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# Heat flux

'''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 $overrightarrow\left\{phi_q\right\}$ 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

## Measuring heat flux

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