Fluids: Liquids and Gases


In the everyday world the word fluid is used only for liquids – they flow, slosh and assume a level surface when undisturbed. In the world of physics, gases are also considered fluids because they also can move about, changing their shape in response to external forces. Gases are even more fluid than liquids because they don’t require a surface to move across, they float in air. A big difference between gases and liquids is that liquids are incompressible – they can’t be squeezed into a smaller volume; obviously gases can!


Fluids differ from solids in that their molecules are only loosely bonded so that fluids can assume the shapes of their containers. Many materials exist only as a solid (for example, wood) but some fluids can be a solid, liquid or a gas, depending on their temperature. Water is the most familiar example, being solid ice when the temperature is colder than 0°C, a gas when it is hotter than 100°C, and a liquid in between.


Fluids are obviously important. We live in one – the atmosphere – and are dominantly made of another - water.


Blaise Pascal (1623-1662)

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