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Pressure

Pressure- Pressure is defined as force per unit area. The standard unit for pressure is the Pascal (Pa) or Newton per square meter (N/m2).

For an object placed on a surface, the force exerted on the surface by the object is the weight of the object, but in different orientations it might have a different area in contact with the surface and therefore exert a different pressure.


Mathematically,    Pressure (Pa) = Force (F) / Area (A)

Atmospheric Pressure –  The Atmospheric Pressure is the pressure exerted by a mercury of 76 cm length at 00C at 450 latitude at the sea level. It is equal to weight of 76 cm column of mercury of cross sectional area 1 cm2

Generally, atmospheric pressure is measured in Bar. 1 bar = 105 N/m2

Atmospheric Pressure, 1 atm = 1.01 bar = 10.0x105 N/m2 = 760 torr

One torr is the pressure exerted by a mercury column of 1mm length.

The small variations in pressure that do exist largely determine the wind and storm patterns of the Earth.

Near the Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for every 30 metres (100 feet). However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air. The pressure at 270,000 metres (10−6 mb) is comparable to that in the best man-made vacuum ever attained. At heights above 1,500 to 3,000 metres (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems.

Characteristics of Atmospheric Pressure


  • Atmospheric pressure decreases with altitude i.e. height from the earth’s surface, this is why, it is difficult to cook on the mountain.
  • Barometer is the instrument used to measure atmospheric pressure and hence weather forecast.
  • Sudden fall in barometer reading indicates storm.
  • Slow fall in barometer reading is indication of rain.
  • Slow rise in barometer reading is indication of clear weather.

Pressure in Liquids

1. Pressure in a liquid Increases with depth
2. Pressure in a liquid acts equally in all directions
3. The greater the density of the liquid, the greater the pressure at a certain depth.

Mathematically, Pressure in liquids can be expressed as

Pressure, P = hdg, where; h = pressure exerted by liquid at depth h below the surface of liquid; d = density of liquid and g = acceleration due to gravity.

Regarding pressure in liquids, following characteristics are important to be understood-

•    In a static liquid at same horizontal level, pressure is same at all points.
•    Pressure at a point in static liquid is proportional to the depth of the point from the free surface.
•    Pressure at a point in a static liquid has same value in all directions.
•    Pressure at a point in a liquid is proportional to the density of the liquid.

Pascal’s Law for pressure of liquids


•    If gravitational attraction is negligible, in equilibrium condition, pressure is same at all points in a liquid.
•    If an external pressure is applied to an enclosed fluid, it is transmitted undiminished to every direction.

Hydraulic Lift, Hydraulic Press, Hydraulic Brake works on Pascal’s Law.

Effect of Pressure on Melting Point and Boiling Point

The melting point (M.P.) of a substance which expands on fusion increases with increase in pressure.
The melting point (M.P.)of a substance which contracts on fusion decreases with increase in temperature.
Boiling point (B.P.) of all substances increases with increase in pressure.

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