Pressure & Pressure Differences in FluidsEdexcel GCSE Physics: Revision notes
Section 1
What is pressure?
Pressure is a measure of how concentrated a force is over a surface.
P = F ÷ A
- P = pressure (Pa or N/m²)
- F = force normal (at right angles) to the surface (N)
- A = area of that surface (m²)
For the same force, a smaller area gives a larger pressure, and a larger area gives a smaller pressure. This is why a sharp knife (small contact area) cuts more easily than a blunt one, and why snowshoes (large area) stop you sinking into snow.
Always check the force is normal (perpendicular) to the surface before using P = F/A — if given at an angle, only the normal component counts.
Section 2
How is pressure transmitted in a fluid?
In a fluid (liquid or gas), pressure is transmitted equally in all directions. This means a pressure applied at one point in an enclosed fluid is felt equally throughout the fluid.
This principle is used in hydraulic systems, which use liquids to transmit force. Because the liquid is (almost) incompressible and transmits pressure equally, a small force applied to a small-area piston creates a pressure that acts on a larger-area piston elsewhere in the system, producing a much larger output force.
Why this works: since pressure = force ÷ area is the same throughout the connected liquid, a small force on a small piston area gives the same pressure as a large force on a large piston area — so hydraulic systems can multiply force.
A car braking system: a small force on the brake pedal creates pressure in the brake fluid, which acts equally on the much larger brake pads to produce a strong braking force.
Section 3
Why does pressure change with depth in a fluid?
Pressure in a liquid increases with depth because there is more liquid (and therefore more weight) pressing down from above at greater depths.
ΔP = h × ρ × g
- ΔP = pressure difference (Pa)
- h = height/depth of liquid (m)
- ρ = density of the liquid (kg/m³)
- g = gravitational field strength (N/kg)
This means pressure difference depends only on depth, density and gravitational field strength — not on the shape of the container or the total volume of liquid.
A common error is forgetting to convert depth into metres or density into kg/m³ before substituting into ΔP = hρg — mismatched units give a wrong answer even with the correct method.
Must Know
- P = F ÷ A; pressure is measured in pascals (Pa) or N/m²
- Smaller area → larger pressure for the same force
- Pressure in a fluid is transmitted equally in all directions
- Hydraulic systems use this to convert a small input force into a large output force
- ΔP = h × ρ × g gives the pressure difference at depth h in a liquid
- Pressure increases with depth because of the increasing weight of liquid above
That's the notes covered.
Carry on to the next subtopic.