heattransfer

Heat transfer is the process by which thermal energy moves from regions of higher temperature to regions of lower temperature. It occurs within materials and between bodies separated by interfaces. Transfers arise from temperature differences and continue until thermal equilibrium is reached or external work is applied.

Conduction: In solids, energy is transferred by molecular interactions and electron movement. Fourier's law describes the rate of conduction: Q = -k A dT/dx, where Q is the heat transfer rate, k is the material’s thermal conductivity, A is the cross-sectional area, and dT/dx is the temperature gradient. Materials with high k, such as metals, conduct well; insulators have low k. Thermal resistance is often used to model layered media.

Convection: Heat transfer between a surface and a moving fluid occurs by advection and diffusion. Newton's law of cooling gives Q = h A ΔT, where h is the convective heat transfer coefficient and ΔT is the surface-fluid temperature difference. Forced convection uses pumps or fans; natural convection relies on buoyancy.

Radiation: All bodies emit electromagnetic radiation that carries energy. The radiative heat transfer rate is Q = ε σ A (T^4 − T_sur^4), with ε emissivity, σ the Stefan-Boltzmann constant, and T and T_sur absolute temperatures. Radiation can occur across a vacuum and between bodies at different temperatures.

Combined and design: In engineering, total heat transfer is the sum from conduction, convection, and radiation. The net rate can be expressed as Q = ΔT / R_total for systems with thermal resistances. Practical topics include heat exchangers, thermal insulation, electronics cooling, and HVAC.