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Cambridge IGCSE Physics · 0625

Chapter 1: Motion, forces and energy — Part 13

Topic 1.8 · Pressure

Pressure

Pressure is force per unit area. p = F / A. Unit: pascal (Pa). 1 Pa = 1 N/m2.

A large force on a small area gives high pressure (drawing pin, knife edge). The same force on a large area gives low pressure (snowshoes, wide tyres).

A drawing pin is pushed with a force of 18 N. The point has area 1.5 × 10−4 m2. Calculate the pressure at the point.

Worked example: pressure is 18 over 1.5 times 10 to the minus 4 equals 1.2 times 10 to the 5 pascals
p = F / A = 1.2 × 105 Pa.
Same force on a small area versus a large area: pressure higher on the small area (drawing pin, sharp knife, camel’s feet as a contrasting large area).
Diagram 1: Same force on a small area versus a large area: pressure higher on the small area (drawing pin, sharp knife, camel’s feet as a contrasting large area).

Pressure in a liquid

Qualitatively, pressure beneath the surface of a liquid increases with depth and is greater if the liquid is denser.

Recall and use Δp = ρgΔh for the change in pressure at depth Δh in a liquid of density ρ.

Calculate the extra pressure 2.5 m below the surface of a lake. Density of water = 1000 kg/m3. Take g = 9.8 N/kg.

Worked example: liquid pressure change is 2.5 times 10 to the 4 pascals
Δp = ρgΔh = 2.5 × 104 Pa.
Liquid pressure increasing with depth: a column of liquid of height Δh, density ρ, with Δp = ρgΔh marked on a dam or measuring cylinder.
Diagram 2: Liquid pressure increasing with depth: a column of liquid of height Δh, density ρ, with Δp = ρgΔh marked on a dam or measuring cylinder.

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