Pressure in Liquids Why do your ears ‘pop’ when you dive to the bottom of a swimming pool?
This is because of the pressure the liquid puts on your eardrums. The size of this pressure depends on how deep you are in the water. Pressure in liquids increases with depth. This means that the bottom of something in a liquid will always feel more pressure than the sides because this point is the deepest.
Fig. 17.6.4 The polystyrene cup on the left was the same as the cup on the right before being taken underwater to a depth of around 3,000 metres by the deep-sea research vessel Alvin
The polystyrene cup that was brought to the bottom of the ocean in Fig. 17.6.4 has not been squashed flat. This is because liquids, at a given depth, exert pressures equally in all directions. They do this because they can flow.
Fig. 17.6.5
At a given depth, pressure is exerted on a diver equally in all directions
Testing pressure and depth
Using a plastic bottle of water, plan, design, carry out and write up a demonstration that shows pressure in liquids increases with depth.
Powering with liquids
Hydraulics use liquid pressure to work machinery. The apparatus in Fig. 17.6.6 shows how hydraulics work. Pressure is applied to the small piston in syringe A and the liquid between the two syringes pushes on the large piston B.
Using the Pressure = Force Area
Force
formula and your knowledge of liquids, predict which piston will experience the larger force.
1. Build this apparatus in class and test it for a range of forces. Remember, you will need to come up with a way of measuring:
a. The force applied to the pistons. b. The area of the pistons.
2. Record your results for a range of data in a clear way. 3. Explain why your predictions did or did not match your results.
298 Water
A Plastic tubing Small piston Large piston Fig. 17.6.6 Hydraulic apparatus
