| English | Pressure in liquids |
|---|---|
| Bahasa Melayu | Tekanan dalam cecair |
| 中文 | 液体压强 |
Definition
The pressure at a point in a liquid, which increases with depth and density (pressure = height × density × gravitational field strength) and acts equally in all directions.
What you need to know
Pressure is defined as force acting per unit area, written as P = F ÷ A, where P is pressure in pascals (Pa), F is force in newtons (N), and A is area in square metres (m²). One pascal is equal to one newton per square metre (1 Pa = 1 N m⁻²).
Pressure in a liquid at a certain depth is given by P = hρg, where h is the depth below the liquid surface in metres (m), ρ is the density of the liquid in kilograms per cubic metre (kg m⁻³), and g is the gravitational field strength, approximately 9.81 m s⁻² (sometimes taken as 10 m s⁻² in calculations).
This formula shows that liquid pressure depends only on depth and density, increasing as depth increases, and does not depend on the shape of the container or the surface area of its base. Two containers of different shapes but holding the same liquid at the same depth therefore have identical pressure at that depth, which explains why liquids always find and maintain a common level.
Worked example
A diver descends to a depth of 5 m below the surface of seawater with density ρ = 1030 kg m⁻³. Calculate the pressure due to the water alone at this depth, taking g = 9.81 m s⁻².
Using P = hρg, the pressure is P = 5 m × 1030 kg m⁻³ × 9.81 m s⁻² = 50 521.5 Pa, which can be rounded to P ≈ 50 500 Pa.
This value represents only the pressure due to the weight of the water above the diver; the total pressure experienced by the diver also includes atmospheric pressure pushing down on the water surface, so the two pressures would need to be added together to find the total pressure at that depth.
Notice that depth is measured straight down from the surface in metres, density is expressed in kilograms per cubic metre, and the final pressure is expressed correctly in pascals throughout the calculation.
How it is examined
In Paper 1, objective questions often ask candidates to calculate pressure using P = F ÷ A or P = hρg, or to identify which factor liquid pressure depends on from a list of options. In Paper 2, structured questions typically ask candidates to state the factors affecting pressure in a liquid, explain why pressure increases with depth in terms of the weight of liquid above a point, and calculate pressure at a given depth.
In Paper 3, practical tasks commonly use a device with holes at different heights on a container to describe how the range of water jets shows that pressure increases with depth.
A common mistake is confusing pressure with force by omitting the area or depth term, or incorrectly assuming that a wider or larger container changes the pressure at a given depth. Candidates should also state clearly whether an answer refers to liquid pressure alone or to the total pressure including the atmosphere, since examiners often distinguish between the two in the mark scheme.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)