Conduction is the transfer of thermal energy through a material or between materials that are in direct physical contact without requiring bulk movement of the material as a whole. At the microscopic level thermal energy is transferred through interactions between neighbouring particles. In solids this commonly involves vibrations of atoms or molecules within a lattice, while in metals mobile electrons provide an additional and often dominant mechanism for heat transfer.
Conduction can also occur in liquids and gases, although molecular collisions and diffusion are generally less effective at transporting thermal energy than the mechanisms available in many solids. The fundamental driving force is a temperature gradient because thermal energy tends to flow from regions of higher temperature toward regions of lower temperature.
The quantitative description of Conduction is given by Fourier law of heat conduction. For one dimensional steady conduction through a homogeneous material the heat transfer rate is proportional to thermal conductivity, cross sectional area and temperature difference and inversely proportional to the distance over which the temperature changes. Thermal conductivity is a material property that measures how effectively a substance conducts heat. Metals such as copper and aluminium generally have high thermal conductivity, whereas materials such as mineral wool, polystyrene and many polymers have much lower thermal conductivity and therefore act as thermal insulators. The thermal conductivity of a substance can vary with temperature, pressure, composition, structure and direction.
Temperature gradients determine the direction and local magnitude of conductive heat transfer. In a simple planar wall the temperature profile can be approximately linear when thermal conductivity is constant and the system is at steady state with no internal heat generation. More complicated geometries produce different temperature distributions. Cylindrical conduction occurs in pipes and insulation around pipes, while spherical conduction is relevant to some particles and shells. When thermal conductivity changes significantly with temperature, the temperature dependence must be included in the governing equation. Internal heat generation from electrical resistance, chemical reactions or nuclear processes can also create curved temperature profiles rather than the simple linear profile associated with uniform conduction.
Thermal resistance provides a useful way to analyse composite systems involving several layers. The conductive resistance of a planar layer depends on its thickness, area and thermal conductivity, and multiple layers arranged in series have resistances that add. This approach is analogous mathematically to resistance concepts in electrical circuits but represents thermal rather than electrical transport. Building walls commonly contain several layers with different thermal properties, and the total resistance determines the heat flow for a specified temperature difference. Contact resistance can also occur at interfaces because surfaces that appear to touch macroscopically may have microscopic gaps containing air or other material.
Conduction is central to engineering applications including building insulation, heat exchangers, electronic cooling, cooking equipment, furnaces, refrigeration systems and thermal protection. In electronic devices heat generated by processors and power components must be conducted toward heat sinks or other cooling structures. In buildings insulation reduces conductive heat loss through walls, roofs and floors. In cookware a metal pan conducts heat from a heated surface into food, although convection and radiation also contribute to the overall heating process. Material selection therefore depends not simply on strength or cost but also on thermal conductivity and the operating temperature range.
The microscopic mechanism of Conduction differs between classes of materials. In crystalline solids atoms occupy relatively ordered positions and thermal energy can propagate through lattice vibrations commonly described using phonons. In metals conduction involves both lattice vibrations and conduction electrons, with electrons often carrying a substantial fraction of the heat. In nonmetallic solids lattice vibrations are generally more important. In gases and liquids energy is transferred through molecular collisions and molecular motion. These microscopic differences explain why thermal conductivity varies greatly between materials and why electrical and thermal conductivity are often related in metals.
Conduction must be distinguished from Convection and Thermal Radiation. Conduction involves local transfer through material interactions, Convection involves heat transport associated with the bulk motion of a fluid, and Thermal Radiation transfers energy through electromagnetic radiation and does not require physical contact or a material medium. Real systems frequently involve all three mechanisms simultaneously. For example a heated wall can conduct heat through its material, transfer heat by convection to surrounding air and exchange thermal radiation with nearby surfaces. Engineers therefore analyse the individual mechanisms and then consider their combined effect when predicting temperatures and heat transfer rates.
The concept of Conduction also extends beyond simple steady state heat transfer. Transient conduction describes situations in which temperatures change with time, such as when a cold object is placed in a warm environment. Thermal diffusivity combines thermal conductivity with density and heat capacity and characterises how rapidly temperature changes propagate through a material. Boundary conditions specify temperatures or heat fluxes at surfaces and are essential for solving conduction problems. Numerical methods are frequently required for complex geometries and variable material properties. The general physical principle remains that microscopic interactions transport thermal energy down temperature gradients while the material structure determines how efficiently that transport occurs.
