'''Heat flux''' or '''thermal flux''' is a flow of energy per unit of area per unit of time In SI units it is measured in

[Watt|W·Metre|m

^{-2}] Since it has both a direction and a magnitude it is a Vector (spatial)|vectorial quantity

Heat

flux is often denoted

$overrightarrow\{phi\_q\}$ the subscript

*q* specifying

*heat* flux as opposed to

*mass* or

*momentum* flux The most important appearance of

heat flux in

physics is in Fourier's law describing

heat conduction

The measurement of

heat flux is most often done by measuring a

temperature difference over a piece of

material with known

thermal conductivity This method is analogous to a standard way to measure an

electric current where one measures the

voltage drop over a known resistor

## Relevance to science and engineering

One of the tools in a scientist's or engineer's toolbox is the

energy balance## See also