Saturday, 23 September 2017

Viscosity With Exams Based MCQ IMP for GATE , SSC JE , PSU

Viscosity : It is the property of the fluid  which opposes the relative motion between the two surfaces of the fluid in a fluid that are moving a different velocity.

Viscosity can be two types:

1: Dynamic Viscosity Or Absolute Viscosity Or Coefficient of Viscosity or Viscosity

2: Kinematic Viscosity ( This is imaginary not really present in real life)

The main difference between Dynamic viscosity and Kinematic Viscosity is:

Dynamic Viscosity: One way is to measure a fluid's resistance to flow when an external force is applied , This is Dynamic viscosity , always it is used for practical purpose.

Units: SI : N.S/ m2

C.G.S Unit: Dyne-Sec/ Cm2 ( It is also called Poise)

M.K.S. Unit: Kgf-Sec/ m2

Kinematic Viscosity: The other way is to measure the resistive flow of a fluid under the weight of gravity , the result is kinematic vicosity . Do not use it by your own, until  there is no mention of particulate word (Kinematic Viscosity) in numerical .

Units:  SI : m2/s

CGS unit: cm2/sec ( It is called stoke )

M.K.S =m2/Sec

IMP Point:-

Two fluid that have the same Dynamic viscosity can have different kinematic viscosity  This is due to kinematic viscosity dependent on the density of the fluid , density is not a factor with Dynamic viscosity .

Some Other Key Points regarding Viscosity:

1: The dynamic viscosity of pure water at 20 degree Celsius is 0.01 poise or 1 centi poise

2: The viscosity of fluid decreases with increasing the temperature ( Because fluid has a tendency to flow in stream line in most of the cases , when velocity of flow is low)

3: The viscosity of gases increases with increasing the temperature ( REASON:  when we heat the gases then gas molecules travel in zig zag motion, so the momentum of gases particles increases and they generate high shear stress due to high momentum)

4: Water has higher viscosity than Mercury.

 Now test youself  : MCQ :  based on Viscosity:

Q1: Poise is the C.G.S unit of:
A: Viscosity
B: Kinematic Viscosity
C: Surface Tension
D: Capillary

 Q2: C.G.S unit of Kinematic Viscosity is:
A: Newton-Sec
B: Stoke 
C: Dyne
D: Poise

Q3: The increase of the temperature :
A: Increase the viscosity of liquid
B Increase the viscosity of gases 
C: Decrease the viscosity of gases
D: No effect

 Q4: Newton law of viscosity states that :

A: shear stress is directly proportional to the velocity
B: Shear stress is directly proportional to velocity
C: shear stress is directly proportional to rate of shear strain
D: Shear stress is directly proportional to the viscosity

Q5: Which has higher viscosity : A : Mercury, B: Water  

Q6:  What measures a material's resistance to flow?

A: Matter , B: Volume C: Viscosity, D: Surface Tension
Q7: When friction increases what else increases?                                                                                        A: Elasticity , B: Density , C: Viscosity, D: Matter



 BEST BOOK FOR FLUID MECHANICS BY R.K. Bansal : Basic and conceptual :

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