Fluid
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2008-06-27T19:21:20Z
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{{Continuum mechanics}}
A '''fluid''' is defined as a substance that continually deforms (flows) under an applied [[shear stress]] regardless of how small the applied stress. All liquids and all gases are fluids. Fluids are a subset of the [[Phase (matter)|phases of matter]] and include [[liquid]]s, [[gas|gases]], [[Plasma physics|plasmas]] and, to some extent, [[plasticity (physics)|plastic solids]].
The term "fluid" is often used as being synonymous with "liquid". This can be erroneous and sometimes clearly inappropriate - such as when referring to a liquid which does not or should not involve the gaseous state. "Brake fluid" is hydraulic oil which will not perform its required function if gas is present. The medical profession relies on the term "fluids" in dietary references ("take plenty of fluids") where the presence of gases is irrelevant or even possibly dangerous.
Liquids form a free surface (that is, a surface not created by the container) while gases do not. The distinction between [[plasticity (physics)|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. However [[Silly Putty]] is correctly termed a [[viscoelastic]] fluid.
Fluids display such properties as:<br />
* not resisting deformation, or resisting it only lightly ([[viscosity]]), and<br />
* the ability to flow (also described as the ability to take on the shape of the container).<br />
These properties are typically a function of their inability to support a [[shear stress]] in static [[Mechanical equilibrium|equilibrium]].
Solids can be subjected to shear stresses, and to normal stresses - both [[Compressive stress|compressive]] and [[Tensile stress|tensile]]. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called [[pressure]]. Real fluids display [[viscosity]] and so are capable of being subjected to low levels of [[shear stress]].
In a solid, shear stress is a function of [[Strain (materials science)|strain]], but in a fluid, shear stress is a function of rate of strain. A consequence of this behavior is [[Pascal's law]] which describes the role of [[pressure]] in characterizing a fluid's state.
Depending on the relationship between [[stress (physics)|shear stress]], and the rate of [[Strain (materials science)|strain]] and its [[derivative]]s, fluids can be characterized as:
*[[Newtonian fluid]]s : where stress is directly proportional to rate of strain, and
*[[Non-Newtonian fluid]]s : where stress is proportional to rate of strain, its higher powers and derivatives.
The behavior of fluids can be described by the [[Navier-Stokes equations]] - a set of [[partial differential equations]] which are based on:<br />
* continuity ([[continuity equation#fluid dynamics|conservation of mass]]),
* conservation of [[linear momentum]]
* conservation of [[angular momentum]]
*[[conservation of energy]].
The study of fluids is [[fluid mechanics]], which is subdivided into [[fluid dynamics]] and [[fluid statics]] depending on whether the fluid is in motion.
==External links==
*[http://hyperphysics.phy-astr.gsu.edu/hbase/fluid.html Fluid mechanics Concept Map from Georgia State University]
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