Solution: First, we must calculate the differential of path, dl, and then evaluate Adl. This is then integrated along the circle (closed contour) to obtain the result. This problem is most easily evaluated in cylindrical coordinates (see Exercise 2.1), but we will solve it in Cartesian coordinates. The integration is performed in four segments: P1 to P2, P2 to P3, P3 to P4, and P4 to P1, as shown in Figure 2.4.