Surface Energy

Surface Energy

Introduction: From the molecular theory of surface tension, it is clear that all the molecules which  are on the surface of the liquid has net force in downward direction. The surface of the liquid layer posses some potential energy due to upward movement of molecules. This energy is stored as surface energy. Let us study this in detail.

Theory : The molecule which is inside the liquid has net force zero while the liquid molecule which is on the surface has net downward force which pulls the molecule downward from the surface as a result, the  potential energy on the surface reduces. As any system tries to attain minimum potential energy level , the surface tries to reduce its potential energy by reducing it's surface area. To increase the surface area work has to be done in sending molecule on the surface of the liquid, this work done is stored as potential energy on the surface called as surface energy.

Relation between Surface energy and Surface Tension

Rectangular Soap Film

Consider a rectangular frame ABCD containing a slider fixed across BC which can be moved freely as shown in fig above. Let l and w be the length and width of the frame. Let this frame is dipped in soap solution and removed so that  a soap film ABCD is formed on the surface of the frame. The area of the film is given by,

A_i = l x w ---(1)

Now the wire AB is displaced to a length   △w, so that the new fil formed is AB'C'D.The final area of the film is given by,

A_f = l x (w + △w ) ---(2)

Thus change in area after displacing the slider by △w is given by,

 A_f - A_i  =[ l x w -  l x (w + △w )] 

A_f - A_i  = l [ w -  w + △w ] 

A_f - A_i  = l x  △w  

As soap film has two surface ,both surface changes it's area and is written as,

△A  = 2  x l x  △w ---(3)

The force acting on both the surface film due to surface tension is given by,

F = 2 x T x l ---(4)

The work done in moving slider through a width  △w is given by,

W  = F x △w 

W  = 2 x T x l x △w  ---(from 4)

W  =   △A  x  T ---(from 3) 

This work done is stored as surface energy

SE =  △A  x  T ---(5)

Equation (5) gives relation between surface energy and surface tension.

│<<< Surface Tension││ Angle of Contact>>>│

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 Related Topic :  Mechanical Properties of fluid      Pressure due to liquid column     Absolute and gauge pressure  Hydrostatic Paradox       Pascal's Law        Measurement of pressure         Surface Tension Surface Energy         Angle of contact          Capillary Action            Motion in a fluid                    Critical velocity and Reynold's Number                      Viscosity                 Stokes' Law            Terminal velocity                 Equation of continuity                  Bernoulli Equation          Speed of Efflux    Ventury Tube