The weight of the air
The Earth is surrounded by a layer of air. The weight of this air pressing down creates atmospheric pressure, which acts in all directions. At sea level it is about 101 325 Pa (roughly 100 kPa), equal to the pressure of a 760 mm (76 cm) column of mercury.
Key formula / 关键公式
Standard atmospheric pressure ≈ 101 325 Pa ≈ 760 mmHg
Pressure of a liquid column: P = ρgh
Atmospheric pressure decreases with increasing altitude.
Measuring atmospheric pressure
A mercury barometer measures atmospheric pressure using the height of a mercury column it can support. Mercury is used because its high density gives a manageable column height (about 76 cm) instead of the ~10 m needed for water.
Worked example / 例题
A barometer shows a mercury column 0.76 m high (ρ = 13 600 kg m⁻³, g = 10 m s⁻²). Find the atmospheric pressure.
P = ρgh = 13 600 × 10 × 0.76 = 103 360 Pa (about 1.0 × 10⁵ Pa).
Worked example / 例题
If atmospheric pressure supports a water column (ρ = 1000 kg m⁻³), how high would the column be for P = 100 000 Pa (g = 10 m s⁻²)?
h = P / (ρg) = 100 000 / (1000 × 10) = 10 m.
Remember / 记住
- Atmospheric pressure ≈ 760 mmHg ≈ 101 325 Pa.
- It decreases as altitude increases (less air above).
- Mercury is used in barometers because of its high density.