# To Determine the Co-efficient of Viscosity by Poiseull’s Capillary Flow method

THEORY: When a liquid flows through a capillary tube of length l and radius r then the volume of liquid flowing per second is given by,

V = $$(πPr^4)/8ηl$$

where,

η = Co-effiecient of viscosity

P = Pressure difference between two ends = hρg

g = acceleration due to gravity

η = $$(πgρr^4)/8l (h/V)$$

where,

h = difference in height between the two arms of the apparatus = (h2-h1)

ρ = density of liquid

Apparatus: Viscosity apparatus, Travelling microscope, cathetometer, measuring cylinder, stop watch.

Procedure:

1. Measure the length of the capillary tube with the help of a metre scale.
2. Measure the radius of the sample capillary tube with the help of the travelling microscope. Determine the Vernier Constant of the microscope. Place the tube on a stand facing the microscope. Focus the orifice. Take reading of the top, down and right ends of the circular orifice.
3. Place the water tank of the viscosity apparatus at a certain level. Allow the water to come out of the tube at a very slow rate (drop by drop). Wait until the water levels at two arms of the manometer take steady values.
4. Collect water from the output end in a measuring cylinder for a fixed time, say T = 4 minutes. Measure the volume. From that calculate the volume of water collected per second.
5. Change the height of the water tank several times and repeat steps 3 and 4.
6. Draw a graph with h in abscissa and V in ordinate. The curve should be a straight line passing through the orifice. If there is a curvature for high values of h, use the linear portion to calculate the slope .

Experimental Results:

1. Vernier constant of the travelling microscope:
2. Measurement of radius of Capillary tube:
 No. of obs. Top (Y1) Bottom(Y2) Left(Y3) Right(Y4) D1 = (Y1~Y2) cm. D2 = (Y3~Y4) cm. D =  cm. Radius (r) =  cm. Mean radius (r) cm. MSR VSR Total MSR VSR Total MSR VSR Total MSR VSR Total

1. Detemination of (V/h) :
 No. of Obs. Reading of the manometer in cm. Volume of water collected per minute Volume of water collected per second (cc/sec.) h1 cm. h2 cm. h = (h2-h1) cm. I II III IV

Calculation:

Percentage error:

η = $$(πgρr^4)/8l (h/V)$$

$$δη/η=δt/t+δh/h+δV/V$$

Conclusion:

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