Second, flux as a scalar field defined along a surface, i.e. a function of points on the surface: j ( p ) = ∂ ∂ I ∂ ∂ A ( p ) , I ( A , p ) = d q d t ( A , p ) . As before, the surface is assumed to be flat, and the flow is assumed to be everywhere perpendicular to it. However the flow need not be constant. q is now a function of p, a point on the surface, and A, an area. Rather than measure the total flow through the surface, q measures the flow through the disk with area A centered at p along the surface.