You might have heard the term of surface tension in various chapters if you are a physics student. Imagine a water droplet on a leaf after a rain shower. The water creates stunning round droplets. Further, you must have also seen the insect water strider walk across the top of water effortlessly. All these things happen due to the surface tension of water. Thus, we will learn surface tension formula in this article.
Surface tension is the attractive force found in liquids which is responsible for pulling surface molecules in the rest of the liquid. Further, it minimizes the surface area. The attractive forces we are talking about here are because of electrostatic forces.
Usually, we refer to this cohesion at the gas-liquid surface; remember not liquid-solid or not liquid-liquid. You might have noticed it take place in water often but it also happens with all other liquids to some extent too.
Furthermore, we see that there are electrostatic forces. These forces attract molecules to each other. The slight dipole with water is able to make the molecules specifically attract to each other. Thus, it enables it to produce a tight-knit cohesive unit.
The force is not able to make much impact when in a liquid state as each molecule pulls all directions towards the other molecules. However, on the surface of the liquid, these molecules do not pull up at all due to the fact that no molecules are there for pulling the others up.
The molecules are pulled down by a strong force between them. Thus, it makes a solid obstacle at the liquid-gas interface. Therefore, a lot of force will be required to break through the liquid’s surface in comparison to going through liquid.
We describe surface tension as the ratio of surface force F which applies on a liquid to the length d along which the force acts. This is the same thing that makes insects like water strider to walk on water effortlessly or the floating of a paperclip. Thus, the surface tension formula is:
Surface tension = (surface force)/ (length force acts)
γ = F /d
Γ refers to the Surface tension
F is the force which applies to the liquid
d refers to the length where the force acts
Solved Examples on Surface Tension
Question- You have a small piece of metal that is 1 cm long and weighs 0.1 N. Find out the surface tension.
Answer- Looking at the figures, we see that we have the F as 0.1 N and d as 1 cm. We can find out the surface tension by applying the above formula:
γ = F/d
γ = 0.1N/1 cm
0.1 N/ 0.01
m = 10 N/m.
Therefore, the surface tension is 10 N/m.
Question- A small insect of about 1.5 cm long is posed upon the water. The surface tension of the water is 5 N/m. Calculate the estimated mass of the insect?
Answer- Looking at the figures, we have our γ as 5 N/m and d as 1.5 cm. We can find out the force, which is the weight, from the surface tension. Thus, we have:
F = γ d
F = 5 N/m 1.5 cm
F= 5 N/m 0.015 m
F= 0.075 N
Further, the mass of the insect will be given by:
M = F/g
Over here, g = 9.8 m/s2 which is the acceleration of gravity.
M = 0.075 N / 9.8 m/s2 = 0.0076 Kg = 7.6 grams
Therefore, the mass of the insect on the water is 7.6 grams.