Water flows through a capillary tube of radius r and length l at a rate of 40 ml per second, when connected to a pressure difference of h cm of water. Another tube of the same length but radius r/2 is connected in series with this tube and the combination is connected to the same pressure head. Calculate the pressure difference across each tube and the rate of flow of water through the combination.  


Correct option is


The volume (quantity) of liquid flowing through a capillary tube per second, 


where P : pressure difference at the two ends of tube  

            l : length of tube 

            r : radius of tube and    

            : coefficient of viscosity of liquid 

When tubes are connected in series, the amount of liquid flowing through them is equal i.e., 



From eq. (ii), 


Substituting for P2 in eq. (iii), 



Now, substituting for P1 or P2 in eq. (ii) 






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