physics_mechanical_properties_fluids
 Mechanical Properties of Fluids FluidAny substance that can flow is a fluid. A fluid is a substance that deforms continually under the action of an external force. A fluid flows under the action of a force or a pressure gradient. Properties of ideal fluid1. Incompressible2. Density is constant.2. Flow is irrotational and smooth4. No turbulences in the flow.5. It is nonviscous i.e. there is no internal friction in the flow, i.e., the fluid has no viscosity.6. Its flow is steady: its velocity at each point is constant in time.7. Fluid do not oppose deformation, they get permanently deformed.8. They have ability to flow.9. They have ability to take the shape of the container.10. Fluid cannot oppose a shear stress when in static equilibrium.11. Ideal fluids, can only be subjected to normal, compressive stress (called pressure). HydrostaticsThe branch of physics which deals with the properties of fluids at rest is called hydrostatics. PressureP=πgh$P=\mathrm{\pi }gh$ Absolute Pressure It is total pressure at depth h below the surface of liquid which includes pressure due to atomosphere and liquid. Gauge PressureThe difference between the absolute pressure and the atmospheric pressure is called the gauge pressure. Pascalβs LawPascalβs law states that the pressure applied at any point of an enclosed fluid at rest is transmitted equally and undiminished to every point of the fluid and also on the walls of the container. Application of Pascal Law1. Hydraulic lift:2. Hydraulic brakes Measurement of Pressure1. Mercury Barometer2. Open tube manometer Surface Tension1. Extra energy is associated with surface of liquids.2. Liquids have no definite shape but have a definite volume. Liquid acquire a free surface when poured in a container. These surfaces possess some additional energy. This phenomenon is known as surface tension.3. Surface tension T is defined as, the tangential force acting per unit length on both sides of an imaginary line drawn on the free surface of liquid.4. Surface tension is also equal to the surface energy per unit area. Cohesive forces result in the phenomenon of surface tension. Molecular Theory of Surface Tension1. Intermolecular forceIt is force between molecules. 2. Range of molecular forceThe maximum distance from a molecule up to which the molecular force is effective is called the range of molecular force. 3. Sphere of influenceAn imaginary sphere with a molecule at its center and radius equal to the molecular range is called the sphere of influence of the molecule. 4. Surface filmThe surface layer of a liquid with thickness equal to the range of intermolecular force is called the surface film. 5. Free surface of a liquidIt is the surface of a fluid which does not experience any shear stress. For example, the interface between liquid water and the air above. 6. Surface tension on the basis of molecular theory Intermolecular force Cohesive forceThe force of attraction between the molecules of the same substance is called cohesive force. Adhesive forceThe force of attraction between the molecules of different substances is called adhesive force. The force due to surface tension acts tangential to the free surface of a liquid. Pressure inside a spherical drop is more than outside. Pressure on the concave side is always greater than that on the convex side Angle of ContactWhen a liquid surface comes in contact with a solid surface, it forms a meniscus, which can be either convex (mercury-glass) or concave (water glass). Shape of meniscusi) Concave meniscus - acute angle of contact.ii) Convex meniscus - obtuse angle of contact. Conditions for concavityAdhesive force >Cohesive Force2$Adhesive\phantom{\rule{0.22em}{0ex}}force\phantom{\rule{0.22em}{0ex}}>\frac{Cohesive\phantom{\rule{0.22em}{0ex}}Force}{\sqrt{2}}$ Conditions for convexityAdhesive force