Pressure in Fluids — AQA GCSE Physics (Separate)
Fluids (liquids and gases) exert pressure on surfaces and on objects within them.
Pressure in a fluid
A fluid is a liquid or a gas. The particles in a fluid collide with surfaces, creating a pressure that acts at right angles (normal) to any surface.
$$\text{pressure} = \frac{\text{force normal to a surface}}{\text{area of that surface}} \qquad P = \frac{F}{A}$$ Pressure is measured in pascals (Pa).
Pressure and depth in a liquid
The pressure in a liquid increases with depth, because there is a greater weight of liquid above. It also increases with the density of the liquid:
$$P = h \times \rho \times g$$
- h = depth (m), ρ = density (kg/m³), g = gravitational field strength (N/kg).
This is why a dam is built thicker at the bottom, where the water pressure is greatest.
Pressure acts in all directions
At a given depth, the pressure in a liquid acts equally in all directions.
Exam tips
- Pressure acts at right angles to a surface; P = F ÷ A (in pascals).
- Pressure in a liquid increases with depth and density: P = hρg.
- Pressure at a point acts in all directions.
- Link greater depth → greater pressure (e.g. dams, submarines).