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The fluid pressure can be measured by open-tube monometer or with the help of mercury barometer. One end of the U-shaped tube containing a liquid (figure a) is open to the atmosphere while the other end is connected to a system of unknown pressure P. The difference in pressure P – Po is ρgh. Hence P = Po + ρgh. The pressure P is called absolute pressure and the difference P – Po is the gauge pressure. The later one is usually measured through gauge scale.
Another type of instrument used to measure the pressure is called barometer (figure b). This instrument follows the rule Po = ρgh; h is the height of the mercury column.
Surface Tension:
Surface tension is defined as the force experienced (F) by a molecule present on a line (l) of the considered fluid. This happens because of the cohesive force between the liking molecules in a fluid.
The cohesive forces between molecules are shared with the neighboring molecules. Those on the surface have no neighboring molecules above and exhibit stronger attractive forces upon the nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension. Surface tension of water at 25oC is 72 dynes/cm. The surface tension of water decreases with increase in temperature.
• Hot water is a better cleaning agent since it has lower surface tension.
• Small insects can walk on the water surface since they have very less weight.
• Soaps and detergents help in lowering the surface tension so that the washing of clothes are possible in better way.
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