Selecting hydraulic fluids
Physics Narrative for 11-14
Liquids are used as the fluid in hydraulic systems. In liquids the particles are close together and so liquids are virtually incompressible. We can therefore assume that the volume of liquid inside the hydraulic system is constant, even when the pistons start moving up and down the cylinders.
In some systems gases are used as the fluid and these are called pneumatic systems. Gases are of low density and cannot therefore be used directly because of their compressibility (the gas in the cylinder would simply compress under the action of the piston). This problem is avoided by using compressed air in pneumatic systems.
The air particles are squeezed closer together to reduce the volume whilst increasing the pressure (there is the same number of particles in a smaller volume, so there are likely to be more collisions with the walls of the container).