Definition of Surface Tension – Properties of Fluid – Fluid Mechanics

hello students here in this video we will see what is surface tension and we will even see the formula of surface tension for water droplet as well as for a soap bubble let us start with the definition part surface tension it is defined as a tensile force acting on the surface of a liquid in contact with air or we can say gas or between two immiscible liquids so here is a simple definition of surface tension surface tension it is defined as a tensile force acting on the surface of liquid suppose we have a liquid which is in contact with air or gas then there is a tensile force a force which is acting at the surface of the liquid that force is called as surface tension surface tension can exist between liquid or gas or between two immiscible liquids like for example one liquid is floating over another liquid then also at the surface we are having some tensile force and that tensile force is called as surface tension no surface tension it is denoted by surface tension is denoted by letter Sigma it’s unit it’s unit is Newton per meter because it is the force per meter distance of length of the surface no if I can explain it in the form of a diagram if I consider here a liquid now this liquid if I am saying that it is water and here I have three different molecules of that liquid this I will call it as molecule number one here molecule two and molecule three now the molecule of liquid Here I am taking water this molecule of water which is denoted by one would be pulled in all its direction by the other molecules that is all the other molecules which are surrounding this molecule one will try to pull this molecule one towards their own direction so the resultant of this will be that this molecule would be stationary it would remain at its position that is the resultant force acting over this molecule is zero now let me take this molecule which is two since it is not completely immersed in water some portion of it is in contact with the air here I can say we are having air so some portion of the molecule is in contact with air so the forces which are acting they would be trying to pull the molecule in the downward direction towards the water so because of that there would be resultant downward force now if I consider the third molecule because of it here there would be a force acting parallel to the surface and similarly there would be forces which would be acting that is the molecules will be attracting this molecule number three now at the surface we are having a parallel force as we can see for molecule number three now this because of the parallel force here this surface appears to be stretched the surface is stretched and when this surface is stretched it means that there is some tenser tension some tensile force which is acting so surface tension is nothing but a tensile force which keeps the surface like it is very much in a tight position if the surface is not slipping it is in a tight position so now I write down the surface tension and pressure force relation for a water droplet is here I have written surface tension and pressure force relation for a water droplet it is the relation which I have that is P is equal to 4 into Sigma divided by D so here is the relation between as we can see surface tension which is denoted by Sigma and intensity of pressure so here I’ll write down where P is the pressure force or we can say intensity of pressure Sigma it is surface tension and D is the diameter of water droplet water droplet Here I am taking water droplet it can apply to any kind of liquid so if we have a liquid water droplet having diameter as small D over which pressure intensity P is acting so for that liquid we can calculate the intensity of surface tension similarly surface tension for soap-bubble it will be denoted by the formula is intensity of pressure is equal to eight times of surface tension divided by D so here also I am having surface tension in the form of Sigma and the surface tension for soap bubble is expressed in the form of intensity of pressure so here I have two different formulas first one to be used for water droplet which is like a sphere and the second formula to be used for a soap bubble so in this video in short we have seen what was surface tension how it acts and we have seen its unit we have seen its how it is denoted and here was the explanation of surface tension so I hope it is very much clear


  1. sir awesome teaching with clear explanation.thank you very much and please upload more videos of your subject

  2. nice lecture sir iam watching ur videos the way teaching and what u r using the sketches is awsome proces

  3. U r videos are very good and we understand very quickly compare to remaining videos these surface tension lecture is not perfact according to my opinion

  4. Kyo sir ander ke kisi bhi point par bhi to sare partical streched hai . It means pani ke ander kisi bhi point par surface tenson. Ho sakata hai

  5. Sir on whom would the tensile force on surface act because on a surface molecule left and right forces will cancel each other out.

  6. सर prast तनाव की वीमा bataiye

  7. अब पता कर लिया सर

  8. Sir aapne btaya ki surface tension liquid-air ya fir two immiscible liquids ke bich me lagega lekin aapne diagram me water ke molecules pe water ka surface tension kaise show kiya?

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  10. Thanks for lectures which are to me, as a being , blessed by the harmony of nature following the rules that fluid mechanics may probably be grasped by honest mind, of Einstein and Newton.

  11. For a free tutorial in udemy on fluid mechanics use the link

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