Properties Of Fluids And Fluids In Relative Equilibrium

PROPERTIES OF FLUID

WHAT IS FLUID?

§  Fluids is a substance that presents in form as a liquid or gas where that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape.

§  When it are at rest, the study of them called it fluid static, but if it are in motion the study of them is called fluid dynamics.

§  The science and technology of mechanical properties of liquids is called hydraulics & in properties of air and other gases is called pneumatics.

THERMODYNAMIC PROPERTIES OF A FLUID

        i.            Density

o   The density of fluid can be defined as mass per unit volume, where:

o   The density of a fluid changes with temperature and pressure.

o   Liquids are essentially incompressible.

o   Density is highly variable in gases nearly proportional to the pressure.

      ii.            Specific weight

o   Fluid Density is the mass per unit volume and is denoted by the Greek letter ρ (rho).

o   The dimensions of density are mass per length cubed or M / L³.

    iii.            Specific gravity

o   Specific gravity is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance and typically water at 4 oC (for liquids) and air (for gases).

    iv.            Viscosity

o   The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.

o   For liquids, it corresponds to the informal concept of "thickness" as example, honey has a much higher viscosity than water.

o   Viscosity is a property arising from collisions between neighbouring particles in a fluid that are moving at different velocities.

o   When the fluid is forced through a tube, the particles which comprise the fluid generally move more quickly near the tube's axis and more slowly near its walls: therefore some stress, (such as a pressure difference between the two ends of the tube), is needed to overcome the friction between particle layers to keep the fluid moving. For the same velocity pattern, the stress required is proportional to the fluid's viscosity.

      v.            Surface tension

o   Surface tension is the elastic tendency of liquids which makes them acquire the least surface area possible. 

o   The cohesive force between liquid molecules are responsible for the phenomenon known as a surface tension

o   It has the dimension of force per unit length (N/m) or of energy per unit area (J/m2).

o   Using a force balance , pressure increase in the interior of a liquid half-cylinder droplet of length L and radius R is :

o   Contact angle appears when a liquid interface intersects with a solid surface.

o   Water is extremely wetting to a clean glass surface with, θ≈0. For a clean mercury-air-glass interface, θ≈130o.