Section 1 of 5The Atmosphere as a Fluid
The atmosphere is a thin layer of gas held around the Earth by gravity.
Fluid
- A fluid is a liquid or a gas, and both can flow and both exert pressure on anything immersed in them.
Cause
- Air particles collide with every surface, and their collisions give a force that acts over the area of that surface.
Sea level value
- At sea level atmospheric pressure is about 100 000 Pa, which is roughly 100 kPa and often quoted as 101 kPa.
Weather
- Pressure varies a little from day to day, and settled, clear weather is usually linked with higher pressure.
Section 2 of 5Pressure and Altitude
Pressure falls steadily as you climb through the atmosphere.
1
At sea level a large depth of air lies above you, and the weight of that air presses down on the surface.
2
As you rise, there is less air above you, so the weight of air pressing down becomes smaller.
3
The air is also less dense at higher altitude, so fewer particles collide with a surface each second.
4
Both effects mean the atmospheric pressure decreases as the height above sea level increases.
5
At great heights the atmosphere becomes so thin that pressure approaches zero, which is why mountaineers need extra oxygen.
Section 3 of 5Adding Liquid Pressure
Adding Liquid Pressure
Under water
- An object under water feels the pressure of the liquid above it plus the pressure of the atmosphere above the surface.
Each 10 m
- Water pressure increases by about 100 kPa for every 10 m of depth, roughly equal to one atmosphere.
Total
- At 10 m depth the total pressure is therefore about 101 kPa + 100 kPa, which is around 200 kPa.
Everyday effects
- Ears feel uncomfortable when diving or flying because the pressure on the eardrum changes.
Section 4 of 5Worked Example
Question
- The atmosphere exerts a pressure of 1.0 × 10⁵ Pa.
- Calculate the force it exerts on a window of area 0.50 m².
- Write the equationpressure = force ÷ area, so force = pressure × area.
- SubstituteF = 1.0 × 10⁵ × 0.50.
- CalculateF = 5.0 × 10⁴ N.
Answer
- 5.0 × 10⁴ N, which is 50 kN.
- Air pressure inside the building usually balances this force.
Section 5 of 5Exam Checklist & Tips
1.3.4 – Key Points for the Exam
- Atmospheric pressure The pressure exerted by the weight of the air above a surface.
- Fluid A substance that can flow, meaning a liquid or a gas.
- Altitude Height above sea level.
- Pascal The unit of pressure, equal to one newton per square metre.
Common Mistakes
- Saying there is no atmospheric pressure inside a building;
- it acts everywhere, and the same pressure acts inside and outside.
- Claiming pressure falls with height because gravity disappears; it is because there is less air above.
- Adding pressures but forgetting to include the atmospheric contribution at depth.
- Mixing units, for example using kPa in the equation without converting to pascals.