| English | Atmospheric pressure |
|---|---|
| Bahasa Melayu | Tekanan atmosfera |
| 中文 | 大气压强 |
Definition
The pressure exerted by the weight of the column of air in the atmosphere on the Earth's surface and on objects in it.
What you need to know
Atmospheric pressure is the pressure exerted on surfaces by the weight of the column of air above them. At sea level, atmospheric pressure is approximately 101 325 Pa, which is also commonly expressed as about 76 cmHg, meaning it can support a column of mercury 76 cm high in a barometer tube.
Atmospheric pressure is measured using instruments such as the mercury barometer, the Fortin barometer, and the aneroid barometer. In a simple mercury barometer, a glass tube filled with mercury is inverted into a mercury reservoir; atmospheric pressure pushing on the reservoir supports the mercury column in the tube, and the height of this column indicates the atmospheric pressure.
Atmospheric pressure decreases as altitude increases, because there is a shorter, lighter column of air above an observer at higher altitude compared with one at sea level. This is why mountaineers experience lower atmospheric pressure at high altitudes, and why aircraft cabins must be pressurised for passenger safety and comfort.
Worked example
A mercury barometer at a weather station shows a mercury column of height h = 75 cm. Convert this reading into pascals, using the density of mercury ρ = 13 600 kg m⁻³ and g = 9.81 m s⁻².
First convert the height into metres: h = 75 cm = 0.75 m.
Using P = hρg, the atmospheric pressure is P = 0.75 m × 13 600 kg m⁻³ × 9.81 m s⁻² = 100 062 Pa, which can be rounded to P ≈ 100 000 Pa.
This value is slightly lower than the standard sea-level atmospheric pressure of 101 325 Pa, which corresponds to a mercury column of about 76 cm, suggesting that this reading may have been taken at a slightly higher altitude or under different weather conditions. Every step keeps consistent SI units: height in metres, density in kilograms per cubic metre, and the final pressure in pascals.
How it is examined
In Paper 1, objective questions often ask candidates to state the approximate value of atmospheric pressure at sea level in pascals or in cmHg, or to identify the correct labelled diagram of a mercury barometer. In Paper 2, structured questions typically ask candidates to explain why atmospheric pressure decreases with altitude, describe how a simple mercury barometer works, and calculate pressure from a given mercury column height using P = hρg.
In Paper 3, practical or application questions may ask candidates to determine readings from a barometer diagram or table and relate changes in atmospheric pressure to weather patterns.
A common mistake is confusing the height of the mercury column with the pressure value itself, or forgetting to convert the column height from centimetres to metres before applying P = hρg.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)