how can the coefficient of viscosity of a highly viscous liquid the determined by stokes method
Answers
hlo mate
1. Objective
To determine the coefficient of viscosity of a known fluid using Stokes’ method.
2. Equipment needed
A glass vessel with glycerine, micrometer calliper, stopwatch, ruler.
3. Theory
Viscosity refers to the friction within a fluid from flowing freely and is essentially a frictional
force between different layers of fluid as they move past one other. The tangential force F
required to move a layer of area S and located a perpendicular distance x from an immobile
surface is given by:
S
dx
dv F , (1)
where is the coefficient of viscosity.
Provided the flow the velocity is not too large, the fluid flows smoothly and the flow is said to
be laminar.
The mutual influences of layers of fluid which move past one another are conditioned by the
molecule intermediate attraction of fluid. This prevents also the motion of solid body in fluid
because the whole surface of body is covered with thin molecular layers by molecules of
fluid. Therefore the force, which prevents the motion of a body in a fluid, can be found by the
force of viscosity. It is possible only then, when the velocity of the body is smaller than the
velocity of laminar flow of fluid. Otherwise the whirls will arise and the formula (1) given by
Newton is useless.
In general it is complicated to find the formula of frictional force Ft
. In case of regular bodies
the problem will simplify. For a spherical bodies Stokes derived the following formula:
F rv t 6 , (2)
where is the coefficient of viscosity, r is the radius of the sphere and v is the velocity of
the sphere.
In this experiment the formula (2) is used to determine the coefficient of viscosity. Computing
the frictional force Ft
the sphere dropping through a column of liquid is observed. (see figure
13.1).