Varmelednings

Varmelednings (heat conduction) is the transfer of thermal energy within a material or between objects that are in direct contact, driven by a temperature difference. It occurs without any bulk motion of the material and is a dominant mode of heat transfer in solids.

Microscopic mechanisms differ by phase. In metals, free electrons rapidly transport energy, while in insulators energy is carried mainly by lattice vibrations (phonons). In liquids and gases, conduction occurs but can be overshadowed by convection, which involves bulk fluid movement.

The governing principle is Fourier’s law: the heat flux per unit area q is proportional to the negative temperature gradient, q = -k ∇T, where k is the material’s thermal conductivity (measured in W/mK) and ∇T is the temperature gradient. Materials are also described by density ρ and specific heat capacity c, with thermal diffusivity α = k/(ρ c). Transient conduction in a uniform medium is described by the heat equation ∂T/∂t = α ∇²T.

Applications and design considerations include the selection of materials with appropriate conductivity. High-k materials (e.g., copper) are used to transfer heat efficiently in heat exchangers and electronic cooling, while low-k or insulating materials (e.g., polyurethane foams, air gaps) reduce heat loss in buildings. For layered structures, total thermal resistance is additive: R_th = ∑ Li/(ki A), where Li and ki are the thickness and conductivity of each layer, and A is the cross-sectional area.

Limitations include interfacial thermal resistance at boundaries and anisotropy in some crystalline materials, where conductivity varies with direction. Understanding varmelednings is essential in thermal engineering, building insulation, electronics cooling, and energy systems.