Heat flow along perfectly insulated wire

Conduction is the transfer of Heat (abbreviated Q, also called heat change) is the transfer of thermal energy between two bodies which are at different temperatures. ...heat or electric current from one substance to another by direct contact. For more information about the electrical sense, see In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ...current (electricity) and Electrical conduction is the current (movement of charged particles) through a material in response to an electric field. ...electrical conduction.

In the case of heat, the transfer is always from a higher Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ...temperature to a lower temperature. Density (symbol: ρ _ Greek: rho) is a measure of mass per unit of volume. ...Denser substances are usually better conductors; For alternative meanings see metal (disambiguation). ...metals are excellent conductors.

The law of heat conduction also know as Fourier's law states that the time rate of This article is in the process of being merged into Heat, and may be outdated. ...heat flow Q through a slab (or a portion of a perfectly insulated wire, as shown in the figure) is The word proportionality may have one of a number of meanings: In mathematics, proportionality is a mathematical relation between two quantities. ...proportional to the In vector calculus, the gradient of a scalar field is a vector field which points in the direction of the greatest rate of change of the scalar field, and whose magnitude is the greatest rate of change. ...gradient of temperature difference:

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 The heat equation or diffusion equation is an important partial differential equation which describes the variation of temperature in a given region over time. ...heat equation.

Conductance

Writing

Fourier's law can also be stated as:

where U is the Conductance can refer to: Electrical conductance, the reciprocal of electrical resistance. ...conductance. The reciprocal of conductance is resistance, equal to '

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:

So, when dealing with a multilayer partition, the following formula is usually used:

When heat is being conducted from one fluid to another through a barrier, it is sometimes important to consider the conductance of the thin For other uses see film (disambiguation) Film refers to the celluliod media on which movies are printed Film — also called movies, the cinema, the silver screen, moving pictures, photoplays, picture shows, flicks, or motion pictures, — is a field that encompasses motion pictures as an art form or as part of...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 Turbulent flow around an obstacle; the flow further away is laminar Laminar and turbulent water flow over the hull of a submarine Turbulence creating a vortex on an airplane wing In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by low_momentum diffusion, high momentum convection, and...turbulence and Viscosity is a measure of the resistance of a fluid to deformation under shear stress. ...viscosity, but when dealing with thin high_conductance barriers it can sometimes be quite significant.

Newton's law of cooling

A related principle, Newton's law of cooling, 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 The Biot number (Bi) is a dimensionless number used in unsteady_state and heat transfer calculations. ...Biot number.

Nevertheless, it is easy to derive from this principle the A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. ...exponential decay of temperature of a body. If T is the temperature of the body, then

where r is some positive constant. From which, it follows that

For example, simplified Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. ...climate models may use Newtonian cooling instead of a full (and computationally expensive) radiation code to maintain atmospheric temperatures.

See also

• In physics, thermal conductivity, λ, is the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area, when the heat transfer is dependent only on the temperature gradient thermal conductivity = heat flow rate / (distance × temperature...Thermal conductivity
• The Mpemba effect is the somewhat surprising phenomenon whereby hot water can, under certain conditions, freeze faster than cold water. ...Mpemba effect
• This is a list of scientific laws named after people. ...Scientific laws named after people