Surface Tension

The surface of a liquid behaves in such a way that it is cover with an elastic skin. This elastic skin allow some tiny objects with a density higher than the density of water (for example) to float on its surface. If a tiny needle is drop on a surface of water  the needle floats despite the density of the needle higher than the density  of the water.

Surface tension can be defined as the force acting parallel on the surface of a liquid which does not allow tiny objects of higher density to sink down the liquid.

Molecular Explanation of Surface Tension.

When a liquid is in a container, there are forces exerting on each of the molecules from other molecules at all angles. The molecules in the liquid are acted upon by equal and opposite forces. These  forces come from molecules in the surroundings . This is not the same for the molecules of the liquid at the surface. There are lots of forces acting on the surface molecules towards the liquid while there are few or no forces acting in opposite direction. As a result of this imbalance in forces, the surface of water (for example) look concave inside because the forces dragging those surface molecules toward the water forced the surface to look concave .

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Coefficient of Surface Tension

The coefficient of surface tension T can be defined as the force per unit length acting parallel to the surface of a liquid in which the force acts.

Mathematically,

The Coefficient of Surface Tension can be defined as      the units is

Where F = force in newton

And L = length of the surface in meter

Suppose L= Length of surface of a liquid in a tube of height h.

Therefore L

   where r = radius of the tube

If this force acts on a needle of Mass = m

The weight of the needle is =mg

The weight of the needle acting downward =opposite force acting upward on the needle

The Coefficient of Surface Tension is

Example

A tube of radius 0.02m and height 0.05m contain mercury. What is the coefficient of surface tension ? Take density of mercury   and

Solution

Factors Affecting Surface Tension

1. Addition of soap or detergent reduces surface tension
2. Increase the temperature of the liquid reduces surface tension
3. Nature of the liquid  determine the surface tension
4. The length or radius of the liquid surface determine the surface tension

Application of Surface Tension

Surface tension is being used in :

1. Umbrella design
2. Washing power of soap and detergent 

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