How Everything Works
How Everything Works How Everything Works

Question 966

An architect friend tells our coffee group that liquids (water, in this case) are compressible to a slight extent. We tell him hydraulics would be impracticable under his thesis. We would appreciate comments or ammunition. — WAW, Brownsville, TX
I'm afraid that your friend is right—liquids are slightly compressible. A compressible material is one that experiences a decrease in volume when it's exposed to an increase in pressure. Gases are highly compressible—they change volume dramatically with changes in pressure. Liquids are said to be incompressible—they change volume very little with changes in pressure. But very little isn't zero. A liquid is essentially incompressible because its atoms and molecules are touching one another and, since those atoms and molecules have relatively fixed sizes, it's hard to pack them closer together than they already are. But increases in pressure do cause those atoms and molecules to move slightly closer together and the liquid does becomes denser and occupies less volume. The effect is small enough that it has almost no effect on most hydraulic systems—the pressurized fluid loses only parts per million of its volume as you squeeze it with normal pressures. All you really care about in a hydraulic system is that over the range of pressures used, the fluid involved doesn't change volumes much. Thus if you keep the pressure changes small enough, even air can be used in a hydraulic system. For example, pneumatic tube delivery systems are essentially air-operated hydraulic systems. But if the pressure changes are large enough, even liquids and solids can be highly compressible. In fact, plutonium-based nuclear weapons use high explosives to crush spheres of solid plutonium, already one of the densest materials in existence, to several times solid density. You wouldn't think of plutonium as compressible, but under these astronomical pressures it compresses almost like a gas.

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