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 amount of heat transferred by conduction can be determined using Fourier's law:

Q = KAΔT/L

where Q is heat transferred per unit time, A is the area perpendicular to the heat flow through which it is passing, L 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 between the hot and cold sides of the substance through which the heat is being transferred. Writing U=K/L this law can also be stated as:

Q = UAΔT

where U is the conductance. The reciprocal of conductance is resistance, equal to AΔT/Q, 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:

1/U = 1/U₁ + 1/U₂ + 1/U₃ ...

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

Q = AΔT/(L₁/K₁ + L₂/K₂ + L₃/K₃ ...)

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.

See also thermal conductivity.