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| Surface Energy |
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| We know that work has to be done in order to bring a molecule, from interior to the surface, against the force of cohesion. If the surface area is increased, more molecules can be accommodated at the surface. Increase in surface area, results in cooling. To maintain the temperature, heat flows from the surroundings to the film and this is added on. This additional energy is termed as 'surface energy'. |
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| Therefore, surface energy is the additional energy per unit area of surface or the work done to increase the area of the surface film by unity. |
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| Consider a liquid film to be formed on a rectangular surface ABCD. On forming a film, the surface film contracts (see arrows) due to surface tension i.e., AB is pulled inwards. To keep AB in its original position, a force F is applied in the outward direction, all along its length. If T is force due to surface tension (i.e. T=F/l). |
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| Then F = T x 2l. Since the film has two free surfaces, the RHS of the equation is multiplied by 2. |
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| If the wire AB is to be pulled by an additional distance dx, then the additional work done W in this case is, |
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| i.e. Work done in increasing the surface area by unity, is equal to the surface tension T at constant temperature. Therefore, this work done, is the surface energy of the film. |
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SI. unit of surface energy is N/m and Dimension of Surface energy [MT-2]
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| We know that in small drops and bubbles, the effect of gravity is negligible on account of surface tension. Small drops do not collapse due to surface tension. This means that the pressure inside the drop is greater than the pressure outside. Let this excess
pressure i.e., pi - po = p. Due to this pressure
difference, let the radius increase from R to R + dR. |
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 Outside force = pressure difference x surface area |
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| Hence, to increase the surface area |
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| Work done by the excess pressure is stored in the form of potential energy. |
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| Increase in P.E. = surface tension x increase in surface area. |
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| dW = increase in P.E. |
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