What Does Fluid Properties Mean
We explain what the properties of fluids are, the primary or thermodynamic and the secondary or specific behavior.
What are the properties of fluids?
Fluids are continuous material media made up of substances in which there is a weak attraction between their particles . For this reason, they change shape without forces occurring inside them that tend to restore their original configuration (as is the case with deformable solids ).
Another important property of fluids is viscosity , thanks to which they can be classified into:
- Newtonian or constant viscosity fluids.
- Non-Newtonian fluids, whose viscosity depends on their temperature and the shear stress applied to them.
- Perfect or superfluid fluids, which present an apparent absence of viscosity.
Recall that only the liquids and gases are considered fluids . Many times we speak of "ideal fluids" because they are easier to study and, although they do not exist in reality, they are an excellent approximation. Solids lack the elemental property of flow and therefore tend to retain their shape, since the attraction between their particles is much more intense.
See also: Properties of matter
Basic characteristics of fluids
Fluids have elementary physical characteristics that define and differentiate them from other forms of matter , such as:
- Infinite deformability . Its molecules follow unbounded movements and between all of them there is no equilibrium position.
- Compressibility . It is possible to compress fluids to a certain degree, that is, make them occupy a volume smaller than the given volume . Gases are more compressible than liquids.
- Viscosity . This is the name given to the internal tension of the fluid that opposes movement , that is, the resistance to movement offered by a fluid and which is much greater in liquids than in gases.
- Lack of shape memory . Fluids occupy the shape of the container that contains them, that is, if they are deformed, they do not return to their original configuration, therefore they completely lack elasticity .
Thermodynamic (or primary) properties
Also called primary properties, they are those that have to do with energy levels in fluids.
- Pressure . Measured in pascals in the International System (SI), pressure is the projection of the force that a fluid exerts perpendicular to a unit area. For example: atmospheric pressure or water pressureon the ocean floor.
- Density . It is a scalar quantity that is generally measured in kilograms per cubic meter or grams per cubic centimeter. It measures the amount of matter per given volume of a substance , regardless of size and mass .
- Temperature . It is related to the amount of internal energy of a thermodynamic system (a body, a fluid, etc.), and it is directly proportional to theaverage kinetic energy of its particles. Temperature can be measured by recording the heat that the system gives up to a thermometer .
- Enthalpy . Symbolized in physics with the letter H, it is defined as the amount of energy that a given thermodynamic system exchanges with its environment, either by losing or gaining heat through different mechanisms but at constant pressure.
- Entropy . Symbolized with the letter S, it consists of the degree of disorder of thermodynamic systems in equilibrium and describes the irreversibility of the processes they undergo. In an isolated system, entropy can never decrease: either it remains constant or it increases.
- Specific heat . It is the amount of heat that a unit of a substance requires to increase its temperature by one unit. Depending on the units used and the scales for measuring temperatures, the unit for specific heat can be cal / gr.ºC, or J / kg.K, for example. It is represented by the letter c.
- Specific weight . It is the ratio between the weight of a quantity of a substance and its volume, measured according to the International System in Newtons per cubic meter (N / m 3 ).
- Cohesion force . The particles of a substance are held together by various intermolecular (or cohesion) forces, which prevent each one from going away on its own. These forces are stronger in solids, less in liquids, and very weak in gases.
- Internal energy . It is the sum of the total kinetic energy of the particles that make up a substance, together with the potential energy associated with their interactions.
Specific (or secondary) behavioral properties
These properties, also called secondary, are typical of the physical mode of behavior of fluids:
- Viscosity . It is a measure of the fluid's resistance to deformations, tensile stresses and movement. Viscosity responds to the fact that the fluid particles do not all move at the same speed, which produces collisions between them that delay the movement.
- Thermal conductivity . It represents the heat transfer capacity offluids, that is, to transfer the kinetic energy of the particles to other adjacent particles with which it is in contact.
- Surface tension . It is the amount of energy necessary to increase the surface of a liquid per unit area, but it can be understood as the resistance that fluids, especially liquids, present when increasing their surface. This is what allows some insects to "walk" on water.
- Compressibility . It is the extent to which the volume of a fluid can be decreased by subjecting it to pressure or compression.
- Capillarity . Linked to the surface tension of fluids (and, therefore, their cohesion), it is the ability of a fluid to go up or down a capillary tube, that is, how much a liquid “wets”. This can be easily seen when we dip the tip of a dry napkin into a liquid and observe how high the liquid stain spreads on the paper against the force of gravity .
- Diffusion coefficient . It is the ease with which a specific solute moves in a given solvent, depending on the size of the solute, the viscosity of the solvent , the temperature of the mixture, and the nature of the substances.
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