Consider the flow field represented by the velocity potential \(\phi=A x+B x^{2}-B y^{2}\), where \(A=1 \mathrm{~m} \cdot \mathrm{s}^{-1}, B=1 \mathrm{~m}^{-1} \cdot \mathrm{s}^{-1}\), and the coordinates are measured in meters. Obtain expressions for the velocity field and the stream function. Calculate the pressure difference between the origin and point \((x, y)=(1,2)\).