Capillary rise — UPSC CSE 2008 Physics PYQ

What do you understand by streamline motion and critical velocity of a viscous liquid through a capillary tube. Capillaries of lengths l, 2l and \frac{l}{2} are connected in series. Their radii are r, \frac{r}{2} and \frac{r}{3} respectively. If the streamline flow is maintained and the pressure across the first capillary is P_1, deduce the pressures across the second and the third capillaries.