14.1 : 流体の特性

Fluids differ from solids mainly in their molecular structure and stress response. Solids have tightly packed molecules with strong intermolecular forces, maintaining their shape and resisting deformation.

In contrast, fluids have molecules that are farther apart with weaker forces, allowing them to flow easily.

A fluid, which includes both liquids and gases, is a substance that deforms continuously under shearing stress.

For example, water and oil are liquids with molecules that can move freely, making them easy to pour but hard to compress.

Gases like air and oxygen have even more spaced-out molecules, making them highly compressible and able to fill any container.

Imagine pouring water from a glass; it flows smoothly, demonstrating its fluid nature. Similarly, air flows effortlessly around a moving car, reducing resistance due to its streamlined shape.

Other fluid examples include honey, which is thicker but still flows, and mercury, a dense fluid used in thermometers. Gases, such as helium and carbon dioxide, flow freely and easily, showing clear fluid behavior.

When studying fluids, their macroscopic behavior is understood by averaging properties over a small volume containing many molecules.