Question

Why the figure "b" has zero divergence but "c" has positive divergence?

Answer 1

The divergence is defined as the derivative of the vector field in the direction of the flow (the flow is 1 dimensional in this case). In fig b the vector field is constant, and therefore its divergence is 0. On the other hand, in fig c the intensity of the vector field increases in the direction of the flow. Its divergence will be positive