| English | Pascal's principle |
|---|---|
| Bahasa Melayu | Prinsip Pascal |
| 中文 | 帕斯卡原理 |
Definition
Pressure applied to an enclosed fluid is transmitted equally and undiminished throughout the fluid in all directions.
What you need to know
Pascal's principle states that pressure applied to an enclosed fluid is transmitted equally in all directions throughout the fluid, without loss. This is possible because a fluid can flow and fill any enclosed space, distributing pressure evenly to every surface it touches.
This principle is applied in hydraulic systems, where a small force applied to a small piston produces a much larger force at a larger piston, because the pressure at both pistons must be equal. This relationship is written as F₁ ÷ A₁ = F₂ ÷ A₂, where F₁ and F₂ are the forces in newtons (N) acting on pistons of area A₁ and A₂ in square metres (m²) respectively.
Because the larger piston has a greater area, it experiences a proportionally larger output force for the same input pressure, allowing a hydraulic system to multiply force. This idea is used in devices such as the hydraulic jack, which lifts heavy vehicles with a small applied force, and in hydraulic brake systems, which transmit braking force efficiently from the pedal to the wheels.
Worked example
A hydraulic jack has a small piston of area A₁ = 0.002 m² and a large piston of area A₂ = 0.08 m². A force of F₁ = 40 N is applied to the small piston.
Calculate the output force produced at the large piston.
Since pressure is transmitted equally throughout the enclosed fluid, F₁ ÷ A₁ = F₂ ÷ A₂, which can be rearranged to F₂ = F₁ × (A₂ ÷ A₁).
Substituting the values gives F₂ = 40 N × (0.08 m² ÷ 0.002 m²) = 40 N × 40 = 1600 N.
This shows that a relatively small input force of 40 N applied to the small piston can produce a much larger output force of 1600 N at the large piston, which is why hydraulic jacks can lift heavy loads such as cars. Both forces are correctly expressed in newtons and both piston areas in square metres throughout the calculation.
How it is examined
In Paper 1, objective questions often ask candidates to calculate an unknown force or area in a hydraulic system using F₁ ÷ A₁ = F₂ ÷ A₂, or to identify which everyday device applies Pascal's principle. In Paper 2, structured questions typically ask candidates to state Pascal's principle, explain how a hydraulic jack multiplies force, and calculate the output force or the required piston area for a given hydraulic system.
In Paper 3, practical or application-based questions may ask candidates to describe the working principle of a hydraulic brake system using a labelled diagram.
A common mistake is inverting the ratio of areas, applying F₂ = F₁ × (A₁ ÷ A₂) instead of F₂ = F₁ × (A₂ ÷ A₁), or forgetting that both pistons must be part of the same enclosed, connected fluid for the principle to apply correctly.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)