= Heat flux density
{title2=$\mathbf q$}
= Heat flux
{synonym}
<Heat flux density> is a <vector> $\mathbf q$ describing the rate of <heat transfer> per unit <area>, measured in $\mathrm{W\,m^{-2}}$. Through a planar face of <area> $A$ with uniform normal component $q$, the signed rate is $\dot Q=Aq$ in $\mathrm W$; positive $q$ means <heat> travels in the chosen positive normal direction. <Fourier's law> determines the conductive contribution. In a bar with a constant cross-sectional <area>, steady <thermal conduction> without internal sources or side losses makes both $q$ and $\dot Q$ constant along the bar. Constant $\dot Q$ need not mean constant $q$ when the cross-sectional <area> varies.
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