Pressure in a fluid 2 (HT only) — AQA GCSE Physics
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Pressure in a fluid 2 (HT only) explained
The pressure at a depth in a liquid comes from the weight of the liquid above, so it depends on how deep you are, on how dense the liquid is and on the gravitational field strength.
Read the full explanation
It does not depend on the width of the container or on the total volume: a narrow tube and a wide tank filled to the same depth give the same pressure at the bottom, because a wider column of liquid has more weight but spreads it over proportionally more area. Every value must be in base units, so a depth in centimetres becomes metres and a density in grams per cubic centimetre becomes kilograms per cubic metre. At 3.0 m down in water of density 1000 kg/m³, with g taken as 9.8 N/kg, the pressure due to the water is about 29 kPa, on top of atmospheric pressure.
Your focus
- Calculate the pressure due to a column of liquid from its depth and density.
- Explain why pressure in a liquid increases with depth and with the density of the liquid.
- Justify why the width of a container does not change the pressure at a given depth.
Pressure in a fluid 2 (HT only) exam tips
Marking Points
- one mark for the equation pressure = height of the column × density × gravitational field strength
- one mark for the height taken as the depth below the surface of the liquid
- one mark for density in kilograms per cubic metre and depth in metres
- one mark for the calculated pressure in pascals or kilopascals
Examiner Tips
- 💡Measure the height downwards from the surface of the liquid to the point in question.
- 💡For an explain question, write more liquid above, so greater weight, so greater pressure.
Common Mistakes
- using the height of the container rather than the depth below the liquid surface
- assuming a wider tank gives a greater pressure at the same depth
- leaving the density in grams per cubic centimetre
- measuring the depth upwards from the bottom of the container