What is Capillarity?
A glass tube of very fine bore throughout the length of the tube is called capillary tube. If the capillary tube is dipped in water, the water wets the inner side of the tube and rises in it [shown in figure
a) If the same capillary tube is dipped in the mercury, then the mercury is depressed shown in figure
b)The phenomenon of rise or fall of liquids in a capillary tube is called capillarity.
8 PRACTICAL APPLICATIONS OF CAPILLARITY
1] The oil in a lamp rises in the wick by capillary action.
2] The tip of nib of a pen is split up, to make a narrow capillary so that the ink raises upto the tin or nib continuously.
3] Sap and water rise upto the top of the leaves of the tree by capillary action.
4] If one end of the towel dips into a bucket of water and the other end hang over the bucket the towel soon becomes wet throughout due to capillary action.
5] Ink is absorbed by the blotter due to capillary action.
6] Sandy soil is drier than clay. It is because the capillaries between sand particles are not so fine as to draw the water up by capillaries.
7] The moisture rises in the capillaries of soil to the surface, where it evaporates. To preserve the moisture m the soil, capillaries must be broken up. This is done by ploughing and leveling the fields
8] Bricks are porous and behave like capillaries.
CAPILLARY RISE (HEIGHT OF A LIQUID IN A CAPILLARY TUBE) ASCENT FORMULA
Consider the liquid which wets the walls of the tube, forms a concave meniscus shown in figure. Consider a capillary tube of radius r dipped in a liquid of surface tension T and density p. Let h be the height through which the liquid rises in the tube. Let p be the pressure on the concave side of the meniscus and pa be the pressure on the convex side of the meniscus. The excess pressure
(p – pa) is given by (p – pa) = 2T/R
Where R is the radius of the meniscus. Due to this excess pressure, the liquid will rise in the capillary tube till it becomes equal to the hydrostatic pressure hpg, Thus in equilibrium state.
Excess pressure = Hydrostatic pressure or 2T/R = hpg
Let be the angle of contact and r be the radius of the capillary tube shown in the fig.
From OAC, OC/OA = cos or R = r/cosθ h = (2T cosθ)/rpg
This expression is called Ascent formula.
