Heat conduction Heat flow along perfectly insulated wire Conduction is the transfer of heat or electric current from one substance to another by direct contact. For more information about the electrical sense, see current (electricity) and electrical conduction. In the case of heat, the transfer is always from a higher temperature to a lower temperature. Denser substances are usually better conductors; metals are excellent conductors. The law of heat conduction also know as Fourier's law states that the time rate of heat flow Q through a slab (or a portion of a perfectly insulated wire, as shown in the figure) is proportional to the gradient of temperature difference: $Q\; =\; K\; A\; \backslash frac$ A is the transversal surface area, Δ x is the thickness of the body of matter through which the heat is passing, K is a conductivity constant dependent on the nature of the material and its temperature, and ΔT is the temperature difference through which the heat is being transferred. This law forms the basis for the derivation of the heat equation. Conductance Writing $U\; =\; \backslash frac,\; \backslash quad$ Fourier's law can also be stated as: $Q\; =\; U\; A\backslash ,\; \backslash Delta\; T\; \backslash quad$ where U is the conductance. The reciprocal of conductance is resistance, equal to ' $\backslash frac,\; \backslash quad$ and it is resistance which is additive when several conducting layers lie between the hot and cool regions, because A and Q are the same for all layers. In a multilayer partition, the total conductance is related to the conductance of its layers by: $\backslash frac\; =\; \backslash frac\; +\; \backslash frac\; +\; \backslash frac+\; \backslash cdots$ So, when dealing with a multilayer partition, the following formula is usually used: $Q\; =\; \backslash frac\; +\; \backslash frac\; +\; \backslash frac+\; \backslash cdots\}$ When heat is being conducted from one fluid to another through a barrier, it is sometimes important to consider the conductance of the thin film of fluid which remains stationary next to the barrier. This thin film of fluid is difficult to quantify, its characteristics depending upon complex conditions of turbulence and viscosity, but when dealing with thin high-conductance barriers it can sometimes be quite significant. Newton's law of cooling There is a related principle, called Newton's law of cooling, which states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings. This form of heat loss principle however is not very precise; a more accurate formulation requires an analysis of heat flow based on the heat equation in an inhomogeneous medium. The general applicability of this simplification is characterized by the Biot number. Nevertheless, it is easy to derive from this principle the exponential decay of temperature of a body. If T is the temperature of the body, then $\backslash frac\; =\; -\; r\; (T\; -\; T\_\})$ where r is some positive constant. From which follows $T(t)\; =\; T\_\}\; +\; (T(0)\; -\; T\_\})\; \backslash \; e^.\; \backslash quad$ For example, simplified climate models may use Newtonian cooling instead of a full (and computationally expensive) radiation code to maintain atmospheric temperatures. See also Thermal conductivity Mpemba effect List of eponymous laws FAVORITE ARTICLES: History of the World    History of the United States    History of Europe    Ancient History    History    Military History    World War I    Bombing of Dresden    California    US Constitution    Frank Sinatra    Boston    Marbury v. Madison This article is licensed under the GNU Free Documentation License at http://www.gnu.org/copyleft/fdl.html You may copy and modify it as long as the entire work (including additions) remains under this license. You must provide a link to http://www.gnu.org/copyleft/fdl.html To view or edit this article at Wikipedia go to http://www.wikipedia.org/wiki/Heat_conduction