Definitions & qualifiers





A red-hot iron rod cooling after being worked: heat is being radiated (visibly) but also conducted along the rod

Thermal conductivity ( W/(mK ) is the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area, when the heat transfer is dependent only on the temperature gradient. The thermal conductivity of a system is determined by how molecules comprising the system interact.

Thermal conductivity = heat flow rate distance / (area temperature gradient).

Thermal resistivity (K.m/W) is the reciprocal of thermal conductivity.

Thermal conductance (WK-1 ) is the quantity of heat that passes in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one degree. For a plate of thermal conductivity λ, area A and thickness L this is λA/L.

Thermal resistance (Km2 /W) is the temperature difference across a unit area of a material of unit thickness when a unit of heat energy flows through it in unit time. It is the reciprocal of thermal conductivity.

Heat transfer coefficient (W/m2 K) is the quantity of heat that passes in unit time through unit area of a plate of particular thickness when its opposite faces differ in temperature by one degree.

Click here for further info and links on the above quantities. Picking your way through the various thermal definitions can be tricky. In some cases (e.g. thermal resistance) there is more than one definition, so take care when thermal quantities are mentioned to understand exactly which definitions are being used.

A photovoltaic panel collects radiant energy from the sun and stores it as electrical energy

Electrical resistivity ( Ωm) is also known as specific electrical resistance and is a measure of how strongly a material opposes a flow of electrical current. A low resistivity indicates a material that readily allows the movement of electrons. For metals and alloys electrical resistance and thermal resistance are connected (see Wiedemann-Franz law) because the flow of both heat and electricity are due mainly to the movement of free electrons.

Thermal insulance (m2 K/W) is the reciprocal of heat transfer coefficient and it is directly proportional to the thickness of the material (very important for insulation).

Thermal diffusivity (m2 /s) is the ratio of thermal conductivity ( λ ) to heat capacity ( ρ c ), i.e. thermal diffusivity = λ / ρ c. It is property that indicates how rapidly heat is conducted in a material. Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings, because they conduct heat quickly.

Thermal effusivity is the square root of the product of thermal conductivity ( λ ) and heat capacity ( ρ c ), i.e. effusivity = ( λ ρ c )1/2 .

It is a heat transfer property that determines the interfacial temperature when two semi-infinite objects at different temperatures touch; for example, imagine the difference between touching a piece of metal as opposed to a piece of wood: the metal effusivity is higher, leading to the sensation that the metal feels colder, even though it was at the same initial temperature as the wood. The effusivity of materials varies due to their differing ability to transfer heat. This is due to differences in heat transfer through and between particles, and is therefore a function of particle size, particle shape, density, morphology, crystallinity and moisture content. For example, powders have effusivities that are strongly correlated with their moisture content. In conclusion, thermal effusivity characterises the transient thermal behaviour that occurs when two material are brought into contact with each other.

For further info on thermal conductivity and related terms try here or here .




The NPL rotating hot-box for measuring the thermal transmittance or U-value of building elements such as walls, windows, doors and skylights

Special definition for buildings

R-value or thermal resistance (m2 K/W) is what is described above as thermal insulance

Thermal conductance (W/m2 K) is the reciprocal of R-value

U-value or thermal transmittance or composite thermal conductance (W/m2 K) incorporates the thermal conductance of a structure along with heat transfer due to convection and radiation. It is measured in the same units as thermal conductance

k value is a synonym for thermal conductivity. The more modern symbol for thermal conductivity is the Greek lambda ( λ )



Heat (J) usually abbreviated Q , is a measure of the amount of energy transferred from one body to another due to the temperature difference between them. (For further info about heat click here . For an interesting comment on the understanding of heat click here .)

Heat transfer rate (W) is heat flow per unit time, usually denoted dQ/dt. (Further info: click here .)

Heat flux (W/m2 ) is defined as the amount of heat per unit time per unit cross-sectional area

Conduction is the means by which heat is transferred inside a solid material as a result of temperature differences within it (further info here ). This form of heat transmission is a result of different microscopic effects like molecular diffusion, electronic diffusion or crystal structural vibrations: these energy transport modes result in the macroscopic effect of conduction. Denser substances are usually better conductors and metals are excellent thermal conductors.



Convection in the atmosphere: a tornado is nature's most violent storm

Convection is usually the dominant form of heat transfer in liquids and gases. This term characterises the combined effects of conduction and fluid flow. In convection, enthalpy transfer occurs by the movement of hot or cold portions of a fluid together with heat transfer by conduction. For example, when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan. Another example can be found in clouds where there are hot and cold air currents in the atmosphere.

Radiation is the only form of heat transfer that can occur in the absence of any form of medium and as such is the only means of heat transfer in a vacuum. Thermal radiation arises form the movement of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which results in emission of thermal energy from the surface.