If the flux j passes through the area at an angle θ to the area normal n ^ ^ , then the dot product j ⋅ ⋅ n ^ ^ = j cos ⁡ ⁡ θ θ . That is, the component of flux passing through the surface (i.e. normal to it) is j cos θ, while the component of flux passing tangential to the area is j sin θ, but there is no flux actually passing through the area in the tangential direction. The only component of flux passing normal to the area is the cosine component.