Pressure in Liquids and Atmosphere

Pressure in Liquids and Atmosphere

Pressure

Definition:

• Pressure is defined as a force acting normally on a unit area of the surface.

Unit of Pressure:

• SI unit of pressure is N/m², also called pascal (Pa). Pressure is a scalar quantity.

Some Applications of the Concept of Pressure

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

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

3. Pins are made to have pointed ends.

4. This explains why a needle is used to burst a gas-filled balloon because the needle exerts more pressure.

Atmospheric Pressure

Definition:

• Atmospheric pressure is the pressure that is exerted by a mercury column of 76 cm length at 0°C at 45° latitude at the sea level. It is equal to the weight of a 76 cm column of mercury of cross-sectional area 1 cm².

• Generally, it is measured in bars.

1 bar = 10⁵ N/m²

Conversion Values:

• 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.

Variation with Altitude:

• Atmospheric pressure decreases with altitude (height from Earth's surface).

• This is why:

1. It is difficult to cook in the mountains.

2. The fountain pen of a passenger leaks in an aeroplane at height.

Measurement of Atmospheric Pressure

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

Barometric Readings:

• A sudden fall in barometric reading is an indication of a storm.

• A slow fall in barometric reading is an indication of rain.

• A slow rise in barometric reading is an indication of clear weather.

Pressure in Liquid

Definition:

• The force exerted on a unit area of the wall or base of the container by the molecules of liquid is the pressure of the liquid.

• The pressure exerted by liquid at depth h below the surface of the liquid is given as:

p = hdg

Where d is the density of the liquid.

Important Points about Pressure in Liquids:

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

2. Pressure at a point in a static liquid has the 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.

Pascal’s Law for Pressure of Liquid

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

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

Applications:

• Hydraulic lift, hydraulic press, and hydraulic brake work on Pascal’s Law.

Final Thoughts

Pressure is the force acting per unit area, measured in pascal (Pa). We see its effects daily—broad handles reduce pressure on hands, while sharp pins increase it.

The atmospheric pressure around us, measured by a barometer, decreases with altitude, explaining why cooking on mountains is harder. In liquids, pressure increases with depth and density and acts equally in all directions.

Pascal’s Law states that pressure applied to a confined liquid is transmitted equally, forming the basis of hydraulic lifts, brakes, and presses used in modern machines.