Determination of Coefficient of Viscosity of Water by Poiseuille's Flow Method - QS Study
QS Study

A capillary tube of very fine bore is connected by means of a rubber tube to a burette kept vertically. The capillary tube is kept horizontal as shown in Figure. The burette is filled with water and the pinch – stopper is removed.


The time taken for water level to fall from A to B is noted. If V is the volume between the two levels A and B, then volume of liquid flowing per second is V /t. If l and r are the length and radius of the capillary tube respectively, then

V/t = πPr4 / 8η∫ … …. (1)

If ρ is the density of the liquid then the initial pressure difference between the ends of the tube is P1 = h1 ρg and the final pressure difference P2 = h2 ρg. Therefore the average pressure difference during the flow of water is P where

P = (P1 – P2)/2

= [(h1 – h2)/2]*ρg = hρg

Substituting the equation (1) we get,

V/t = πhρgr4 / 8η∫

Or, η = (πhρgr4t) / (8∫V)