Atmospheric Pressure

Atmospheric pressure is the force per unit area exerted by the weight of the air above a given point, caused by the Earth’s gravitational pull on the air molecules.

• The Earth’s atmosphere is composed of gases that have mass and therefore exert weight.
• Air exerts pressure in all directions—upward, downward, and sideways—because gas molecules move randomly and collide with surfaces.
• At sea level, the standard atmospheric pressure is approximately \( 1.013 \times 10^5 \, \text{Pa} \) (101.3 kPa), which is also equivalent to \( 760 \, \text{mmHg} \) or \( 1 \, \text{atm} \).
• Atmospheric pressure decreases with altitude because there is less air above a given point, meaning less weight pressing down.
• Pressure is measured using devices such as barometers and manometers.

Mathematical Relation:

\( P = \rho g h \)

Where: \( P \) = pressure due to air column (Pa) \( \rho \) = density of air (kg/m³) \( g \) = gravitational field strength (m/s²) \( h \) = height of air column (m)

Measurement:

Mercury Barometer:

Uses a column of mercury; atmospheric pressure is indicated by the height of the mercury column.

Aneroid Barometer:

Uses a sealed metal chamber that expands/contracts with changes in pressure; movement is transferred to a dial.

Effects of Atmospheric Pressure:

• Boiling point of liquids changes with altitude—lower pressure means lower boiling point.
• Breathing at high altitudes becomes harder due to reduced oxygen partial pressure.
• It can crush objects if internal pressure is less than external atmospheric pressure (e.g., a sealed can cooled after heating).

Applications:

• Weather prediction using barometric pressure trends.
• Vacuum packaging and sealing.
• Aircraft cabin pressurization for passenger comfort at high altitudes.