Thermal conductance and resistance

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Thermal conductance
Common symbols	G
SI unit	watt per kelvin (W/K)
In SI base units	kg⋅m2⋅s−3⋅K-1
Dimension	${\displaystyle {\mathsf {L}}^{2}{\mathsf {M}}{\mathsf {T}}^{-3}{\mathsf {\Theta }}^{-1}}$

Thermal resistance
Common symbols	R
SI unit	kelvin per watt (K/W)
In SI base units	kg-1⋅m-2⋅s3⋅K
Dimension	${\displaystyle {\mathsf {L}}^{-2}{\mathsf {M}}^{-1}{\mathsf {T}}^{3}{\mathsf {\Theta }}}$

In heat transfer, thermal engineering, and thermodynamics, thermal conductance and thermal resistance are fundamental concepts that describe the ability of materials or systems to conduct heat and the opposition they offer to the heat current. The ability to manipulate these properties allows engineers to control temperature gradient, prevent thermal shock, and maximize the efficiency of thermal systems. Furthermore, these principles find applications in a multitude of fields, including materials science, mechanical engineering, electronics, and energy management. Knowledge of these principles is crucial in various scientific, engineering, and everyday applications, from designing efficient temperature control, thermal insulation, and thermal management in industrial processes to optimizing the performance of electronic devices.

Thermal conductance (G) measures the ability of a material or system to conduct heat. It provides insights into the ease with which heat can pass through a particular system. It is measured in units of watts per kelvin (W/K). It is essential in the design of heat exchangers, thermally efficient materials, and various engineering systems where the controlled movement of heat is vital.

Conversely, thermal resistance (R) measures the opposition to the heat current in a material or system. It is measured in units of kelvins per watt (K/W) and indicates how much temperature difference (in kelvins) is required to transfer a unit of heat current (in watts) through the material or object. It is essential to optimize the building insulation, evaluate the efficiency of electronic devices, and enhance the performance of heat sinks in various applications.

Objects made of insulators like rubber tend to have very high resistance and low conductance, while objects made of conductors like metals tend to have very low resistance and high conductance. This relationship is quantified by resistivity or conductivity. However, the nature of a material is not the only factor as it also depends on the size and shape of an object because these properties are extensive rather than intensive. The relationship between thermal conductance and resistance is analogous to that between electrical conductance and resistance in the domain of electronics.

Thermal insulance (R-value) is a measure of a material's resistance to the heat current. It quantifies how effectively a material can resist the transfer of heat through conduction, convection, and radiation. It has the units square metre kelvins per watt (m²⋅K/W) in SI units or square foot degree Fahrenheit–hours per British thermal unit (ft²⋅°F⋅h/Btu) in imperial units. The higher the thermal insulance, the better a material insulates against heat transfer. It is commonly used in construction to assess the insulation properties of materials such as walls, roofs, and insulation products.