The surface s is in the rz plane and is therefore perpendicular to the ϕ direction, as shown in Figure 2.9a. Thus, the element of surface is: ds1 ¼ ^ ϕdrdz or ds2 ¼^ ϕdrdz. In this case, because the flux must be positive, we choose ds1 ¼ ^ ϕdrdz. The flux is