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Fluids
Introduction
Physics Textbooks Boundless Physics Fluids Introduction
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Concept Version 8
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What is a Fluid?

A fluid is a substance that continually deforms (flows) under an applied shear stress.

Learning Objective

  • Explain the properties of substances under an applied shear stress


Key Points

    • Fluids are a subset of the states of matter and include liquids, gases, and plasma.
    • Fluids display properties such as: not resisting deformation or resisting it only lightly (viscosity), and the ability to flow (also described as the ability to take on the shape of the container).
    • Ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and thus are capable of being subjected to low levels of shear stress.

Terms

  • fluidity

    A measure of the extent to which something is fluid. The reciprocal of its viscosity.

  • viscosity

    A quantity expressing the magnitude of internal friction in a fluid, as measured by the force per unit area resisting uniform flow.

  • shear stress

    The component of stress that causes parallel layers of a material to move relative to each other in their own planes.


Full Text

A fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the states of matter and include three of the four states—liquids, gases, and plasma (shown in ).

Four Fundamental States of Matter

Four fundamental states of matter: 1) top left corner corresponds to solid; 2) top right corner corresponds to liquid; 3) bottom left corner corresponds to gas; 4) bottom right corner corresponds to plasma.

Liquids form a free surface (i.e., a surface not created by the container) while gases do not. The distinction between solids and fluid is not entirely obvious. The distinction is made by evaluating the viscosity of the substance. Silly Putty can be considered to behave like a solid or a fluid, depending on the time period over which it is observed. It is best described as a viscoelastic fluid.

Fluids display properties such as:

  • a) not resisting deformation or resisting it only lightly (viscosity), and
  • b) the ability to flow (also described as the ability to take on the shape of the container).

This also means that all fluids have the property of fluidity. These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and normal stresses—both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress (called pressure). Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

Although the term fluid includes both the liquid and gas phases, it is also commonly used as a synonym for liquid, with no implication that gas could also be present. For example, "brake fluid" is hydraulic oil and will not perform its required function if there is gas in it. This colloquial usage of the term is also common in the fields of medicine and nutrition (e.g., "take plenty of fluids").

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