A diver descends from the surface of the sea to a depth of 30 m. Explain why the pressure acting on the diver increases as the diver descends.

Pearson Edexcel International GCSE Physics (4PH1) — 5.1 Density and pressure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Seawater has a density of approximately 1025 kg/m³, which is greater than that of fresh water.

Model answer (4 marks)

Pressure in a liquid increases with depth because the weight of the liquid above the diver adds to the pressure.

At 30 m depth the diver is supported by a column of seawater that is 30 m high. The weight of this column is greater than that of the column at the surface, so the pressure is higher.

Pressure also depends on the density of the liquid; seawater (ρ≈1025 kg m⁻³) is denser than fresh water, so the pressure at a given depth is larger.

The pressure acts equally in all directions and acts perpendicular to any surface of the diver.

Examiner tips

  • Use the formula p=ρgh to show the depth dependence; mention ρ for seawater.
  • Explain that the weight of the overlying water increases with depth.
  • State that pressure is isotropic – acts at right angles to all surfaces.

Common mistakes

  • Confusing pressure with force; forgetting to include density.
  • Saying pressure only depends on depth and ignoring density.
  • Claiming pressure acts only vertically, not in all directions.

Mark scheme (4 marks)

  1. Pressure in a liquid increases with depth
  2. There is a greater weight / more liquid above the diver at greater depth
  3. Pressure in a liquid also depends on the density of the liquid
  4. Pressure acts equally in all directions / acts at right angles to any surface

Key terms in this question

pressure · depth

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