The property of surface tension gives rise to an interesting phenomenon called capillarity. When a capillary tube is dipped in water, the water rises up in the tube. The level of water in the tube is above the free surface of water in the beaker (capillary rise). When a capillary tube is dipped in mercury, mercury also rises in the tube. But the level of mercury is depressed below the free surface of mercury in the beaker (capillary fall).
The rise of a liquid in a capillary tube is known as capillarity. The height h in Fig. indicates the capillary rise (for water) or capillary fall (for mercury).
Illustrations of capillarity
(i) A blotting paper absorbs ink by capillary action. The pores in the blotting paper act as capillaries.
(ii) The oil in a lamp rises up the wick through the narrow spaces between the threads of the wick.
(iii) A sponge retains water due to capillary action.
(iv) Walls get damped in rainy season due to absorption of water by bricks.
Surface tension by capillary rise method
Let us consider a capillary tube of uniform bore dipped vertically in a beaker containing water. Due to surface tension, water rises to a height h in the capillary tube as shown in Fig.. The surface tension T of the water acts inwards and the reaction of the tube R outwards. R is equal to T in magnitude but opposite in direction. This reaction R can be resolved into two rectangular components.
(i) Horizontal component R sin θ acting radially outwards
(ii) Vertical component R cos θ acting upwards.
The horizontal component acting all along the circumference of the tube cancel each other whereas the vertical component balances the weight of water column in the tube.
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