Surface tension is the property of a liquid by virtue of which free surface of liquid at rest tries to have minimum surface area. In doing so, the free surface of liquid at rest behaves as if it is covered with a stretched membrane. Surface tension (S) of a liquid is measured by from (F) acting on unit length fo a line (l) imagined to be drawn tangentially anywhere on the free surface i.e., `S = (F)/(l).` S is measured in `N m^(-1)` Read the above passage and answer the following question : (i)What is the cause of surface tension ? (ii) A wire ring of 30 mm diameter resting flat on the surface of a liquid is raised. The pull required is 1.5 gf more before the film breaks then it is after. What is surface tension of the liquid ? (iii) What are the implication of this phenomenon in day to day life ?

Explanation

(i) Every individual molecule in the free surface of a liquid has maximum potential energy. And the free surface on the whole tends to have minimum potential energy for stability. This is possible when free surface of liquid contains minimum number of molecules. As area of each molecule is fixed. the free surface of liquid at rest tries to have minimum area. This is the cause of surface tension. (ii) Here, `r = (30)/(2) = 15 mm = 1.5 cm. F = 1.5 gf = 1.5 xx 980` dyne As liquid is touching the ring inside as well as outside, therefore, `F = S xx 2l= S xx (2pi r)`
`S = (F)/(4pi r) = (1.5 xx980)/(4 xx 3.14 xx 1.5) = 78.0 dyn e//cm` (iii) Suppose you are the owner of a factor where 20 people are working. Each of them wants to get maximum wages and you want to save maximum. The only practical solution is to pay them the desired wages with the promise that each one will put in the best of efforts, so that you are not to engage extra labour. This is the practical impication of the concept of surface tension, where free surface tries to contain minimum number of molecules, with each molecule having maximum energy.