thermal properties. []
  • 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 D5334-08, "Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure"
  • 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" []
  • ISO 22007-2:2008 "Plastics -- Determination of thermal conductivity and thermal diffusivity -- Part 2: Transient plane heat source (hot disc) method" []
  • Note: What is called the k-value of construction materials (e.g. window glass) in the U.S., is called λ-value in Europe. What is called U-value (= the inverse of R-value) in the U.S., used to be called k-value in Europe, but is now also called U-value in Europe.


The reciprocal of thermal conductivity is thermal resistivity, usually measured in kelvin-metres per watt (K·m·W−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.


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 kelvin. For a plate of thermal conductivity k, area A and thickness L this is kA/L, measured in W·K−1 (equivalent to: W/°C). Thermal conductivity and conductance are analogous to electrical conductivity (A·m−1·V−1) and electrical conductance (A·V−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 kelvin. 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 (equivalent to: °C/W)
    • 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


When thermal resistances occur in series, they are additive. So when heat flows through two components each with a resistance of 1 °C/W, the total resistance is 2 °C/W.

A common engineering design problem involves the selection of an appropriate sized heat sink for a given heat source. Working in units of thermal resistance greatly simplifies the design calculation. The following formula can be used to estimate the performance:

R_{hs} = \frac {\Delta T}{P_{th}} - R_s


  • Rhs is the maximum thermal resistance of the heat sink to ambient, in °C/W
  • \Delta T is the temperature difference (temperature drop), in °C
  • Pth is the thermal power (heat flow), in watts
  • Rs is the thermal resistance of the heat source, in °C/W

For example, if a component produces 100 W of heat, and has a thermal resistance of 0.5 °C/W, what is the maximum thermal resistance of the heat sink? Sup

British thermal unit

The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1 055.05585 joules. It is approximately the amount of energy needed to heat 1|lb|kg|3 of water 1|F-change|lk=on. It is used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric English-speaking countries (such as Canada), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.

In North America, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU per hour (BTU/h) is the correct unit, though this is often abbreviated to just "BTU".

The unit MBTU was defined as one thousand BTU, presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SImega (M) prefix, which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a thermis used representing 100,000 or 105 BTU, and a quadas 1015 BTU.


A BTU is defined as amount of heat required to raise the temperature of one 1|lb|kg|3 of liquid water by 1|F-change at a constant pressure of one atmosphere. As is the case with the calorie, several different definitions of the BTU exist, which are based on different water temperatures and therefore vary by up to 0.5%: A BTU can be approximated as the heat produced by burning a single wooden match or as the amount of energy it would take to lift a one-pound weight to a height of 778|ft|m|0.


One BTU is approximately:

  • 1.054 to 1.060 kJ (kilojoules)
  • 0.293071 W·h (watt hours)
  • 252 to 253 cal (calories, or "little calories")
  • 0.25 kcal (kilocalories, "large calories," or "food calories")
  • 25 031 to 25 160 ft·pdl (foot-poundal)
  • 778 to 782 ft·lbf (foot-pounds-force)

Other conversions:

  • In natural gas, by convention 1 MMBtu (1 million BTU, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m3 of natural gas at defined temperature and pressure. So, 1 MMBtu = 28.263682 m3 of natural gas at defined temperature and pressure.
  • 1 standard cubic foot of natural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)

Associated units

The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU. The term is sometimes shortened to BTU hour (BTU.h) but both have the same meaning.

  • 1 watt is approximately 3.41214 BTU/h
  • 1000 BTU/h is approximately 293.071 W
  • 1 horsepower is approximately 2,544 BTU/h
  • 1 "ton of cooling," a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h. It is the amount of power needed to melt one short ton of ice in 24 hours, and is approximately 3.51 kW.
  • 1 thermis defined in the United States and European Union as 100,000 BTU—but the U.S. uses the BTU59 Â°F whilst the EU uses the BTUIT.
  • 1 quad (energy)(short forquadrillion BTU) is defined as 1015 BTU, which is about one exajoule (1.055 × 1018 J). Quads are used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used 99.75 quads/year in 2005. One quad/year is about 33.43 gigawatts.

The BTU should not be confused with the Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy (1 kW·h, or about 3412 BTU).

The BTU is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in BTU required to generate 1 kWh of electrical energy. A typical coal-fired power plant works at 10,500 BTU/kWh, an efficiency of 32-33% .

Thermal radiation - Wikipedia, the free encyclopedia

Examples of thermal radiation are an incandescent light bulb emitting ... Its emissivity, however, at a temperature of about -5 C, peak wavelength of about 12 .... Radiant heat panel for testing precisely quantified energy exposures at ...

From Yahoo Answers

Question:Please be clear about what the original form was, how it was converted, and whether the resulting thermal energy would be easy to detect with a thermometer.

Answers:All energy from any source eventually dissipates into thermal energy!!! 1) Electrical energy can be converted to heat with resistance coils. Examples: toaster, hair dryer, electric blanket. 2) Chemical. Any material that burns will produce heat. This is chemical energy converted to heat. Examples: wood, paper, gas, oil, wax, thermoplastics. 3) Solar (light) energy. The original source, the sun, is thermonuclear energy emitting light. A large percentage of the light striking Earth is absorbed and converted to heat. Solar heaters for heating your home or producing hot water work on the principle that dark colors (black) are very good at converting light to thermal energy. 4) Nuclear energy. Nuclear is the process of splitting atoms (fission) or joining atoms together (fusion). Nuclear energy produces enormus quantities of thermal energy in the process, and in controlled conditions can be used to generate steam to generate electricity. 5) Mechanical. Mechical energy is any object that is in motion (kinetic energy) or any object that has potential to move (potential energy). Kinetic energy always gives off some thermal energy due to friction. If you rub your hands together, you can feel the friction producing heat on the palms of your hands. Most other forms of energy are variations of the above. For example, wind energy is mechanical (moving air). It becomes useful when the wind drives a windmill turbine to do some work such as grinding flour or generating electricity. Another example, hydroelectrical is falling water (mechanical energy) used to turn a turbine and generate electricity. For both wind and hydroelectrical, the original source is the weather, and the source of the weather is solar energy.

Question:not geothermal just thermal.

Answers:Thermal energy is the easiest to find and create. It is necessary in some situations (for example, chemical processes where molecular motion is necessary) but it cannot be applied to most of our modern technology. it has to be converted to electrical or kinetic energy, and this is a touchy and very dissipating process. For example, the pistons in a car convert thermal energy to motion by expanding the gasses within, generating a linear motion, and transforming this mechanically into a circular motion. Coal power plants generate electricity by burning coal, but I don't know exactly what process they use. I have to note that this thermal energy is originally chemical energy, so it's more like an intermediate. But thermal energy in general can be converted to other energy forms through expansion of gasses etc.


Answers:Answer regarding density is not correct, at all. Density is not a factor. Aluminum, for example, is a MUCH better heat conductor than lead. Lead is pretty bad, in fact. The answer to your question is: Becase the same freedom of electrons to conduct electrical energy promotes good conduction of thermal energy. But there is much more to the story. Basic thermal conductivity apart from free electrons in a bulk, pure material results from the specific nature of the coupling of adjacent molecules or atoms, as the case may be. For crystalline materials, this can be predicted and analysed mathematically by looking at how a quantum of vibrational energy, called a phonon (to parallel the light analogue of a photon) is transmitted through the crystal lattice. This property of the material can dominate the free electron effect, hence the fact that diamond, an insulator, has higher thermal conductivity than silver, the best conductor. So most metals have generally good conductivity from the crystal structure, and a second added thermal conduction mechanism of the free electrons. The methods used to predict thermal conductivity of crystals of pure material can be used to give approximations in the real world, where all materials have impurities, have trapped gas bubbles, have flaws in the crystal structure, etc. Organic materials, such as wood, pretty much have to be measured experimentally. I am sure a number of people every year earn their PhD's by trying to predict the thermal concudtivity of wood from a theoretical standpoint.

Question:Ok I have a school project due tomorrow (January 5, 2010) and we have to make a poster with the name of the energy, a picture showing an example, and the definition. I have all the titles and definitions but only 6 out of the 7 pictures. I need some examples for Nuclear Energy. These are what I used for the others. Please tell what I can use for Nuclear Energy really soon. Electrical Energy- T.V. Radiant Energy- Burning Candle Chemical Energy- An Unlit Match Mechanical Energy- Wind Turbine Acoustic Energy- Radio Thermal Energy- Boiling Water For Nuclear my friend was using a Power Plant but I didn't think it would work. Please tell me if it would and I'll use it but if you have a better one I'll use it. Thanx if you can help.

Answers:Every nuclear power plant in the world uses nuclear fission to heat water, to make steam, to drive a turbine that generates electricity. Nuclear weapons (usually fission-fusion-fission) are very vigorous uses of nuclear energy. Our Sun and every other healthy main sequence star is just a massive nuclear furnace, where hydrogen is being fused into helium, releasing the energy that we use as heat, light, and is we wanted, conversion to solar-electric power.

From Youtube

Thermal Equilibrium :Check us out at In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states. The local state of a system at thermodynamic equilibrium is determined by the values of its intensive parameters, as pressure, temperature, etc. Specifically, thermodynamic equilibrium is characterized by the minimum of a thermodynamic potential, such as the Helmholtz free energy, ie systems at constant temperature and volume: A = U TS; Or as the Gibbs free energy, ie systems at constant pressure and temperature: G = H TS. The process that leads to a thermodynamic equilibrium is called thermalization. An example of this is a system of interacting particles that is left undisturbed by outside influences. By interacting, they will share energy/momentum among themselves and reach a state where the global statistics are unchanging in time. Thermal equilibrium is achieved when two systems in thermal contact with each other cease to exchange energy by heat. If two systems are in thermal equilibrium their temperatures are the same. The word equilibrium means a state of balance. In an equilibrium state, there are no unbalanced potentials (or driving forces) with the system. A system that is in equilibrium experiences no changes when it is isolated from its surroundings. The opposite of ...

Solar Thermal Energy: Harnessing the Power of the Sun :The video shows the use of solar thermal energy for heating in Europe. Examples from Denmark and Spain show the use in different climates. Interviews with users and manufacturers give a good overview of this clean energy technology. The video is provided for free by the Sustainable Energy Europe Campaign of the European Union.