Water pressure increases with depth because each lower layer must support the weight of the water above it. A useful rule of thumb is that pressure rises by about 1 atmosphere for every 10 meters of seawater depth. At 100 meters, the total pressure is about 11 atmospheres because the ocean adds about 10 atmospheres on top of the 1 atmosphere from the air.
This increasing pressure affects submarine design, human diving safety, and the survival of deep sea organisms.
Key Facts
- Water pressure increases with depth because deeper water supports the weight of more water above it.
- Pressure in a fluid at rest is given by P = P0 + rho g h.
- In seawater, pressure increases by about 1 atm every 10 m of depth.
- 1 atm = 101,325 Pa, which is the approximate air pressure at sea level.
- At 30 m depth, the total pressure is about 4 atm: 1 atm from air plus 3 atm from water.
- Pressure acts in all directions on a submerged object, not just downward.
Vocabulary
- Pressure
- Pressure is force spread over an area, calculated as P = F/A.
- Hydrostatic pressure
- Hydrostatic pressure is the pressure caused by a fluid at rest due to the weight of the fluid above a point.
- Atmosphere
- An atmosphere is a unit of pressure equal to the average air pressure at sea level.
- Pressure hull
- A pressure hull is the strong sealed structure of a submarine designed to resist crushing by outside water pressure.
- Depth
- Depth is the vertical distance below the water surface.
Common Mistakes to Avoid
- Forgetting surface air pressure: total pressure underwater includes 1 atm from the atmosphere plus the pressure added by the water.
- Thinking pressure only pushes downward: in a fluid, pressure acts in all directions, so a submarine is squeezed from the top, bottom, and sides.
- Using 1 atm per 10 meters as an exact law: it is a useful rule of thumb for seawater, but accurate calculations use P = P0 + rho g h.
- Confusing pressure with buoyant force: pressure increases with depth, while buoyant force depends on the weight of displaced water.
Practice Questions
- 1 A submarine is 50 m below the ocean surface. Using the rule of thumb, what is the approximate total pressure in atmospheres?
- 2 A diver is at 20 m depth. If 1 atm equals 101,325 Pa, estimate the total pressure in pascals using the 1 atm per 10 m rule.
- 3 Explain why a surface ship does not need the same kind of pressure hull as a submarine, even though both are in contact with seawater.