<P> Heat flux or thermal flux, sometimes also referred to as heat flux density or heat flow rate intensity is a flow of energy per unit of area per unit of time . In SI its units are watts per square metre (W ⋅ m). It has both a direction and a magnitude, and so it is a vector quantity . To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small . </P> <P> Heat flux is often denoted φ q → (\ displaystyle (\ overrightarrow (\ phi _ (\ text (q))))), the subscript q specifying heat flux, as opposed to mass or momentum flux . Fourier's law is an important application of these concepts . </P> <P> For most solids in usual conditions, heat is transported mainly by conduction and the heat flux is adequately described by Fourier's law . </P> <P> The heat flux associated with a temperature profile T (x) (\ displaystyle T (x)) in a material of thermal conductivity k (\ displaystyle k) is given by </P>

Which direction does thermal energy flow in the following diagram