Viscosity - Definition, Formula, Unit, Principle, FAQs

What is viscosity? Or Define Viscosity

Viscosity definition and Viscosity meaning or Viscous meaning: The resistance of a fluid to gradual deformation by shear or tensile stress is characterized as viscosity. To put it another way, viscosity refers to a fluid's resistance to flow. Simply put, honey is thicker than water and, as a result, honey is more viscous than water.

Viscosity examples-

1. Syrup, for instance, has a higher viscosity than water.
2. Pitch, peanut butter, and glass are examples of fluids that have a high viscosity and behave like solids. At a standard temperature of 20°C, water has a viscosity of 1.002 cPs.
3. Molasses has a viscosity that is higher than that of water.

Formula of viscosity

The ratio of shearing stress to velocity gradient in a fluid is used to calculate viscosity. The viscosity of a fluid can be determined by dropping a sphere into it and using the following formula:

$\eta=\frac{2 g a^2(\Delta \rho)}{9 v}$

Where,

• ∆ρ is the density difference between the fluid and the sphere being investigated
• a is the radius of the sphere,
• the gravitational acceleration is g.
• The sphere's velocity is v.

How to measure viscosity?

Viscosity can be measured in a variety of ways, depending on the materials used and the conditions. Choosing the proper viscometer for your experiment is challenging. For viscosity measurements, there are many different types available; simple ones entail counting seconds as a liquid drop from a stick, whereas complex devices require sophisticated automatic recorders. This makes deciding on an instrument type while experimenting with accurate measurement is challenging.

What are the benefits of measuring viscosity?

Obtaining data on a substance's viscosity aids manufacturers in predicting how the material will behave in the real world. For example, if the toothpaste does not have the necessary viscosity, pumping out the paste from the tube will either be too difficult or too much will be squeezed out. Furthermore, knowing a material's viscosity has an impact on how production and transportation operations are built.

Unit of viscosity

1. SI Unit:

Pascal second (Pa$\cdot \mathrm{s}$ ) or newton second per square metre ( $\mathrm{N} \cdot \mathrm{s} / \mathrm{m}^2$ )

2. CGS Unit:

Poise (P)

Relation between SI and CGS units:

1 Poise $=0.1 \mathrm{~Pa} \cdot \mathrm{~s}$

Law of Viscosity

The law of viscosity was given by Sir Isaac Newton. It states that:
"The force of friction between two adjacent layers of a liquid is directly proportional to the area of contact and the velocity gradient between the layers."

$
F \propto A \times \frac{d v}{d x}
$

or

$
F=\eta A \frac{d v}{d x}
$
Where:

• $F=$ tangential force
• $A=$ area of the layer
• $\frac{d v}{d x}=$ velocity gradient (rate of change of velocity)
• $\eta=$ coefficient of viscosity

Also Read:

• NCERT solutions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids
• NCERT Exemplar Class 11 Physics Solutions Chapter 10 Mechanical Properties of Fluids
• NCERT notes Class 11 Physics Chapter 10 Mechanical Properties of Fluids

Principle of Viscosity

The principle of viscosity is based on the internal friction that occurs when one layer of a fluid moves over another.

It states that:
"When a fluid flows, the adjacent layers of the fluid exert frictional forces on each other, which oppose their relative motion." This internal resistance to flow is called viscosity.

The greater the viscosity, the stronger the internal friction between layers - meaning the fluid flows more slowly (like honey).

Applications of Viscosity

1. Lubrication: Oils with suitable viscosity reduce friction between machine parts.
2. Vehicle Engines:- Engine oils are chosen based on viscosity to ensure smooth movement of parts in hot or cold weather.
3. Medicine: Viscosity helps in controlling the flow of blood, syrups, and injections.
4. Food Industry: Used to check the thickness of liquids like honey, sauces, and juices.
5. Paints and Cosmetics: Correct viscosity ensures smooth application of paints, creams, and lotions.
6. Air and Water Flow: Viscosity helps design aircraft, ships, and pipelines by studying fluid resistance.

Short Summary of Viscosity

Semi-solids and gases, as well as some types of solids, all have viscosity. The viscosity of a product is measured in relation to its quality and efficiency. Anyone working in fluid motion research and development, fluid transfer, or quality control encounters viscosity on a regular basis.

The viscometer is considered by manufacturers and scientists to be an important part of their research and development, as well as other control programs. One of the most efficient ways to examine the key parameters that determine product performance is to measure viscosity.

The study of fluids in motion allows us to better understand their working structure and behaviour so that we can manipulate them.

Also check-

• NCERT Exemplar Class 11th Physics Solutions
• NCERT Exemplar Class 12th Physics Solutions
• NCERT Exemplar Solutions for All Subjects

NCERT Physics Notes:

• NCERT Notes Class 11th Physics
• NCERT Notes Class 12th Physics
• NCERT Notes For All Subjects

Related Topics Link,

• Derivation of Continuity Equation
• Fluid Flow
• Sphygmomanometer
• Surface Energy
• Specific Gravity