The emf in section bc and da is zero because dl and v B are perpendicular to each other throughout the length of these two
segments as can be seen in Figure 10.7b (dl is along the segments, whereas v B is perpendicular to the segments, as can be
verified using the right-hand rule). However, v B and dl are in the same direction on segment ab and in opposite directions
on segment cd, producing a nonzero emf. The velocity of segments ab and cd (in the system of coordinates shown in
Figure 10.7b) is