How to find the coefficient of Viscosity of a highly viscous liquid using Stoke's method

 

    The TANSCHE syllabus for first year B.Sc. Physics highlights the significance of hands-on learning in the field of physics. A major experiment included in the curriculum is Stoke's Method, which enables students to measure the viscosity of a liquid by applying fluid dynamics concepts.

    In this blog post, we will walk you through the Stoke's Method experiment, detailing each step along the way. This procedure reinforces your understanding of fluid mechanics, especially Stoke's Law, which explains the forces acting on a sphere as it moves through a viscous substance. This experiment offers important insights into how frictional forces operate in actual fluid systems.

    By the end of this post, you will know exactly how to perform the experiment, from setting up the equipment to accurately determining the liquid's viscosity. Whether you're a student gearing up for your first practical or a teacher leading your class, this guide will be an essential tool for mastering Stoke's Method.

Viscosity - Stoke's method

Aim:

    To determine the coefficient of viscosity of a highly viscous liquid.

Apparatus required:

    Lead shots or glass beads, long cylinder, screw gauge, stop watch, metre scale etc

Formula:

            Coefficient of viscosity is,

                            

Here,

         is the coefficient of viscosity of a highly viscous liquid Ns/m2

        S is the distance travelled by the lead shots m

         is the density of the lead shot kg/m3

         is the density of the highly viscous liquid kg/m3

        g is the acceleration due to gravity m/s2

        r is the radius of the lead shot m

Procedure:

    A glass jar that is 1 meter tall and has a diameter of 5 cm is completely filled with a highly viscous liquid, and we want to find out its viscosity. To measure the radius (r) of a glass bead or lead shot, we use a screw gauge. The bead is carefully placed on the surface of the liquid so that it sinks straight down. 
    Start a stopwatch when the bead passes the first mark, labelled A. We then record the time it takes for the bead to reach each of the following marks. At first, the bead speeds up, but eventually, it slows down and reaches a constant terminal velocity due to the viscous force. The time it takes for the bead to travel the distance between two marks is noted as ‘t’ seconds. 
    This experiment is repeated with glass beads of various sizes, and we calculate the average value of n.

Tabular Column 1: To find r2t

Least count (LC) = 0.01 mm
Zero error = ____ div                  Zero correction = ____ div

S.No	Distance S cm	PSR mm	CHSR mm	d = TR mm	r = d/2 mm	r2 mm2	Time t s	r2t mm2s

Result: 

The coefficient of viscosity of the highly viscous liquid is = ______ Ns/m2