Question

What is the unit of surface tension?
A. $$Nm$$
B. $$N{m}^{-1}$$
C. $$N{m}^{-2}$$
D. $$N{m}^2$$

Answer 1

Hint:Surface tension of any liquid is that property of liquid which tends to minimize the area occupied by the liquid. This happens because the molecules inside the liquid are in equilibrium position as all the forces are balanced on them. But the molecules which are at the surface have a net force downwards which makes the surface to act like a stretched membrane.

Complete step by step answer:
 To measure surface tension of a liquid we draw an imaginary line which is tangent to the liquid surface. Surface tension is then equal to force per unit length on both sides of the imaginary line and the force acts perpendicular to the imaginary line.
$$T={\displaystyle \frac{\text{total force on either side of tangent(F)}}{\text{length of the tangent(l)}}}$$
The S.I. the unit of surface tension is $$N{m}^{-1}$$ and the CGS unit is $$dyn\text{c}{\text{m}}^{-1}$$. The forces responsible for surface tension are mainly cohesive and adhesive forces. The value of surface tension depends on impurities present.

Hence, the correct answer is option B.

Note:The shape of rain water is spherical only due to the surface tension, which trends to minimize the area of raindrop. At boiling temperature surface tension becomes zero. Detergents and soap also work on the principle of surface tension when they are added to water the tension of the water surface gets reduced and soap gets soaked into the pores of cloth.