Atmospheric pressure is the force per unit area caused by the weight of air above a surface. Even though air feels light, the whole column of air from the ground to the top of the atmosphere has mass and is pulled downward by gravity. This pressure matters because it affects weather, breathing, boiling points, flight, and many everyday tools.
At sea level, the atmosphere pushes on every square meter with a force of about 101,325 newtons.
Key Facts
- Pressure is force per unit area: P = F/A.
- Standard atmospheric pressure at sea level is 1 atm = 101,325 Pa.
- A mercury barometer balances air pressure with a mercury column: P = ρgh.
- Standard air pressure supports about 760 mm of mercury in a barometer.
- Atmospheric pressure decreases with altitude because there is less air above you.
- Pressure differences cause fluids and air to move from higher pressure toward lower pressure.
Vocabulary
- Atmospheric pressure
- The pressure exerted by the weight of the air in the atmosphere on surfaces.
- Pascal
- The SI unit of pressure, equal to one newton per square meter.
- Barometer
- An instrument that measures atmospheric pressure, often using the height of a mercury column.
- Altitude
- The height of a location above sea level.
- Pressure difference
- A difference in pressure between two regions that can cause air or fluids to move.
Common Mistakes to Avoid
- Thinking suction pulls objects together. Suction cups and straws work because higher outside air pressure pushes fluid or surfaces toward a lower pressure region.
- Forgetting that pressure depends on area. The same force can create a larger pressure when it acts on a smaller area because P = F/A.
- Assuming atmospheric pressure is the same at all heights. Pressure decreases with altitude because the column of air above becomes shorter and less massive.
- Using the wrong units in P = ρgh. Density must be in kg/m^3, g in m/s^2, and height in meters to get pressure in pascals.
Practice Questions
- 1 A force of 500 N acts evenly on an area of 0.25 m^2. What pressure is produced in pascals?
- 2 A mercury barometer has a mercury height of 0.760 m. Using ρ = 13,600 kg/m^3 and g = 9.8 m/s^2, calculate the atmospheric pressure.
- 3 Explain why it is harder to drink through a straw on a very high mountain than at sea level.