what does cross section of wire really mean?
E.g If you imagine a cone, with the circular base resting on a surface. You now slice the cone horizontally, so that you now have a very small cone (on top), and a "truncated cone" (2 circular faces, top one slightly smaller, with sloping sides joining them) below. The "cross section" is the top face of the truncated cone, and the lower face of the small cone (both cross sections are the same!). i.e they are the new "sides" that you have created. These new "faces" are both circular, so if you were to find the area you would use the formula pi*radius^squared = area