Question

One end of a towel dips into a bucket full of water and the other end hangs over the bucket. It is found that after some time the towel becomes fully wet. It happens
(A) Because viscosity of water is high
(B) Because of the capacity action of cotton threads
(C) Because of gravitational force
(D) Because of evaporation of water

Answer 1

Hint:Capillary action is the phenomenon of rise of liquid inside a capillary tube when it is dipped in the liquid.

Complete step by step answer:
A glass tube having a very fine bore is called capillary and the capillary action primarily occurs due to adhesive and cohesive forces.
Capillary action Explanation:
Let consider the situation before the water is inside the capillary tube and the shape of surface of water is concave as shown in fig

Point \[A\]is above the concave surface and inside the capillary tube. Point \[B\]is below the concave surface inside the capillary tube. Point C is above and point \[D\]is below the surface of the liquid outside the capillary tube.
Let, \[{P_A},{P_B},{P_C},{P_D}\]be the pressure at points A, B, C, D respectively.
Observations are as follows.
(i) The pressure at the concave side of the free surface of water is greater than that on the convex side.
${P_A} > {P_B}$ . . . (1)
(ii)${P_A}$ and ${P_C}$ atmospheric pressure so
${P_A} = {P_C}$ … (2)
(iii) Pressure is the same on both sides of the surface above and below, outside the capillary. Hence,
${P_C} = {P_D}$ … (3)
From (2) and (3), we get
${P_A} = {P_D}$ . . . (4)
And from (1) and (4), we get
\[{P_D} > {P_B}\]
Thus, under the liquid surface, pressure outside the capillary is greater than the pressure inside the capillary. Therefore, to maintain the equilibrium, the liquid level rises in the capillary tube to a certain height h until the pressure at both the points, B and D becomes equal. This is the reason why there is a rise of liquid inside the capillary tube.
When one end of a towel is dipped into the bucket, it would act as a capillary. And hence, it would be fully wet after some time due to rise in the water level over the towel to maintain the pressure inside the bucket.
Hence the correct answer is option (B) Because of the capacity action of cotton threads.

Note:In capillary action, the liquid does not always rise. In some cases, when the pressure under the layer of the liquid inside the capillary tuber is higher than the pressure outside the capillary, then the height of the liquid inside the capillary will reduce or fall down.