Note on Some Examples Explaining Surface Tension and Surface Energy

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Examples of Surface Tension

Thread on a soap film

Take a frame of wire, dip it in soap solution and then take it out. A soap film will be formed in the frame. Place a loop of wet thread gently on the film. It will remain in the form as we place it. Now pierce the film will a pin at any point inside the loop, the lop will immediately take circular form as shown in figure.

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Floating needle

Take a greased needle on a piece of blotting paper and place it gently over water surface. Blotting paper soaks water and soon sinks down but the needle keeps floating. The neeedle creates depression in the liquid surface so that surface tension forces have an upward component which is capable to support the weight of needle. If we add oil in water the needle sinks. It is because oil lowers the surface tension of water and weight of needle exceeds the upward directed component of surface tension force.

When a dry brush is diped into water, its hair spread out

When a brush is dipped into water, the hair do not cling together but when the brush is removed from water, its hair cling together as shown in figure. When the brush is in water, there is water all around its hair. Consequently, there is no free surface and the effect of surface tension is absent. However when the brush is taken out from water, the force of surface tension contract the hair.

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Formation of lead shots

Inorder to manufacture lead shots, melted lead is allowed to fall in water by spraying it from a sufficient height. During its fall, the melted lead forms small spherical drops due to surface tension forces and on entering water, they become solid.

Oil has less surface tension than water

When a drop of oil id dropped on the surface of water, the oil is streched in all directions as a thin film. Mosquitoes breed on the free surface of stagnant water. Due to surface tension, the liquid layer supports the eggs laid by the mosquitoes. Whenil is spread in water, there is little surface tension and the mosquitoes cannot bread.

Surface Energy

The free surface of a liquid always has a tendency to contract and occcupies minimum surface area. If the surface area of a liquid has to be inceased, work has to be done. The work done is stored in the liquid. The work done is stored in the liquid surface film as its potential energy.The potential energy per unit area of the surface film is surface energy. It is also defined as the amount of workdone in increasing the area of a surface film through unity.

$$\text {Surface energy,} \sigma = \frac {\text {work done in increasing surface area} }{\text {increase in surface area}}$$

It is also called free energy because the mechanical work done can be released when the surface contracts.

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Relation between Surface Tension and Surface Energy

Let us consider a rectangular frame of wire ABCD made up of a metal wire in which position EF is movable. It is dipped in a soap solution and is taken out.The force due to surface tension on \(EF = T \times 2l\) in ward direction on there exists two free surfaces.

The part EF is moved to a distance x to the position E'F' by external agent \(=F \times x = T \times 2l \times x \)

Equation (i) becomes

$$\sigma = \frac {T\times 2l \times x}{2lx}$$

$$\therefore \sigma = T$$

Thus surface tension T is numerically equal to surface energy.

Surface tension is applied for thread on a soap, floating needle, formation of  lead shots, dipping of brush in water, adding oil in stagnant water and so on. This note provide us an information on some examples explaining surface tension and surface energy.

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Very Short Questions

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  • Additional force needed to pull a needle of 2.5 cm long floating on water of surface tension 7.2 × 10-2 N/m is ______.

    5.6 × 10-3 N


    1.8 × 10-3 N


    3.6 × 10-3 N


    7.2 × 10-3 N


  • The oil spreads over the surface of water due to ______.

    elastic force


    buoyance


    surface tension


    viscosity


  • Detergents makes easier to wash clothes because it ______.

    decrease surface tension
    density hardness
    increases density
    increase surface tension
  • The quantity with same dimension as surface tension is ______.

    force


    spring constant


    young's modulus


    strain


  • Shaving brush cling together on taking out of water due to ______.

    viscoscity


    fluid pressure


    upthrust


    surface tension


  • Shaving brush cling together on taking out of water due to ______.

    surface tension


    fluid pressure


    viscoscity


    upthrust


  • On increasing temperature the surface tension of liquid ______.

    increases


    none


    decreases


    remain constant


  • On adding salt in pure water, surface tension of it ______.

    increases


    becomes zero


    remain constant


    decreases


  • On adding salt in pure water, surface tension of it ______.

    becomes zero


    decreases


    remain constant


    increases


  • Ink flow in tip of fountain pen due to ______.

    viscoscity


    none of the answers


    capillary action


    surface tension


  • The surface tension of water increases when ______.

    salt is added


    phenol os added


    detergent is added


    temperature is increased


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Sangam Baral

where if ef in se


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