Page 107 - Physics - XI

EXPERIMENT - 4

Aim

To determine the surface tension of water by capillary rise method.

Apparatus and Materials Required

Three glass capillary tubes of diff erent diameters, travelling microscope, adjustable stand, a fl at bottom
glass dish, clamp stand, a glass strip, a needle, thin rubber bands, a thermometer, and clean tap water
whose surface tension is to be determined

Principle/Theory Clamp stand

Surface Tension Rubber band
It is the fundamental property of a liquid surface. It is the
property of a liquid by virtue of which its free surface behaves
like a stretched membrane which try to acquire the minimum R Glass
surface area. plate
Surface tension is defi ned as the force acting per unit length on R
either side of an imaginary line drawn on the liquid surface at
rest. The S.I. unit of the surface tension is Nm . C Needle
–1
Capillary tube and capillarity
Glass
A glass tube of very fi ne bore (very small bore) is called capillary dish
tube. When a capillary tube is dipped in a liquid, the liquid level
either rises or falls in the capillary tube. The phenomenon of rise Adjustable
or fall of a liquid level in a capillary tube dipped in it is called stand
capillarity or capillary action.
When a liquid rises in a capillary tube, the surface tension of the
liquid is given as

rh r g Fig. 4.1: Determination of surface

T = 3 tension by capillary rise
2 cos
For water, the value of angle of contact θ is very small.
Hence, cos θ ≈ 1 [θ ≈ 0; cos θ ≈ 1]

The surface tension of water can be given as

rh r g

T = 3
2
where T is the surface tension of water.
r is the radius of capillary tube.
h is the rise of water in capillary tube above the free surface of the water.
ρ is the density of the water at room temperature.

g is the acceleration due to gravity.

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