6. Class 11th Physics | Surface Tension | Concept of Surface Energy | by Ashish Arora

6. Class 11th Physics | Surface Tension | Concept of Surface Energy | by Ashish Arora


lets discuss the concept of surface energy.
. when we talk about the stretched spring it always stores. some potential energy in
it. so whenever we talk about an elastic body , if it is an undeformed state we can say
that there is no potential energy in it but when we talk about its stretched state. whenever
we stretch a spring some work is done and that is stored in form of potential energy
in it which we term as elastic potential energy. similarly when we talk about , free surface
. of a liquid . as we have discuss that due to surface tension the free surface of a liquid
also behave like a stretched membrane , so we can say in free surface of liquid also
some. potential energy stored in it. and this energy is only stored in the surface of the
liquid which is behaving like a stretched membrane. that’s why this potential energy
is termed as . surface energy. so lets analyze the surface energy mathematically and, to
understand the surface energy lets have an experiment. say we are given with a u shaped
wire frame . on which, a slider is placed. and say the length of this slider is l. and
in the enclose part of this u tube and this slider we make, a soap film, say for example
we are making a soap film here . then obviously we can say the soap film will act like a stretched
membrane due to its surface tension. and if the soap film is having surface-tension t.
then this will exert a force on the slider in leftward direction as on the right side
there is no liquid. in a left side of the slider there is a soap film. as the film is
having two surfaces one is above the film and other is below the film. so net force
on it we can simply write . force on slider. due to surface tension this force we can write
as twice of t-l. as we already discuss that t is force acting per unit length, normal
to this. or normal to any line considered in contact with liquid . so here due to the
film there are two surface in contact with a slider , so net force on the slider in inward
direction will be “ 2”-t-l. now if we wish to move slider in outward direction we
need to apply an external force. and we can say to slowly. move the slider out. this external
force must be equal to the force which is applied by the surface tension that is two-t-l.
and say if we displace the slider by distance delta x, in outward direction. so we can say
if it is pulled outward by distance delta-x, the surface area of film will also increase.
and some work is done by the external force in pulling the slider outward , so whatever
work is done by the external force will be stored in the film in form of its surface
energy . in the similar way like if we are having a spring , which is already stretched
and if we further stretch this by an external force . then whatever work is done by the
external force will be stored in the, spring in form of elastic potential energy. so here
we can calculate work done. in displacing. slider by delta-x. is. this work delta w can
be written as actually it’ll be f-external into delta-x. and if it is slowly being pulled
in that case f-external we can substitute as, two t-l delta x . and in this situation
if we talk about its increase in area we can simply write increase in . surface area of
film. this delta s it can be written as twice of l delta x, because as we have discussed
this is a film, one surface is above the film and other is below the film. so total increase
in surface area of film will be twice of l delta-x l delta-x on one side l delta-x on
other side . so here twice l delta-x can be written as increase in area so here work done
can be written as t delta s. and as we already discussed whatever work is done , in pulling
the slider by distance delta-x , by the external agent , the work will be stored in form of,
elastic potential energy of the object being stretched and here, the film is being stretched
, so the surface of film will store this much of amount of work in form of potential energy.
so we can write this work as delta u which we can write as , increase in. surface energy
of film. and in this situation if we just rearrange this expression. here we can state
t we can write as delta u by delta, s. which gives us an idea about , the numerical value
of surface tension as. surface energy per unit area because, here the increase in area
is delta-s and increase in surface energy is delta-u, that means this delta u energy
is being stored in . this much amount of area as the whole film is uniform. so we can write
the surface tension as. surface energy. per unit area, this is another way how we can
define surface-tension. and lets have some example based on the concept of surface tension
defined as surface energy per unit area this will be more clear to u.

23 Replies to “6. Class 11th Physics | Surface Tension | Concept of Surface Energy | by Ashish Arora”

  1. Sir, How do we know that force due to surface tension is conservative force. Because we only use the concept of energy stored in case of conservative force

  2. sir here if we take external force as 2TL the how can we pull the film as we appyl force 2TL in opposite direction to the surface tention and the force will be neutralized and now we have to apply some force to do work in short net force will be 0 the why will it move

  3. Sir please give answer: Apart from liquid at the boundary, surface tension everywhere in the free surface cancels out due to opposite forces acting perpendicular to the hypothetical line but at the boundary, liquid molecules are present only in one side of the hypothetical line drawn along the boundary and because of this surface is stretched inside from all the boundaries .If Iam wrong please correct me?

  4. A soap film consists of two layers of soap molecules separated by a thin layer of fluid, which may vary in thickness from . The largest thickness wills occur immediately after the formation of the film.

    Once the film is formed it will begin to thin. The surplus water will drain away from the film by various draining processes. The thickness of the film will decrease until a final equilibrium thickness is reached.

    In a state of equilibrium the surface tension is the same in all points on the surface.

    When the film has reached equilibrium its surface area will have a minimal value, this minimum area property of soap films can be used to solve some mathematical minimization problems.
    To understand consider a soap solution film. Suppose it’s thickness is 1 micron. Now, size of molecule as building blocks of the film is of the order of 10^(-10) m. This means that when we go from one side of the surface to the other side and count the number of molecules, it comes out to be roughly10,000. Keeping this in mind imagine a wall whose thickness is 10,000 bricks. In this situation if you disturbe one side of the wall the other side is not affected. In this sense two surfaces are INDEPENDENT of each other. Exactly the same situation exists in film of soap solution or in the wall of soap solution bubble in air. Therefore, we say that a film and a bubble in air has TWO independent surfaces.

  5. Dear sir on moving the slider will it not be more difficult to move a slider over the distance (i.e force req . is directly proportional to distance .)

    So we need to integrate. Or it is not what I am thinking.πŸ€”

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