8.10 - Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air above us. It is a result of the gravitational pull on the air molecules in the Earth's atmosphere. This pressure decreases with an increase in altitude, as the air becomes thinner with height.

The air around us is made up of various gases such as nitrogen, oxygen, and carbon dioxide, which all contribute to the overall weight of the atmosphere. Atmospheric pressure acts in all directions and is responsible for various phenomena like the weather and movement of fluids in pipes and the human respiratory system.

Definition: Atmospheric pressure is the pressure exerted by the weight of the air above a given point. It is generally measured using a device called a barometer.

Unit of Measurement: The SI unit of atmospheric pressure is the Pascal (Pa). One Pascal is equivalent to one Newton per square meter (N/m²).

Factors affecting Atmospheric Pressure:

• Altitude: As altitude increases, atmospheric pressure decreases because there is less air above to exert pressure.
• Temperature: Warm air is lighter and rises, causing a decrease in pressure, while cold air is denser and sinks, increasing the pressure.
• Humidity: More water vapor in the air leads to lower pressure because water vapor is less dense than dry air.

Formula for Atmospheric Pressure:

The pressure exerted by a column of air can be calculated using the formula:

$P = \dfrac{F}{A}$

Where:

• $P$ is the atmospheric pressure,
• $F$ is the force exerted by the air column,
• $A$ is the area on which the force is acting.

At sea level, the average atmospheric pressure is 101325 Pa (Pascal) or 1 atmosphere (atm). In different regions, this pressure can vary depending on local weather conditions.

Barometer: A barometer is an instrument used to measure atmospheric pressure. There are two types of barometers: the mercury barometer and the aneroid barometer.

Mercury Barometer: In this barometer, a column of mercury in a tube is supported by the atmospheric pressure. The height of the mercury column changes with variations in atmospheric pressure. Higher pressure pushes the mercury up, while lower pressure causes the mercury to fall.

Aneroid Barometer: This barometer contains a small metal box that is sensitive to pressure changes. The box expands or contracts as atmospheric pressure changes, and this movement is translated into readings on a scale.

Effect of Atmospheric Pressure: Atmospheric pressure plays a crucial role in the movement of fluids and gases. For example, in the human respiratory system, air enters the lungs due to a pressure difference between the atmospheric pressure and the pressure inside the lungs.

Example of Atmospheric Pressure: The phenomenon of sucking water through a straw is a result of atmospheric pressure. When you create a lower pressure inside the straw by sucking, the higher atmospheric pressure pushes the water up into the straw to equalize the pressure.

In conclusion, atmospheric pressure is an important concept in understanding various natural and physical phenomena. It is crucial for various scientific applications and the study of weather patterns, aviation, and even in our everyday activities.