Fluid, Characteristic of fluid, Ideal fluid, real fluid, viscosity, cause of viscosity, Newton’s law of viscosity, kinematic viscosity

Definition of

A fluid is a substance which deforms continuously
when subjected to external shearing forces.

of fluid

It has no definite shape of its own, but conforms to
the shape of the containing vessel.

2.  Even a small
amount of shear force exerted on a fluid will cause it to undergo a deformation
which continues as long as the force continues to be applied.

3.  It is interesting
to note that a solid suffers strain when subjected to shear forces whereas a
fluid suffers Rate of Strain i.e. it flows under similar circumstances.

Ideal and Real

Ideal Fluid

An ideal fluid
is one which has no viscosity, no surface tension and incompressible

2.  Real Fluid

An Real fluid is one which has viscosity, surface tension and compressible


Viscosity is the property
of a fluid which determines its resistance to shearing stresses.



Cause of

It is due to cohesion and molecular momentum exchange between fluid

Newton’s Law of Viscosity: 

It states that the shear stress (τ) on a fluid element layer is
directly proportional to the rate of shear strain. 

                 Velocity gradient = du/dy

                  According to Newton’s lawtm du/dy

Velocity Variation near a solid boundary

Variation near a solid boundary

Units of

S.I. Units: Pa.s or N.s/m

C.G.S Unit of viscosity is Poise= dune-sec/cm

1/100 Poise is called centi-poises.

viscosity of water at 200 degree centigrade is approx= 1 cP


It is the ratio between the dynamic viscosity and
density of fluid.

 kinematic viscosity= 

Units of
Kinematic Viscosity

S.I units: m

C.G.S units: stoke = cm

One stoke = 10-4 m

Thermal diffusivity and molecular diffusivity have same dimension,
therefore, by analogy, the kinematic viscosity is also referred to as the

Effect of
Temperature on Viscosity

With increase in

Temperature response are
neglected in case of Mercury.

The lowest viscosity is reached at the critical

Effect of
Pressure on Viscosity

Pressure has very little effect on viscosity.

But if pressure
increases intermolecular gap decreases then cohesion increases so viscosity
would be increase.

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