Surface Tension, Elasticity | Science & Technology for UPSC CSE PDF Download

An insect called ‘pond skater’ can easily walk on the surface of the water. A slight depression of the surface is produced by the legs of the insect, showing that the surface acts like an elastic “skin”. 

If a needle is placed on small piece of blotting paper, which is then placed on the surface of the water,the paper sinks in a few seconds leaving the needle floating on water. A close examination reveals that the needle rests in a slight depression as if lying on an elastic skin.

Thus the surface of a liquid behaves like an elastic membrane and, therefore, has a tendency to contract.

This property of a liquid is called surface tension. Surface tension is caused by molecular attractions. When a paint brush is dipped in water all its hair spread out but when it is taken out it is covered with a thin film of water which contracts due to surface tension and pulls the hair together. Liquid drops, such as raindrops, oildrops, drops of molten metals, dewdrops etc. are all spherical because their surface tend to contract in order to have minimum surface area. For a given volume, a sphere has the minimum surface area.
Soaps and detergents lower the surface tension of water. This increases the wetting power of water or its ability to detach dirt particles from clothes and utensils.
Mosquitoes breed on stagnant water. Their larvae keep floating on water due to surface tension. When oil is sprinkled on the stagnant water its surface tension is lowered resulting the drowning and death of the larvae.

Capillarity If a clean glass tube having a small inside diameter (called a capillary tube) is dipped in water, the water rises in the tube. This phenomenon is called capillarity.
Water rises in the capillary tube because water molecules are attracted to glass more than to each other.

If the same capillary tube is dipped in mercury, the level of mercury in the tube is lower than the level outside because mercury molecules are less attracted to glass than to each other. The force of attraction between unlike molecules is called adhesion and that between like molecules cohesion. The melted wax of a candle is drawn up into the wick by capillary action. Oil rises up a lampwick for the same reason. If one end of a sugar cube is dipped into tea, the entire cube is quickly wet on account of capillary action.
The fine pores of a blotting paper act as tiny capillary tubes. The ink rises into the blotting paper through these pores. The capillary action in soils is important in bringing water to the roots of plants.

Bricks are porous and, therefore, subsoil water can seep up them by capillary action. To avoid dampness in a building, a layer of non-porous material, such as slate, is necessary in its foundation.

Elasticity is the property of a body or material of resuming its original form and dimensions when the forces acting upon it are removed. If the forces are sufficiently large for the deformation to cause a break in the molecular structure of the body or material, it lose its elasticity and the elastic limit is said to have reached.
Elasticity

(i) Stress: It is defined as the force per unit area

(ii) Strain: When a body is deformed by an applied stress. The strain is the ratio of the dimensional change to the original or unstrained dimensions. The strain may be a ratio of lengths, areas, or volumes.

(iii) Young’s Modulus: The ratio of stress to strain is called the Young’s Modulus.

(iv) Ductility: It is property, especially of metals, of being capable of being drawn out into a wire. Hooke’s Law states that provided the elastic limit is not exceeded, the deformation of a material is proportional to the force applied to it.

Hooke illustrated the law by reference to four different experiments:

(i) Loading a wire (deformation = Increase in length).

(ii) Loading a spiral spring (deformation = Increase in length).

(iii) Loading a horizontal beam fixed at one end (deformation = depression of the free end).

(iv) Tightening a watch spring (deformation = angular rotation).

Bernoulli’s Principle: He found that in a streamline fluid-flow, the pressure is lower in the region where the flow is faster. This is called Bernoulli’s Principle. When the speed of the fluid (liquid or gas) in a pipe line increases, the pressure decreases and conversely when the speed of the fluid decreases, the pressure increases.

Applications of Bernoulli’s Principle:

(i) Filter Pump or Aspirator,

(ii) Atomizer or Sprayer,

(iii) The Bunsen Burner,

(iv) Lifting a disc,

(v) Magus effect,

(vi) Streamlining,

(vii) Lift on an aircraft wing