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From Wikipedia

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 (WÂ·Kâˆ’1Â·mâˆ’1, i.e. W/(KÂ·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 [http://www.energystar.gov/index.cfm?c=windows_doors.pr_ind_tested 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/hÂ·ftÂ²Â·Â°F). The lower the U-factor, the better the window insulates.

The reciprocal of thermal conductivity is thermal resistivity.

## Measurement

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.

### Standards

• IEEE Standard 442-1981, "IEEE guide for soil thermal resistivity measurements", ISBN 0-7381-0794-8. See also soil thermal properties. [http://ieeexplore.ieee.org/servlet/opac?punumber=2543]
• 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 [http://ieeexplore.ieee.org/servlet/opac?punumber=7893]
• 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" [http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D5470.htm?E+mystore]
• ASTM Standard E1225-04, "Standard Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique" [http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/E1225.htm?L+mystore+wnox2486+1189558298]
• ASTM Standard D5930-01, "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique" [http://www.astm.org/cgi-bin/SoftCart.exe/STORE/filtrexx40.cgi?U+mystore+wnox2486+-L+THERMAL:CONDUCTIVITY+/usr6/htdocs/astm.org/DATABASE.CART/REDLINE_PAGES/D5930.htm]
• ASTM Standard D2717-95, "Standard Test Method for Thermal Conductivity of Liquids" [http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/D2717.htm?L+mystore+wnox2486+1189564966]
• ISO 22007-2:2008 "Plastics -- Determination of thermal conductivity and thermal diffusivity -- Part 2: Transient plane heat source (hot disc) method" [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40683]
• Note: What is called the k-value of construction materials (e.g. window glass) in the U.S., is called Î»-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.

## Definitions

The reciprocal of thermal conductivity is thermal resistivity, usually measured in kelvin-metres per watt (KÂ·mÂ·Wâˆ’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.

### Conductance

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 WÂ·Kâˆ’1 (equivalent to: W/Â°C). Thermal conductivity and conductance are analogous to electrical conductivity (AÂ·mâˆ’1Â·Vâˆ’1) and electrical conductance (AÂ·Vâˆ’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 WÂ·Kâˆ’1
• thermal resistance = L/(kA), measured in KÂ·Wâˆ’1 (equivalent to: Â°C/W)
• heat transfer coefficient = k/L, measured in WÂ·Kâˆ’1Â·mâˆ’2
• thermal insulance = L/k, measured in KÂ·mÂ²Â·Wâˆ’1.

The heat transfer coefficient is also known as thermal admittance

### Resistance

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

where:

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

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

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.

Question:I'm doing a science project on it and really need to know. Thanks!!! :)

Answers:I think you're refering to geothermal energy. If so, the advantage is that it's a clean potential energy source which already exists. The disadvantage is that it still costs much more than burning fossil fuels.

Question:what are thermal power plants and what countires that have thermal powerplants?

Answers:In Iceland, a country in the North Atlantic, they use thermal energy to heat their homes as well as a lot of other things. Look up Iceland on the Internet. Advantages are that it has no negative effects on the earth's environment. Disadvantages may be costs of harnessing but that should be reconfirmed. The costs of finding oil and getting it up to the surface may be just as high plus we are polluting the planet which has an incalculable cost.