·          Take a thread on a circular wire frame which has been dipped into soap solution then removed, it is covered with a soap film.

·          When the soap film is broken (pierced by using a pin) the loop becomes a circle.

·          This shows that forces of surface tension are contractive forces, which act to reduce the surface area.

·          The forces of the surface tension act in the surface of the liquid & perpendicular to its boundaries.

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Q: Why a needle can be carried by the surface of water ?

A:  Because the surface of water act as if it is covered with a thin elastic membrane & the weight of the needle is balanced  by the vertical component of the reaction of the force of surface tension.

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The coefficient of surface tension:

·          Suppose we have a frame ABCD in the shape of U.

·          A wire XY can move on the two sides (AD, BC).

·          Immerse the frame in the soap solution.

·          Remove the frame gently.

·          We notice that there are two on both sides of the frame.

·          The forces of thee surface tension try to decrease the area of the two films.

·          It is necessary to apply a force “F” to hold the surface from shrinking this force acts against the force of surface tension.

·          The results:

1.         This force is proportional to the length of wire.

    F a L

Since there are two films.
F a 2L
F= constant x 2L
F= g x 2L
g = F/2L

2.         The work done is proportional to the area.

    W a 2A
    W= constant x 2DA
    W= g x 2D A
     g= W/ 2D A

3.         Let us stretch the soap film so that the wire XY could move a distance dx Since the work done  ] W= F. dx

The increase in area ] A= 2 x L x dx

 Since    g= W/ 2D A =  F. dx/2L. dx

Then     g = F/2L 

Coefficient of surface tension:

        ·          It is the force per unit length acting perpendicular to it's surface.

        ·          It is the work done on the surface to produce an increase of unit area.

        ·          Unit:          J/m^2           N/m