properties of fluid

Properties of fluid (part 3)

Viscosity- Resistance to flow of fluid is called viscosity.

Reason for viscosity- 1) Cohesive force. 2) Intermolecular momentum transfer.

If we plot a graph between perpendicular distance to solid boundary and velocity at that distance, this curve is called velocity profile (or) velocity distribution.

NEWTON’S LAW OF VISCOSITY- Newton’s law of viscosity states that shear stress is directly proportional to rate of shear strain or velocity gradient.

The equation of newton’s law of viscosity is τ = μ du/dy

Where τ= shear stress, μ= dynamic viscosity, and du/dy= velocity gradient.

Note: SI unit of dynamic viscosity is Pascal second.

Properties of fluid (part 2)

1) Surface Tension- Free surface of a fluid behaves like a stretched membrane and tries to minimise its area upto maximum possible extent. This property is known as surface tension.

Note: 1)Free surface is a surface with constant normal stress and zero shear stress. 2) Normal stress is always equal to the pressure and zero shear stress means no external force in tangential direction.

Mathematically, Surface Tension= Force (F)/ Length (l)

2) Adhesive and Cohesive force- Adhesive force is the attraction force between the unlike molecules. Cohesive force is the attraction force between the molecules of same substance.

Note: 1) If adhesive force>> cohesive force, liquid will wet the solid boundary and the liquid is called wetting liquid. Eg: water glass interface 2) If cohesive force >> adhesive force, liquid is called non-wetting liquid. Eg: Glass mercury interface.

Property of fluid (Part 1)

1) Density or Mass density (ρ) – mass per unit volume is called density.

ρ= mass/volume

If density increases, so mass increases , so structure is compact or dense medium. If density decreases mass decreases.

SI unit of density is kg/m³

Note: Density of water is maximum at 4°C. Density of water at 4°C is 1000 kg/m³

2) Weight density– It is called weight per unit volume.

Weight density= Weight(W)/Volume(V) = N/m³

Weight density= ρ*g where ρ is mass density and g is gravitational acceleration.

3) Specific Gravity or Relative Density- Specific Gravity= Density of fluid / Density of standard fluid at STP

For liquid- Specific Gravity= Density of liquid/ Density of water at 4°C

For gas- Specific Gravity= Density of gas/ Density of air at STP

Note: 1) Density of water at 4°C is 1000 kg/m³ 2) Density of air at STP is 1.21 kg/m³