Why is the fluid in the hydraulic machine a liquid
than a gas?
Fluids are incompressible, relatively speaking, hence the pressure
applied through the system is directly transmitted to the object which
you want to lift. If you used a gas, you would waste a lot of energy
compression the gas-hence the system would be inefficient.
Gases are compressible; liquids generally are not. Compressible fluids
store energy. A hydraulic machine using a compressible working fluid
would be dangerous and also difficult to manage. Try attaching your
hands to a lawnmower with springs and cutting the grass that way.
Gases are more compressible than liquids. When you exert pressure on a
gas, it can just scrunch down to occupy less volume. Liquids are not so
compressible, so they can more reliably convert the motion of a piston at
one end of the apparatus into motion of another piston at the other end.
Compressed gases can be used to drive machinery; lots of tools are driven
by compressed air. They tend to involve letting the air escape at the
businesss end, while hydraulic apparatus heep the fluid in a sealed
Richard E. Barrans Jr., Ph. D.
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Update: June 2012