Pressure

⇒ The bags and suitcases are provided with broad handles so that small pressure is exerted on the hand while carrying them.

⇒ Railway tracks are laid on large sized wooden sleepers so that thrust due to weight of train is spread over a large area.

⇒ Pins are made to have pointed ends. This explains why needle is used to brust a gas filled balloon because needle exerts more pressure.

Atmospheric Pressure : Atmospheric pressure is that pressure which is exerted by a mercury column of 76 cm length at 0°C at 45° latitude at the sea-level. It is equal to weight of 76 cm column of mercury of cross-sectional area 1 cm². Generally it is measured in bar. 1 bar = 10⁵ N/m²

Atmospheric pressure 1 atm = 1.01 bar = 1.01 × 10⁵ N/ m² = 760 torr

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

⇒ Atmospheric pressure decreases with altitude (height from earth’s surface). This is why 1. It is difficult to cook on the mountain 2. The fountain pen of a passenger leaks in aeroplane at height.

⇒ Atmospheric pressure is measured by barometer. With the help of barometer, weather forecast can be made.

⇒ Sudden fall in barometric reading is the indication of storm.

⇒ Slow fall in barometric reading is the indication of rain.

⇒ Slow rise in the barometric reading is the indication of clear weather.

Pressure in liquid : Force exerted on unit area of wall or base of the container by the molecules of liquid is the pressure of liquid.

The pressure exerted by liquid at depth h below the surface of liquid is given as p = hdg where d is the density of liquid.

⇒ Regarding pressure, the following points are worth noting :

1. In a static liquid at same horizontal level, pressure is same at all points.

2. Pressure at a point in a static liquid has same value in all directions

3. Pressure at a point in a liquid is proportional to the depth of the point from the free surface.

4. Pressure at a point in a liquid is proportional to the density of the liquid.

1. If gravitational attraction is negligible, in equilibrium condition, pressure is same at all points in a liquid.

2. If an external pressure is applied to an exclosed fluid, it is transmitted undiminished to every direction.

⇒ Hydrolic lift, hydrolic press, Hydrolic brake work on Pascal law.

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