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Explain surface energy and surface tension.

Answer» <html><body><p></p>Solution :An extra energy is associated with surface of liquid , the creation of more surface keeping volume fixed requires additional energy.<br/><img src="https://doubtnut-static.s.llnwi.net/static/physics_images/KPK_AIO_PHY_XI_P2_C10_E01_059_S01.png" width="80%"/><br/>As <a href="https://interviewquestions.tuteehub.com/tag/shown-1206565" style="font-weight:bold;" target="_blank" title="Click to know more about SHOWN">SHOWN</a> in figure (a) a U- shape frame is made from wire , the wire PQ slide without firction over the rides AP and BQ.<br/>This frame dipped and taken out of the soap solution , thin film APQB is formed from the frame.<br/>In figure (a) film is in equilibrium. <br/>In figure (b) it is shown that the film is <a href="https://interviewquestions.tuteehub.com/tag/stretched-7711564" style="font-weight:bold;" target="_blank" title="Click to know more about STRETCHED">STRETCHED</a> at extra distance d.<br/> Since the area of the surface increases ,the system now more energy , means that some work has been done against an internal force.<br/>Let this internal force be F.The work done by the applied force is , <br/> `W=vecF*vecd=Fd` ...(1)<br/>From the <a href="https://interviewquestions.tuteehub.com/tag/conservation-929810" style="font-weight:bold;" target="_blank" title="Click to know more about CONSERVATION">CONSERVATION</a> of energy , this is stored as additional energy in the film.<br/>If the surface energy of the film is S per unit area , the extra area is 2ld. (Because `DeltaA`=length `xx` breadth ld and film has two free surface,so 2ld is present.<br/> The liquid in between , so there are two surfaces and the extra energy is`=(2ld)S`.<br/>`thereforeW=(2ld)S=SDeltaA[because2ld`= increase in area]<br/>`thereforeS=(W)/(DeltaA)`<br/>`thereforeS=(fd)/(2ld)=(F)/(2l)` <br/>This quantity S is the magnitude of surface tension .It is <a href="https://interviewquestions.tuteehub.com/tag/equal-446400" style="font-weight:bold;" target="_blank" title="Click to know more about EQUAL">EQUAL</a> to the surface energy per unit area of the liquid interface and is also equal to the forceper unit lenght exerted by the <a href="https://interviewquestions.tuteehub.com/tag/fluid-993483" style="font-weight:bold;" target="_blank" title="Click to know more about FLUID">FLUID</a> on the movable bar.</body></html>


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