Viscosity Experiment (Class 11)
Viscosity Experiment for Class 11
Aim:
To determine the coefficient of viscosity of a viscous liquid by measuring the terminal velocity of a given spherical body.
Apparatus:
A tall glass jar, viscous liquid (ex: castor oil or diesel), 6 or 7 identical lead shots, screw gauge, stop watch etc.
Principle:
Terminal velocity, v= [2(ρ  σ)g/9η] x r^{2}
Therefore, coefficient of viscosity of a viscous liquid, η = 2r^{2}(ρ – σ)g/9v
where, ρ – density of lead shot
σ – density of viscous liquid
r – radius of lead shot
v – terminal velocity of lead shot
Also, v = distance travelled by lead shot/time taken = AB/t
Procedure:
Two marks, A and B, at a measured distance S (say, 10 cm apart) are marked one vertically below the other on the outside surface of the jar by tying cotton threads around it. The mark B should be sufficiently well down the jar to ensure that the spherical body acquires terminal velocity when it fall through the viscous liquid in the jar before passing A. The jar is filled with the given liquid.
The radius r of a lead shot is measured with the screw gauge. It is dropped gently into the liquid. The time of descend t of the lead shot between A and B is noted using the stopwatch. The terminal velocity v of the lead shot is calculated (v = S/t). The experiment is repeated with lead shots of different radii.
Plot a graph between the terminal velocity v along the yaxis and the square of the radius r^{2} of lead shots along the xaxis. The graph is a straight line.
The terminal velocity is given by the equation v= [2(ρ  σ)g/9η] x r^{2}
The slope m of the v — r^{2} graph gives 2(ρ  σ)g/9η. From this η, the coefficient of viscosity of the liquid is calculated.
η = 2(ρ – σ)g/9 m
where ρ is the density lead shots and σ is the density of the liquid. (They are noted from the physical table).
Observations and Readings
(a) Radius of the lead shots
Pitch of the screw = _____ mm
Least count = ____ mm
Zero correction = ____ div
Lead Shot 
Trial 
PSR (mm) 
Observed HSR 
Corrected HSR 
Diameter (mm) 
Mean Diameter 
Mean radius 
1 
1 2 3 






2 
1 2 3 






3 
1 2 3 






4 
1 2 3 






Distance between A and B = S = ___
Lead shot 
r^{2}(m^{2}) 
Time t(s) 
Terminal velocity 
1 



2 



3 



4 



Density of the material of lead shot, ρ = ___ kg/m^{3}
Density of the liquid, σ = ___ kg/m^{3}
Slope of v – r^{2} graph, m = BC/AB = ___
g = 9.8 m/s^{2}
η = 2(ρ – σ)g/9m = ___ Nsm^{2}
Result:
Coefficient of viscosity of the liquid = ___ Nsm^{2}
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