First, flux as a (single) scalar: j = I A , where I = lim Δ Δ t → → 0 Δ Δ q Δ Δ t = d q d t . In this case the surface in which flux is being measured is fixed and has area A. The surface is assumed to be flat, and the flow is assumed to be everywhere constant with respect to position and perpendicular to the surface.