5.1.3Solids, Liquids and Gases

Pressure — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

10 min read 12 parts Lesson 4 of 11

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision

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Part 1 of 10Pressure and its equation

Pressure

Pressure the force acting normally per unit area, p = F ÷ A.
What the symbols mean: p = pressure (Pa = N m⁻²) · F = force acting at right angles to the surface (N) · A = area (m²)
- (at right angles)
Force is in N, area is in m² and pressure is in pascals (Pa), where 1 Pa = 1 N/m².

Larger units

1 kPa = 1000 Pa and 1 MPa = 10⁶ Pa.

Reading the equation

Reading the equation for a given force, a smaller area gives a larger pressure.
For a given area, a larger force gives a larger pressure.

Force from weight

Force from weight an object resting on a surface exerts a force equal to its weight, F = mg.
A 60 kg student exerts about 590 N on the floor.

Area units

Area units convert cm² to m² by dividing by 10 000, because 1 m² = 10 000 cm².

Part 2 of 10Changing pressure in everyday life

Increase pressure

Increase pressure make the area small.
Knives, nails, needles and cutting wires have sharp edges so a modest force produces a large pressure.

Decrease pressure

Decrease pressure make the area large.
Snowshoes, wide tractor tyres, skis and the broad base of buildings spread the weight so the ground does not sink.

Table of examples

Table of examples each links force, area and pressure.

Situation	What changes	Result
Walking in snowshoes	Area is larger	Pressure is smaller, less sinking
Pressing a drawing pin	Tip area is tiny	Very large pressure, it enters wood
Caterpillar tracks	Area is large	Heavy vehicle does not sink
Stiletto heel	Area is small	Large pressure, can mark floors

Part 3 of 10Pressure in liquids

1Pressure in a liquid

Pressure in a liquid it acts in all directions and increases with depth.
A dam wall is thicker at the bottom because the pressure there is greatest.

2Equation

Equation the change in pressure with depth is Δp = ρgΔh.
ρ is the density of the liquid in kg/m³, g is 9.8 N/kg and Δh is the vertical depth in metres.

3Where it comes from

Where it comes from the pressure at depth h is the weight of the column of liquid above divided by the area, p = ρVg ÷ A = ρgh.

4Factors

Factors pressure depends on the depth and density but not on the shape of the container or the amount of liquid.
A narrow tube and a wide lake with the same depth have the same pressure at the bottom.

5Total pressure

Total pressure the pressure at depth in an open liquid is the sum of atmospheric pressure and ρgh.

6Liquids are almost incompressible

Liquids are almost incompressible they transmit pressure through the whole liquid, which is used in hydraulics.

Part 4 of 10Atmospheric pressure and the barometer

1Atmospheric pressure

Atmospheric pressure the pressure caused by the weight of the air above us.
At sea level it is about 100 000 Pa (1.0 × 10⁵ Pa), or 760 mm of mercury.

2Variation with height

Variation with height it decreases as altitude increases because there is less air above.
Climbers and aeroplanes experience lower pressure, which is why cabins are pressurised.

3Barometer

Barometer an instrument that measures atmospheric pressure.
A simple mercury barometer has a tube of mercury inverted in a dish and the column height, about 760 mm, shows the pressure.

4Why mercury is used

Why mercury is used it is very dense, so the column is a manageable height.
With water the column would need to be about 10 m high.

5Aneroid barometer

Aneroid barometer it uses a sealed metal box that changes shape with pressure and moves a pointer.

6Effects of atmospheric pressure

Effects of atmospheric pressure a drinking straw works because suction lowers pressure and atmospheric pressure pushes the liquid up.

Part 5 of 10Manometer and pressure differences

Manometer

Manometer a U-tube containing liquid, used to measure the pressure of a gas supply or the difference between two pressures.

How it works

How it works one side is connected to the gas and the other is open to the air.
The liquid is pushed down on the gas side and rises on the other side until the pressures balance.

Equation

Equation the gas pressure above atmospheric pressure equals ρgh, where h is the difference in the heights of the liquid levels.

Using the manometer

Using the manometer follow the steps.

Connect one arm to the gas supply and leave the other open to the atmosphere.

Measure the difference in heights of the liquid surfaces.

Calculate Δp = ρgΔh.

Add atmospheric pressure to find the absolute gas pressure.

Same level rule
- points at the same height in the same continuous liquid have the same pressure.

Part 6 of 10Hydraulic machines

Hydraulic machines

Hydraulic system

Hydraulic system a closed system of liquid in connected cylinders that transmits pressure.
The same pressure acts everywhere in the liquid because it is incompressible.

Principle

Principle a small force on a small piston creates a pressure, p = F₁ ÷ A₁, which gives a large force on the large piston, F₂ = p × A₂.
So F₁ ÷ A₁ = F₂ ÷ A₂.

Force multiplier

Force multiplier the large piston has a larger force but moves a shorter distance.
The work done on both pistons is equal, so energy is conserved.

Applications

Applications car brakes, hydraulic jacks, lifts and excavators.

Brake fluid

Brake fluid a liquid is used rather than a gas because it does not compress, so the force is transmitted immediately.
Air bubbles in the pipes make brakes spongy because the air compresses.

Part 7 of 10Solving pressure problems

Step-by-step method
- follow the same pattern for every pressure question.

Write the known quantities with units.

Identify whether the pressure is due to force on an area or liquid depth.

Convert areas to m² and depths to metres.

Substitute into p = F ÷ A or p = ρgh.

Give the answer in Pa or kPa.

Link to density

Link to density liquid pressure questions usually need a density from the table: water 1000 kg/m³, mercury 13 600 kg/m³.

Link to forces

Link to forces once you know the pressure and area, the force on a surface is F = pA.
A submarine window of area 0.20 m² at 5.0 × 10⁵ Pa carries a force of 1.0 × 10⁵ N.

Part 8 of 10Direction of pressure and a demonstration

1Pressure acts at right angles

Pressure acts at right angles the pressure at a surface produces a force at 90° to that surface.
A liquid pushes outward on the walls of its container perpendicular to the wall at each point.

2Experiment to show it

Experiment to show it a tall can of water with small holes at different depths.

Make three holes in a vertical line down the side of a plastic bottle and cover them with tape.

Fill the bottle with water and keep it topped up.

Remove the tape and watch the water jets.

Each jet leaves at right angles to the wall, and the lowest jet goes farthest because the pressure there is greatest.

Second demonstration

Second demonstration a thin rubber sheet over a funnel bulges outward in every direction when the funnel is lowered into water.
The bulge is greater at greater depth.

Everyday link

Everyday link water towers are built high so that the pressure in the pipes at the bottom is large enough to push water to taps.

Part 9 of 10Reading a barometer

Method
- follow the steps to find the atmospheric pressure from a mercury barometer.

Fill a long glass tube with mercury and invert it in a dish of mercury.

The mercury falls until its column is held up by the atmosphere, and a vacuum forms at the top.

Measure the vertical height h of the column above the mercury surface in the dish.

Atmospheric pressure = ρgh, or the height of the column in mm of mercury.

-- A column of 760 mm of mercury gives 13 600 × 9.8 × 0.760 = 1.01 × 10⁵ Pa.

Tilting the tube
- it does not change the vertical height, so the pressure reading stays the same.

Part 10 of 10Worked examples

A 70 kg man stands on both feet, each with an area of 0.020 m². Find the pressure he exerts on the ground (g = 9.8 N/kg). [3]3 marks
Calculate the pressure at a depth of 25 m in the sea of density 1030 kg/m³, and the total pressure if atmospheric pressure is 1.0 × 10⁵ Pa (g = 9.8 N/kg). [4]4 marks
In a hydraulic jack the small piston has an area of 4.0 cm² and the large piston 100 cm². A force of 50 N is applied to the small piston. Find the force on the large piston. [3]3 marks
The mercury levels in a manometer connected to a gas supply differ by 80 mm. Find the gas pressure above atmospheric (ρ = 13 600 kg/m³, g = 9.8 N/kg). [3]3 marks
Explain why a camel can walk on sand but a person with narrow shoes sinks in. [2]2 marks
A mercury barometer column is 745 mm high. Find the atmospheric pressure in pascals (ρ = 13 600 kg/m³, g = 9.8 N/kg). [3]3 marks

Key terms

Term	Meaning
Pressure	The force acting normally per unit area, p = F ÷ A, in pascals.
Pascal	The unit of pressure, 1 N/m².
Atmospheric pressure	The pressure due to the weight of the air above, about 1.0 × 10⁵ Pa at sea level.
Barometer	An instrument that measures atmospheric pressure.
Manometer	A U-tube liquid gauge used to measure pressure differences.
Hydraulic system	A liquid-filled system that transmits pressure to multiply a force.
Incompressible	Cannot easily be squashed into a smaller volume, as with liquids.

Avoid these mistakes

Using cm² directly with N to get Pa — convert area to m² first.

Saying liquid pressure depends on the volume of liquid — it depends on depth, density and g only.

Forgetting atmospheric pressure when asked for the total pressure at depth — add it to ρgh.

Thinking hydraulic systems give free energy — the larger force moves a proportionally smaller distance.

Measuring manometer height along the tube — use the vertical difference between levels.

Last updated: 20 October 2026

Part 1 of 10Pressure and its equation

Part 2 of 10Changing pressure in everyday life

Part 3 of 10Pressure in liquids

Part 4 of 10Atmospheric pressure and the barometer

Part 5 of 10Manometer and pressure differences

Part 6 of 10Hydraulic machines

Part 7 of 10Solving pressure problems

Part 8 of 10Direction of pressure and a demonstration

Part 9 of 10Reading a barometer

Part 10 of 10Worked examples

A 70 kg man stands on both feet, each with an area of 0.020 m2. Find the pressure he exerts on the ground (g = 9.8 N/kg). [3]3 marks

Calculate the pressure at a depth of 25 m in the sea of density 1030 kg/m3, and the total pressure if atmospheric pressure is 1.0 × 105 Pa (g = 9.8 N/kg). [4]4 marks

In a hydraulic jack the small piston has an area of 4.0 cm2 and the large piston 100 cm2. A force of 50 N is applied to the small piston. Find the force on the large piston. [3]3 marks

The mercury levels in a manometer connected to a gas supply differ by 80 mm. Find the gas pressure above atmospheric (ρ = 13 600 kg/m3, g = 9.8 N/kg). [3]3 marks

Explain why a camel can walk on sand but a person with narrow shoes sinks in. [2]2 marks

A mercury barometer column is 745 mm high. Find the atmospheric pressure in pascals (ρ = 13 600 kg/m3, g = 9.8 N/kg). [3]3 marks

Key terms

Avoid these mistakes

A 70 kg man stands on both feet, each with an area of 0.020 m². Find the pressure he exerts on the ground (g = 9.8 N/kg). [3]3 marks

Calculate the pressure at a depth of 25 m in the sea of density 1030 kg/m³, and the total pressure if atmospheric pressure is 1.0 × 10⁵ Pa (g = 9.8 N/kg). [4]4 marks

In a hydraulic jack the small piston has an area of 4.0 cm² and the large piston 100 cm². A force of 50 N is applied to the small piston. Find the force on the large piston. [3]3 marks

The mercury levels in a manometer connected to a gas supply differ by 80 mm. Find the gas pressure above atmospheric (ρ = 13 600 kg/m³, g = 9.8 N/kg). [3]3 marks

A mercury barometer column is 745 mm high. Find the atmospheric pressure in pascals (ρ = 13 600 kg/m³, g = 9.8 N/kg). [3]3 marks