SURFACE TENSION

Surface Tension



“Surface tension is the property of a liquid by virtue of which its free surface behaves like a stretched membrane and supports, comparatively heavier objects placed over it”. It is measured in terms of force of surface tension.

The free surface of a liquid contracts so that its exposed surface area is a minimum, i.e., it behaves as if it were under tension, somewhat like a stretched elastic membrane. This property is known as surface tension. The surface tension of a liquid varies with temperature as well as dissolved impurities, etc. When soap is mixed with water, the surface tension of water decreases. Also, the surface tension decreases with increase in temperature.












Force of cohesion:- It is force between two molecules of similar nature.


Force of adhesion:- It is the force between two molecules of different nature.


Molecular range:- The maximum distance between two molecules so that the force of attraction between them remains effective is called molecular range.


Sphere of influence:- Sphere of influence of any molecule is the sphere with molecule as its center and having a radius equal to molecular range (=10-7 cm).


Surface film:- Surface film of a liquid is defined as the portion of liquid lying on the surface and caught between two parallel planes situated molecular range apart.


Surface tension:-





Surface tension is the property of a liquid by virtue of which its free surface behaves like a stretched membrane and supports, comparatively heavier objects placed over it. It is measured in terms of force of surface tension.


Force of surface tension:- It is defined as the amount of force acting per unit length on either side of an imaginary line drawn over the liquid surface.



(a) T = Force/length = F/l

(b) T = Surface energy/Surface area = W/A

Units:- S.I – Nm-1

C.G.S- dyn cm-1







Properties of Surface Tension:-


Scalar quantity.


Temperature sensitive.


Impurity sensitive.


Depends only n the nature of the liquid.


Unit of surface tension, N/m.


 Dimension of surface tension, ML0T-2.  








































How do detergents clean dirty clothes?

Consider a wire frame (see the adjacent figure) equipped with a sliding wire AB. It is dipped in a soapy water. A film of liquid is formed on it. A force F has to be applied to hold the wire in place. Since the soap film has two surfaces attached to the wire, the total length of the film in contact with the wire is 2L.



T (surface tension) = F/2L.






Surface tension of a liquid is measured by the normal force acting per unit length. On either side of an imaginary line drawn on the free surface of a liquid, the direction of this force is perpendicular to the line and tangential to the free surface of liquid.