We defined the gradient of a scalar and the divergence of a vector in the previous two sections. Both of these have physical
meaning, and some applications of the two were shown in examples. In particular, the divergence of a vector was shown to
be an indication of the strength of the scalar source of the vector. The question now is the following: If a vector can be
generated by a scalar source (for example, a water spring is a scalar source, but it gives rise to a vector flow which has both
direction and magnitude), is it also possible that a vector source gives rise to a vector field? The answer is clearly yes.
Consider again the flow of a river; the flow is never uniform; it is faster toward the center of the river and slower at the banks.
If you were to toss a stick into the river, perpendicular to the flow, the stick, in addition to drifting with the flow, will rotate
and align itself with the direction of the flow. This rotation is caused by the variation in flow velocity: One end of the stick is
dragged down the river at higher velocity than the other as shown in Figure 2.19. The important point here is that we cannot
explain this rotation using the scalar source of the field. To explain this behavior, and others, we introduce the curl of a
vector. The curl is related to circulation and spatial variations in the vector field. To define the curl, we first define the
circulation of a vector. In the process, we will also try to look at the meaning of the curl and its utility.