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Question
Explain the capillary action.
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Solution
If one end of a capillary tube is dipped in a liquid which partially or completely wets the surface of the capillary, the level of liquid in the capillary rises. On the other hand, if the capillary tube is dipped in a liquid which does not wet its surface, the level of liquid in the capillary drops. The phenomenon of the rise or fall of a liquid inside a capillary tube when it is dipped in the liquid is called capillarity.
Consider the points A, B, C, and D such that
i) Point A is just above the convex surface and inside the capillary.
ii) Point B is just below the convex surface inside the capillary.
iii) Point C is just above the plane surface outside the capillary.
iv) Point D is just below the plane surface and outside the capillary, and below the point C.
Let PA, PB, PC, and PD be the pressure values at points A, B, C, and D, respectively. The pressure on the concave side is always greater than that on the convex side.
Capillary fall:
Consider a capillary tube dipped in a liquid which does not wet the surface. The shape of the liquid meniscus in the capillary is upper convex.
∴ PB > PA
As points A and C are at the same level, the pressure at both these points is the same, and it is the atmospheric pressure.
∴ PA = PC
Between the points C and D, the surface is plane.
∴ PC = PD = PA
∴ PB > PD.
But the points B and D are at the same horizontal level. Thus, in order to maintain the same pressure, the liquid in the capillary rushes out of the capillary. Because of this, there is a drop in the level of liquid inside the capillary, as shown.
Capillary rise:
Consider a capillary tube dipped in a liquid which wets the surface. The shape of the liquid meniscus in the capillary is concave.
∴ PA > PB
As points A and C are at the same level, the pressure at both these points is the same, and it is the atmospheric pressure.
∴ PA = PC
Between the points C and D, the surface is plane.
∴ PC = PD = PA
∴ PD > PB.
But the points B and D are at the same horizontal level. Thus, in order to maintain the same pressure, the liquid in the capillary rushes into the capillary. Because of this, there is a rise in the level of liquid outside the capillary, as shown.
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