Difference Between Kinematic and Dynamic Viscosity     Viscosity is defined as the elemental property while studying the flow of liquid for any application. The two basic types of viscosity are kinematic and dynamic. The association between these two properties is quite simple. It seems like a simple concept at first glance. But in reality, there are numerous terms that come under the definition of it. These terms determine the measurement of it.

Dynamic viscosity, which is also known as absolute viscosity, evaluates the internal resistance of a fluid to flow; in contrast, kinematic one describes the ratio of dynamic viscosity to density. Two fluids with the same value of dynamic thicknesses can have a different value of kinematic densities based on density and vice versa. However, to have a broader knowledge regarding the difference between kinematic and dynamic viscosity, students can follow the tabular representation of differences:

Difference Between Kinematic Viscosity and Dynamic Viscosity

 Kinematic Viscosity Dynamic Viscosity This is defined as the diffusivity of momentum. To be precise, it explains how fast the liquid is moving when a certain amount of external force is applied. This is defined as absolute viscosity. It gives more information about the force required to make the liquid flow at a specific rate. It represents the inertia as well as the viscous force of the fluid. Whereas, dynamic viscosity represents the viscous force of the liquid The symbol of the kinematic viscosity is V. The symbol of dynamic viscosity is μ. This represents the ratio between dynamic viscosities to density. This represents the ratio between shear stress to shear strain. It is utilized when inertia and viscous force are dominant. Dynamic force is utilized only when viscous force is dominant. Kinematic viscosity is a more fundamental property. Dynamic viscosity is a derived property. Unit of kinematic viscosity is m2/s Unit of Dynamic Viscosity is Ns/m2

Apart from the difference between dynamic viscosity and kinematic viscosity, a few relations of this concept should be cleared. The internal resistance of a liquid flow suggests an external force applied in the movement of a liquid. That external force (F) is proportional to Shear rate (SR), Dynamic Viscosity (η), and Surface area (A).

Now that students have collected some knowledge about viscosity and the difference between kinematic and dynamic viscosity, students must know about the different viscosity units.

CGS Unit of Different Viscosities

Sometimes students are asked about units of viscosities. Since there are several types of density and each has its unit, to differentiate between dynamic viscosity and kinematic viscosity in units, students can use Poise (P) as the CGS unit of dynamic density and Stokes (St) as the CGs unit of kinematic viscosity. Poise (P) is used explicitly in ASTM standards as centipoises (cP). The unit centistokes (cST) has its applications in various fields.

After knowing units of densities, it is essential to learn how to calculate densities. Below explained the symbols and terms used to calculate viscosity.

Calculation of Viscosity

The density of a liquid is estimated based on a ratio of shearing stress to its velocity gradient. If we rest a sphere, into a liquid, we can evaluate the density by using the formula mentioned below:

Note: Shearing stress- If a direction of external force on an object is parallel to an object's plane, deformation will be along the plane and pressure felt on the object is considered shear stress.

• η = 2ga2(Δρ)/9v

η= viscosity

Δρ= difference of density of the fluid and tested sphere

a = radius of a sphere

v = velocity of sphere

Viscosity is measured in Pascal seconds, i.e. Pa s. Moreover, the velocity of the spheres increases with the density of a fluid. However, temperature increases with the decreasing density of a liquid.

Apart from the difference between kinematic and dynamic viscosity, students can get a precise idea about the definition of viscosity and how the concept of density differs from the kinematic density of a liquid, students can follow the table below:

Difference Between Viscosity and Kinematic Viscosity

 Viscosity Kinematic viscosity The theory of viscosity indicates a struggle against a flowing which is being misshapen due to some external shear force applied to it. Kinematic viscosity is a measurement of a fluid’s internal resistance to go along under a gravitational force. The formula of Viscosity: F= µA u/yF- Force, A- Area of each plate, u/y- a rate of shear deformation, µ- viscosity of the fluid. The formula of kinematic viscosity: v=µ/ρWhere, µ- dynamic or absolute viscosity, ρ- density SI unit of viscosity is (Pa·s) or kg·m⁻¹·s⁻¹. SI unit of kinematic viscosity is m²/s. Observing viscosity is important to oil analysis. The kinematic viscosity of an oil is explained according to  its resistance to flow and shear force under gravity

The definition and difference between kinematic and dynamic viscosity have been discussed above in length. To know more about the same topic, and others from physics, one can visit Vedantu’s official website. Students can also register for online classes where our expert teachers and professionals will guide them to solve sample papers for examination.

1. What is the Relationship Between Kinematic and Dynamic Viscosity?

Ans. Fluid density includes itself as a part of measurement in kinematic viscosity. However, dynamic viscosity is the measurement of force, whereas kinematic density is the measurement of velocity. Here comes the difference. If the kinematic thickness is divided by the fluid density, you will get absolute viscosity. Kinematic viscosity: v (v= ηu) is the dynamic viscosity of the medium η, and it is divided by the density ρ. Hence, the equation is v= η/p.

2. What are the Factors Affecting Viscosity?

Ans. The factors affecting Viscosity include colloid systems where the lyophilic colloid solution has mostly high viscosity. Also, in the case of chemical composition, the density of liquids generally depends on molecular size, shape, and chemical nature— density of a fluid increases with smaller molecules than with larger; also with elongated molecules than spherical ones—generally, large quantities of liquefied solid increase a velocity. Moreover, suspended particles increase the density of a liquid. Temperature is considered one of the primary factors that influence a fluid's thickness

3. What Do You Understand By Temperature-compensated Viscosity?

Ans. Temperature-compensated viscosity (TCV) is a precise evaluation of a fluid's density at a reference temperature that is different from the process temperature. The mathematical association is based on the ASTM and standard D341 and is also appropriate for liquid hydrocarbons and other fluids. TCV is used to express process measurements to standard laboratory values and lessen process temperature fluctuations for tighter viscosity control.

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