The volume of water flowing per second through a tube of length l and radius r under a pressure difference P is given by
v = πpr4/8ηl,
Where η = coefficient of viscosity.
Given that the pressure difference is equal to height of water i.e.,
p = egλ
and volume flowing per second is 40 ml. Hence.
When another tube of same length but radius (r/2) is connected in series with this tube, let P1 and P2 be the pressure difference across these tube respectively. They are connected to the same pressure head.
Dividing equation (i) in equation (ii), we get