Students should be able to calculate the differences in pressure at…

The idea

A pressure difference between two depths needs only the difference in height, because the density and the field strength are the same at both. A diver ten metres deeper than another is at a pressure about 101 kPa greater in sea water, which is roughly one extra atmosphere for every ten metres of depth. The same idea explains why a dam is built much thicker at its base than at the top, why a water tank in a roof space gives good pressure at a ground-floor tap, and why the lowest hole in a punctured bottle squirts furthest. If a question asks for the absolute pressure at a depth rather than the difference, add atmospheric pressure, normally taken as about 100 kPa, to the pressure due to the liquid.

What earns the marks

• Using the difference in depth between the two points.
• A correct substitution into pressure = height × density × gravitational field strength.
• The pressure difference in pascals or kilopascals.
• Adding atmospheric pressure where the total pressure is asked for.

Common mistakes and how to put them right

• Using the deeper depth alone when the question asks for a difference.

• Adding atmospheric pressure to a difference, where it cancels out.

• Converting kilopascals into pascals the wrong way round.

• Assuming the pressure difference depends on the shape of the container.

Exam tips

• Underline whether the question wants the pressure at a depth or the difference between two depths.
• Use one hundred kilopascals for every ten metres of water as a quick check on the answer.