Soojusvoogu

Soojusvoog, in thermodynamics and heat transfer, refers to the rate at which heat passes through a surface per unit area. It is a vector quantity, typically denoted q″, with units of watts per square meter (W/m²). The direction of the heat flux points from regions of higher temperature to lower temperature. The total heat transfer through a finite surface S is Q = ∬S q″ · n dA, where n is the surface normal.

In many solids, the flux of heat conducted through a material is described by Fourier’s law. For isotropic, homogeneous materials, q″ = −k ∇T, where k is the thermal conductivity (W/m·K) and ∇T is the temperature gradient. In anisotropic materials, heat conduction is written as q″ = −K ∇T, with K a thermal conductivity tensor. The negative sign reflects the direction of heat flow from hot to cold, consistent with the temperature gradient.

Practically, boundary conditions relate the heat flux to surface temperatures and environments. For convection, q″ = h (T_s − T∞), where h is the convective heat transfer coefficient, T_s is the surface temperature, and T∞ is the ambient temperature. Radiative exchange follows radiation laws that depend on emissivity and the fourth power of temperatures. In engineering, these concepts are used in building insulation design, electronic device cooling, and heat exchanger analysis.

Measurement and estimation of soojusvoog employ sensors and techniques such as thin-film thermopile flux sensors, guarded hot plates, or infrared thermography, often combined with material property data and numerical modeling. The concept is fundamental across disciplines, linking temperature fields to actual heat transfer rates.