Surface tension is the force acting on unit length of an imaginary line drawn on the free surface of a liquid, the force acting normal to the line and parallel to the surface. The unit of surface tension is Nm-1 and its dimension is MT-2

The free surface of a liquid at rest behaves like a stretched elastic membrane with a tendency to contract in area. The following simple experiments will illustrate this property.

1. When a camel hairbrush is dipped into water, the hairs remain spread out. When the brush is taken out, the hairs cling together on account of the films of water between them contracting.

2. A film of the soap solution is formed in a wire ring. If a wetted loop of thread is gently placed on the film, the film is unaffected and the thread remains in any form in which it is placed on the film. But if the film is pricked inside the loop, the film outside contracts on all sides and hence the circular loop is formed. The area of the film is reduced to a minimum.

The above examples show that a force acts on the surface of liquid to reduce the surface area to a minimum. This force is known as surface tension.

A water drop takes the spherical shape because for a given volume, sphere has the minimum surface area. This is due to surface tension.

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