IGCSE Physics / Density and pressure
Pressure in liquids and gases
Edexcel International GCSE Physics (4PH1), chapter 18: Density and pressure, specification point 5.6, 5.7. Free lesson.
Read the revision notes · IGCSE Physics past papers
Pressure in a fluid at rest acts equally in all directions. Increases in depth or fluid density increase the pressure, which you can calculate using the pressure difference equation.
How pressure acts in a fluid
Liquids and gases are fluids because they can flow. At any point in a fluid at rest, pressure acts equally in all directions, not just downwards. This explains why water pushes sideways on a dam and upwards on the bottom of a boat. Pressure in a fluid acts upwards, downwards and sideways.
How depth and density affect pressure
The deeper you go in a liquid, the more liquid there is above you. Its weight creates greater pressure. A denser liquid also creates more pressure at the same depth. For water, each metre of depth adds 10 kPa of pressure. Oil is less dense, so each metre adds 8 kPa. At 5 m below the surface, the pressure difference from water is 50 kPa. Pressure from a liquid increases in a straight line as depth increases. At the same depth, a denser liquid gives a greater pressure difference.
How to calculate pressure difference
Use p = h × ρ × g to calculate the extra pressure caused by a fluid. The equation is on the equation list. It gives the pressure difference between two points, not the total pressure. To find total pressure, add atmospheric pressure. p is pressure difference, in pascals (Pa). h is the height difference, or depth below the surface in a liquid, in metres (m). ρ is the fluid density, in kilograms per cubic metre (kg/m³). g is gravitational field strength, in newtons per kilogram (N/kg). Use 10 N/kg here.
How pressure explains what you observe
The pressure differences in fluids explain several familiar effects. The same pattern applies to air: at the top of a mountain, there is less air above you and it is less dense, so the air pressure is lower. Water spurts furthest from the lowest hole in a bottle because that hole is deepest and has the greatest pressure. A dam wall is thicker at the bottom because water pressure is greatest there. A diver’s ears hurt more at greater depths because there is more water above her. Air pressure is lower at the top of a mountain because there is less air above and it is less dense.
Why container shape does not matter
At the same depth in the same liquid, the pressure difference is the same whatever the shape or width of the container. The equation depends on depth, density and g. It contains no area or volume. A narrow tank does not have less pressure at the bottom than a wide tank if both contain the same liquid to the same depth. If a diver moves from 5 m to 10 m deep, the pressure from the water increases because there is more water above her. Doubling the depth doubles the pressure from the water.