Viscosity: Definition, Types Formula & their Examples

In this article, you will learn what viscosity is, how to measure viscosity and the formula for calculating fluid viscosity.

Viscosity: Definition, Types Formula & their Examples
Viscosity: Definition, Types Formula & their Examples

What is Viscosity?

Most fluids offer some resistance to movement. This resistance is called “viscosity”. Viscosity occurs when there is relative motion between fluid layers. More precisely, it measures the resistance to flow due to internal friction between layers of fluid as it flows. Viscosity can also be thought of as the thickness of a fluid, or a measure of the resistance to objects passing through it.

Viscous fluids resist motion because strong intermolecular forces create a lot of internal friction that prevents the layers from moving against each other. In contrast, low-viscosity liquids flow more easily because their molecular structure creates less friction during motion. Gases are also vicious, but they are barely noticeable under normal circumstances.

Viscosity Definition

The definition of viscosity is as follows:

Viscosity is a measure of a fluid’s resistance to flow.

The SI unit of viscosity is poiseiulle (PI). Its other units are newton-second per square meter (N s m-2) or pascal-second (Pa s.) The dimensional formula of viscosity is [ML-1T-1].

Viscosity Formula

Viscosity is measured in terms of a ratio of shearing stress to the velocity gradient in a fluid. If a sphere is dropped into a fluid, the viscosity can be determined using the following formula:

η=2ga2(∆ρ)9v

Types of Viscosity

Viscosity is the measure of a fluid’s friction to its flow. There are two ways to measure the fluid’s viscosity as follows:

  • Dynamic Viscosity (Absolute Viscosity)
  • Kinematic Viscosity

One method is to measure the fluid’s resistance to flow when an external force is applied. This is called kinematic viscosity. Another method is to measure the resistance of liquids under the action of gravity. This measure of fluid viscosity is known as kinematic viscosity.

Many people confuse the two viscosity measurements, which he considers one and the same. In fact, there is a big difference between the two. For some applications, kinematic viscosity is more useful than absolute or dynamic viscosity.

Types of Viscosity & When to Apply Them

  • Dynamic (absolute) viscosity – the measurement of the fluid’s internal resistance to flow when force is applied.
  • Kinematic viscosity – refers to the ratio of dynamic viscosity to density or a measurement of the viscosity of a fluid in motion.

Frequently Asked Questions – FAQs

What is Viscosity?

Viscosity is a measure of a fluid’s resistance to flow.

Why is viscosity an intensive property?

Viscosity does not change as the number of matter changes, therefore it is an intensive property.

How does viscosity vary with temperature?

The viscosity of liquids decreases rapidly with an increase in temperature, and the viscosity of gases increases with an increase in temperature.

How are viscosity and flow rate related?

As the viscosity increases, the flow rate decreases. The flow rate is inversely proportional to viscosity.

What is Kinematic Viscosity?

Kinematic viscosity is a measure of a fluid’s internal resistance to flow under gravitational forces.

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