Atmosphere Pressure

Behaviour of gases in the atmosphere

• The gases in the atmosphere constantly mix or diffuse into one another.
• Atmospheric gases can be compressed.
• The heat energy of the sun warms the atmospheric gases. This in turn, changes the temperature and density of the air, which further results in the formation of wind (moving air).
• The molecules of the gases strike continuously on the sides of the vessel and in doing so exert a certain force on the walls of the vessel.

DEFINITION:

Pressure is the force (thrust) acting on a unit area of cross-section. e.g. if a force of 1 newton acts perpendicular on an area of 1 m², then the pressure is said to be one pascal (Pa).
Thus, the force (thrust) of the atmosphere on a unit cross- section area is called atmospheric pressure. The atmospheric pressure changes with the change in altitude. It is because, on higher altitudes the weight of the air responsible for the pressure, decreases.

UNIT OF ATMOSPHERIC PRESSURE:

The unit of pressure in SI system is Pascal. However, it is too small unit. Thus, atmospheric pressure is measured in a bigger unit called kilopascal (kPa).

1 kilopascal (kpa) = 10³ Pa = 1000 Pa

A Pascal is defined as the pressure exerted by a force of one Newton (N) acting over an area of one square metre.

ATMOSPHERIC PRESSURE:

The pressure exerted by the atmosphere at sea level is called atmospheric pressure.

The magnitude of normal pressure is 1.03 × 10⁵ Pa. The normal atmospheric pressure is also expressed in terms of vertical height of a pure mercury column. Its magnitude is 760 mm of Hg. Or 760 torr (1mm is equal to 1 torr).

Atmospheric pressure is slightly lower than 775mm and higher than 737mm of mercury.

MEASUREMENT OF ATMOSPHERIC PRESSURE

The atmospheric pressure is measured by an instrument called barometer. Commonly used barometer is as follows:-

MERCURY BAROMETER

CONSTRUCTION:

An ordinary mercury barometer consists of a graduated glass tube of about 840mm. Height, closed at the one end and open at the other. This tube is filled with mercury and then inverted in a trough full of mercury. The level of the mercury falls to a height of about 760mm above its level in the trough (i.e. atmospheric pressure at the sea level is 760 mm of hg.)

WORKING:

The atmospheric pressure measured by observing the height of a column of mercury that its pressure exactly balances the pressure of atmosphere.

USES:

Mercury barometer is used in school laboratories and in weather stations.
Reasons for using mercury in barometer:

• It has a high density (13.6 g/cm₃), so it permits a short column.
• It does not stick to the walls of the glass tube.
• Mercury does no vaporise under vacuum conditions. Thus, we can record the correct atmosphere pressure.