Pressure

What these notes cover

These notes cover pressure for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): pressure as force per unit area, pressure in liquids, atmospheric pressure and why it falls with height, and measuring atmospheric pressure with a mercury barometer. They include worked examples of calculating the pressure an object exerts on a surface.

Key formulas and definitions

• Pressure is the force acting per unit area: p = F / A. Unit: pascal (Pa), where 1 Pa = 1 N m⁻². Pressure is a scalar.
• The same force spread over a larger area produces a smaller pressure; concentrated on a smaller area, it produces a larger pressure.
• Pressure in a liquid increases with depth and with the density of the liquid: Δp = ρgΔh, where ρ is the density and g the gravitational field strength.
• Atmospheric pressure is caused by air molecules colliding with surfaces. At sea level it is about 100 000 Pa (1.0 × 10⁵ Pa).
• Atmospheric pressure decreases with height because the air is less dense, so fewer molecules collide with a surface each second.
• In a mercury barometer, atmospheric pressure supports a column of mercury about 760 mm high at sea level: the column stops falling when its pressure equals atmospheric pressure.