Newton's Law of Viscosity

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. The constant of proportionality is called the co-efficient of viscosity. Mathematically, it is expressed as given by equation 

       
                               τ  =  μ (du/dy)

                Fluid which obey the above relation are known as Newtonian fluids and the fluids which do not obey the above relation are called Non-Newtonian fluids.

 VARIATION OF VISCOSITY WITH TEMPERATURE. 

                                Temperature affects the viscosity. The viscosity of liquids decreases with the increase of temperature while the viscosity of gases increases with the increase of temperature. This is due to reason that the viscous forces in a fluid are due to cohesive forces and molecular momentum transfer. In liquids, the cohesive forces predominates the molecular momentum transfer, due to closely packed molecules and with the increase in temperature, the cohesive decreases with the result of decreasing viscosity. But in case of gases the cohesive forces are small and molecular momentum transfer predominates. With the increase in temperature, molecular momentum transfer increases and hence viscosity increases.

TYPES OF FLUIDS. 

                              The fluid may be classified into the following five types :

1. Ideal fluid, 

2. Real fluid, 

3. Newtonian fluid

4. Non-Newtonian fluid, and 

5. Ideal plastic fluid. 

1.  IDEAL FLUID.  

                       A fluid, which is incompressible and is having no viscosity, is known as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluids, which exists, have some viscosity. 

2.  REAL FLUID. 

                    A fluid, which possesses, is known as real fluid. All the fluids, in actual practice, are real fluids.

3. NEWTONIAN FLUID. 

                   A real fluid, in which the shear stress is directly proportional to the rate of shear strain (or velocity gradient) , is known as a Newtonian fluid. 

TYPES OF FLUID

4.  NON-NEWTONIAN FLUID. 

                  A real fluid, in which the shear stress is not proportional to the shear strain (or velocity gradient) , known as a Non-Newtonian fluid. 

5.  IDEAL PLASTIC FLUID. 

                   A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain ( or velocity gradient) , is known a ideal plastic fluid. 

                                     The End. 

    Please comment if you have any query. 

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