# 2. Class 11th Physics | Surface Tension | Significance of Surface Tension | by Ashish Arora

let us talk about significance of surface
tension. to understand the significance of surface tension lets consider a container
. in which , say a liquid is filled. say this is a container in which liquid is filled up
to the top. and in this situation we already discuss that, every point on surface of liquid,
will behave as a if it is under tension . or every point will be experiencing an outward
force of attraction by its neighboring particles. now in this situation if i consider a hypothetical
line say this line ay-b on the. liquid surface . then every particle on this line will be
experiencing the force on left-and-right side. of this line. so we can say the resulting
effect of all these forces will be acting in a direction normal to the line. so we can
say in this situation the total force due to surface tension acting on the line will
be zero as it is being pulled by equal forces in opposite direction. but if i talk about
a line, which is there on the. edge of the container. and only on one side of this container.
only on one side of the edge as liquid is present and on the other side there is no
liquid. this line will be experiencing the force in the direction . only along the surface
of liquid. so we can say net force due to surface tension on this line say this line
is x-y it will be nonzero . so using surface tension we can find out the total force. for
this purpose we need to give a mathematical . significance or an analytical significance
for surface tension. and its numerical definition i’ll right , so very carefully you just
noted down . here numerically we can say . surface tension. is. numerically defined as. the force
acting. per unit length. of a, hypothetical line. drawn. on a liquid surface. normal to
it. in other direction. here we can say for hypothetical line is drawn. and it experiences
a force net force f and f on the two sides and say its length is l. so here we can say
. the surface tension of liquid. can be given as, t, t is the symbol we use to denote a
surface tension for the liquid it can be given as f upon l. and its unit we can write as
newton per meter obviously . and using this relation if we wish to find out the force
acting , on line in one direction , then we can say if this line is of length l-one we
can say the total force f-one acting on it will be t l-one. if t is the surface tension
of the liquid as force in either direction can be given as . the product of surface tension
in the length of the line on which it is acting. similar analysis we can , understand here
with help of this situation here you can see , if there is wire frame on which there is
a soap film. if we place a, loop of a thread on the soap film it’ll be arbitrary in shape.
and say if we just pick the soap film between the loop by a needle as you can see here . the
force of surface tension will start acting on the loop segments in outward direction
where liquid exist. and like now there is no film in between so force can not be balanced
, earlier when the film was there inside also the force on the two sides were balanced . and
the loop were in arbitrary shape. now due to this prick, the inner film will burst and
, only outer film exist and we can see , due to the forces automatically it is pulled in
outward direction and finally the loop will take a shape of a circle. because we can see
here if outward forces are acting on the loop it will have a tendency to acquire a shape
in which , all the forces will balanced, and that’s why it’ll take, the shape of a
circle. this experiment you can also perform at your home. and understand and this is the
very good experiment to understand . how the force of surface tension act on a line, or
a segment, on which only one side , the film or liquid surface exist .

### 14 Replies to “2. Class 11th Physics | Surface Tension | Significance of Surface Tension | by Ashish Arora”

1. Asshwin M says:

really a nice explanation sir. the experiment made everything clear. thank you very much sir

2. Syed Sumaid says:

Sir, in that experiment how can we calculate tension across the loop thread after it forms circle?

3. Rohan Patil says:

sir…one of my book says that free surface behaves as an elastic membrane trying to reduce its surface area to minimize its potential energy and gain stability.this proper is called surface tension ….So i have doubt that if it reduces its surface area than why its equal to work done in increasing surface area..???

4. Arun teltia says:

sir why we consider a hypothetical line why not we consider and thing else for eg triangle and sir is the line u had considered is the straight line water molecules or any other liquid plz elaborate

5. Vansh Mishra says:

why is it not moving if net force is not equal to 0?

6. Ritesh Uppal says:

Sir i am having a silly doubt!..in starting of the chapter…we said that the molecules due to their contact have contact forces causing a molecule to have net force 0 but inward pressure….so similarly here at line XY (1:52)..the net force should be on left i.e. repulsive not attractive.plz clear it!..i am confused

7. Naveen Kumar says:

Sir I am having doubt that why we subtract Pexcess from P atm when we study ascent formula?

8. prabhat kumar Verma says:

Great work sir
Thanks a lot for this quality education.

10. Vedant Kaushik says:

Sir, if a net force on edge line is acting, then how is it in equilibrium?

11. Shivam Raj says:

For all those who want to see soap film and thread experiment :
https://youtu.be/e0fhh1830Kc

12. KESHAV MAHESHWARI says:

sir what is soap film

13. Suman Singh says:

Your voice is pathetic and distracting

14. aspirant Aj says:

Sir , I have a doubt for line AB surface tension should be 0 right because net force is 0 sir? But you wrote t= F/L plz elaborate