Careers360 Logo
Viscosity - Definition, Formula, Unit, Principle, FAQs

Viscosity - Definition, Formula, Unit, Principle, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:40 PM IST

When you pour 500 mL of water on someone's head, what happens? (apart from them getting angry). It spills over their faces after running through their hair. What happens if you pour 500 ml of honey on someone's head, though? Doesn't it take its sweet time to run its course through that person's mind? Why is there a distinction? This is due to a fluid feature known as viscosity. Let us learn about viscosity in this article.

This Story also Contains
  1. What is viscosity? Or define viscosity
  2. Formula of viscosity-
  3. How to measure viscosity?
  4. Unit of viscosity-
  5. Principle of viscosity-
Viscosity - Definition, Formula, Unit, Principle, FAQs
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.

Background wave

Also read -

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.
NEET Highest Scoring Chapters & Topics
This ebook serves as a valuable study guide for NEET exams, specifically designed to assist students in light of recent changes and the removal of certain topics from the NEET exam.
Download E-book

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:

η=2ga2(Δρ)/9v

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.

Where,

v = sphere's distance travelled divided by the time it takes to cover that distance

As you can see from this equation, as the viscosity increases the sphere's speed decreases. The higher a fluid's viscosity, the more resistance it causes to any object moving through it. Although all liquids have some viscosity, liquid viscosity is frequently classed as high or low, with the viscosity of water acting as a benchmark.

Water viscosity is 0.001 Pa s, air viscosity is 0.000019 Pa s, and motor oil viscosity is 1. So, you can very much figure it out on your own. In addition, the viscosity of liquids reduces as the temperature rises, but the viscosity of gases rises as the temperature rises.

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-

The Stokes unit of kinematic viscosity is named after Sir George Gabriel Stokes, a British physicist.

One centimetre squared per second is defined as a stroke.

Law of viscosity-

Consider a liquid in which the fluid particles flow in straight parallel lines in a given direction. Sir Isaac Newton made certain assumptions about the viscosity of fluids.

The flow velocity in fluids changes linearly at different places, with 0 at the bottom and u at the top. The viscous force F acting on the fluid particles is proportional to the fluid velocity ‘u' and the layer area ‘A', but not to the distance ‘y' between them.

The viscosity formula (viscous force formula), according to Newton, is:

F=μA(u/y)

The unit is Pa.s, and μ is the viscosity factor for a fluid.

Shear deformation, or the rate of shear velocity, is defined as the u/y ratio.

This is a derivative of fluid velocity in perpendicular to the direction of the plate.

What if the fluid velocity isn't proportional to y? The generalised equation in this scenario is as follows:

τ=μ(∂u/∂y)

Where u/y is the local shear velocity and =F/A is the viscous force per unit area. Newton's viscosity law is represented by this equation.

Importance of viscosity-

Liquids used for lubrication, such as lubricating oils and grease, must have a certain viscosity. Viscosity is the amount of resistance a liquid produces when it flows. The viscosity of fast-moving liquids like water is low, while the viscosity of slow-moving liquids like honey is high.

A fluid cannot be pumped if its viscosity is too high at low temperatures. It cannot be used for lubrication if the viscosity is too low at high temperatures.

Lubrication necessitates a fluid with a reasonable viscosity. When it comes to lubrication, oil isn't always the best solution. Lubricant quality is important, and it should have an appropriate viscosity and have a multi-grade, such as API and SN, supplied by a reputable company. This improves lubrication and prevents wear and tear.

Also Read:

Principle of viscosity-

The viscosity principle is stated as follows:

When a layer of liquid is moved across a surface or another layer of the same liquid, the fluid particles tend to resist the movement; this resisting viscous force is known as viscosity.

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 Physics Notes:

Frequently Asked Questions (FAQs)

1. What factors influence viscosity?

Temperature and silica content are the two key elements that influence viscosity.

2. What role does viscosity play in everyday life?

You should be aware of the viscosity of the oil you use in your car or truck. This is because viscosity influences friction, which influences heat. Furthermore, viscosity influences the pace of oil consumption as well as the ease with which your car starts in hot or cold weather.

3. What are the different kinds of viscosity?

The viscosity of a fluid is a measurement of its ability to flow. Kinematic and dynamic viscosity are the two types of viscosity that are typically reported.

4. Which fluid has the highest viscosity?

Honey, for example, has a viscosity substantially higher than water. A viscometer is used to measure viscosity. The range of values measured is several orders of magnitude. Gases have the lowest viscosities of all fluids, whereas thick liquids have the highest.

5. Is viscosity a measure of how thick something is?

A liquid's viscosity determines how thick it is and how much difficulty it has to flow. Most materials' viscosity is affected by temperature.

6. What is the viscosity coefficient, and how is it used in calculations?
The viscosity coefficient, typically denoted as μ, is a measure of a fluid's resistance to flow. It's used in various fluid mechanics calculations, such as determining shear stress (τ = μ(du/dy)), pressure drop in pipes, and drag forces on objects moving through fluids. The viscosity coefficient is crucial for predicting fluid behavior and designing efficient systems in engineering applications.
7. What is the difference between dynamic viscosity and kinematic viscosity?
Dynamic viscosity (μ) is the ratio of shear stress to shear rate in a fluid, measuring its resistance to flow. Kinematic viscosity (ν) is the ratio of dynamic viscosity to fluid density. While dynamic viscosity is an inherent property of the fluid, kinematic viscosity takes into account the fluid's density, making it useful in calculations involving fluid flow and motion.
8. How does viscosity affect the onset of turbulence in fluid flow?
Viscosity plays a crucial role in determining the onset of turbulence:
9. What is the significance of viscosity in the study of plate tectonics and mantle convection?
Viscosity is crucial in understanding plate tectonics and mantle convection:
10. What is the role of viscosity in blood flow and cardiovascular health?
Viscosity plays a crucial role in blood flow and cardiovascular health:
11. How is viscosity measured experimentally?
Viscosity can be measured using various methods, including:
12. What is the concept of apparent viscosity in non-Newtonian fluids?
Apparent viscosity is the effective viscosity of a non-Newtonian fluid at a specific shear rate. Unlike Newtonian fluids, the viscosity of non-Newtonian fluids changes with shear rate. Apparent viscosity allows us to describe the fluid's behavior at a particular condition, making it easier to analyze and predict non-Newtonian fluid flow. This concept is crucial in industries dealing with complex fluids like polymers, suspensions, and some food products.
13. How does viscosity affect the terminal velocity of objects falling in fluids?
Viscosity plays a crucial role in determining the terminal velocity of objects falling in fluids. Higher viscosity fluids exert more drag force on falling objects, resulting in lower terminal velocities. This relationship is described by Stokes' law, which states that the drag force is directly proportional to the fluid's viscosity. Understanding this concept is important in various fields, from meteorology to chemical engineering.
14. How does viscosity affect the formation and stability of emulsions?
Viscosity plays a crucial role in emulsion formation and stability:
15. How does pressure affect the viscosity of liquids and gases?
For most liquids, viscosity increases slightly with pressure due to decreased intermolecular spacing. This effect is generally small at moderate pressures but can become significant at very high pressures. For gases, viscosity is largely independent of pressure at moderate pressures, as predicted by kinetic theory. However, at very high pressures or near the critical point, gas viscosity can increase with pressure. Understanding these relationships is important in high-pressure applications and geological processes.
16. How does temperature affect the viscosity of liquids and gases?
For most liquids, viscosity decreases as temperature increases. This is because higher temperatures give molecules more energy to overcome intermolecular forces. In contrast, for gases, viscosity typically increases with temperature due to increased molecular motion and collisions. Understanding this relationship is crucial for many industrial processes and natural phenomena.
17. How does the concept of viscosity relate to Newton's law of viscosity?
Newton's law of viscosity states that the shear stress between adjacent fluid layers is proportional to the negative velocity gradient between the two layers. This relationship is expressed as τ = μ(du/dy), where τ is shear stress, μ is dynamic viscosity, and du/dy is the velocity gradient. This law forms the basis for understanding fluid flow behavior and is crucial in many engineering applications.
18. What is a Newtonian fluid, and how does it differ from a non-Newtonian fluid?
A Newtonian fluid is one in which the viscosity remains constant regardless of the applied shear stress or shear rate. Examples include water and most gases. Non-Newtonian fluids, on the other hand, have viscosities that change with applied stress or shear rate. Examples include ketchup (shear-thinning) and cornstarch in water (shear-thickening). Understanding these differences is crucial for predicting fluid behavior in various applications.
19. How does viscosity affect the flow rate in pipes?
Viscosity directly impacts flow rate in pipes by creating friction between the fluid layers and the pipe walls. Higher viscosity fluids experience more internal friction, resulting in slower flow rates for a given pressure difference. This relationship is described by the Hagen-Poiseuille equation, which shows that flow rate is inversely proportional to viscosity. Understanding this concept is crucial for designing efficient piping systems.
20. What is the relationship between viscosity and Reynolds number?
The Reynolds number (Re) is inversely proportional to viscosity. It's calculated as Re = ρvD/μ, where ρ is density, v is velocity, D is characteristic length, and μ is dynamic viscosity. Lower viscosity leads to higher Reynolds numbers, indicating a greater tendency for turbulent flow. This relationship is crucial for predicting flow regimes and understanding fluid behavior in various engineering applications.
21. What is the unit of viscosity in the SI system?
The SI unit for dynamic viscosity is pascal-second (Pa·s) or newton-second per square meter (N·s/m²). For kinematic viscosity, the SI unit is square meters per second (m²/s). However, other units like poise (P) for dynamic viscosity and stokes (St) for kinematic viscosity are also commonly used in various fields.
22. How does viscosity affect the power required to pump fluids?
Viscosity directly impacts the power required to pump fluids. Higher viscosity fluids require more power due to increased internal friction and resistance to flow. The relationship is described by the power equation: P = Q∆p, where P is power, Q is flow rate, and ∆p is pressure drop. As viscosity increases, the pressure drop for a given flow rate increases, resulting in higher power requirements. This concept is crucial for designing efficient pumping systems.
23. What is the relationship between viscosity and the speed of sound in fluids?
Viscosity affects the speed of sound in fluids, particularly in highly viscous liquids. In general, the speed of sound decreases with increasing viscosity due to increased internal friction and energy dissipation. This relationship is described by the Stokes-Kirchhoff equation, which accounts for both thermal conductivity and viscosity effects on sound propagation. Understanding this concept is important in fields like acoustics, underwater communications, and non-destructive testing.
24. How does viscosity affect heat transfer in fluids?
Viscosity influences heat transfer in fluids by affecting fluid motion and mixing. Higher viscosity fluids tend to have lower heat transfer rates due to:
25. How does viscosity affect the formation and behavior of vortices in fluids?
Viscosity plays a crucial role in vortex formation and behavior:
26. What is viscosity and why is it important in fluid mechanics?
Viscosity is a measure of a fluid's resistance to flow. It's important in fluid mechanics because it determines how easily a fluid flows and how it interacts with surfaces. Viscosity affects everything from blood flow in our bodies to oil flow in engines, making it crucial for understanding and predicting fluid behavior in various applications.
27. What is the concept of extensional viscosity, and how does it differ from shear viscosity?
Extensional viscosity is a measure of a fluid's resistance to stretching or extensional deformation, while shear viscosity (commonly referred to simply as viscosity) measures resistance to shear deformation. For Newtonian fluids, extensional viscosity is typically three times the shear viscosity. However, for many non-Newtonian fluids, especially polymer solutions, extensional viscosity can be much higher and behave differently from shear viscosity. Understanding extensional viscosity is crucial in processes involving stretching flows, like fiber spinning and inkjet printing.
28. What is the relationship between viscosity and surface tension?
While viscosity and surface tension are both properties of fluids, they are distinct concepts. Viscosity measures a fluid's resistance to flow, while surface tension is the tendency of a fluid's surface to resist an external force. However, both properties arise from intermolecular forces. Generally, fluids with stronger intermolecular forces tend to have both higher viscosity and higher surface tension. Understanding this relationship is important in various applications, from designing inkjet printers to studying fluid behavior in microgravity.
29. What is the significance of viscosity in lubrication?
Viscosity is crucial in lubrication as it determines a lubricant's ability to form and maintain a protective film between moving surfaces. Higher viscosity oils generally provide better protection but may increase energy consumption. The ideal viscosity depends on factors like load, speed, and temperature. Understanding viscosity's role in lubrication is essential for minimizing wear and maximizing efficiency in mechanical systems.
30. What is the significance of the viscosity index in lubricating oils?
The viscosity index (VI) is a measure of how much an oil's viscosity changes with temperature. A higher VI indicates that the oil's viscosity changes less with temperature, which is desirable in many applications. Oils with high VI provide more consistent performance across a wide temperature range, making them crucial in applications like automotive engines that experience significant temperature variations. Understanding VI is essential for selecting appropriate lubricants for specific operating conditions.
31. What is the concept of eddy viscosity in turbulent flows?
Eddy viscosity is a concept used to model the effects of turbulent mixing in fluid flows:
32. How does the molecular structure of a fluid affect its viscosity?
The molecular structure of a fluid significantly influences its viscosity. Factors include:
33. How does viscosity affect the behavior of bubbles in fluids?
Viscosity significantly influences bubble behavior in fluids:
34. How does viscosity affect the efficiency of wind turbines and hydroelectric power generation?
Viscosity impacts the efficiency of wind turbines and hydroelectric power generation:
35. What is the significance of viscosity in the study of magma behavior and volcanic eruptions?
Viscosity is crucial in understanding magma behavior and volcanic eruptions:
36. How does viscosity affect the behavior of polymers in solution?
Viscosity significantly influences polymer behavior in solution:
37. How does viscosity affect the formation and stability of foams?
Viscosity plays a significant role in foam formation and stability:
38. What is the relationship between viscosity and the glass transition in amorphous materials?
Visc

Articles

Questions related to

Have a question related to ?

Correct Answer: George Gabriel Stokes


Solution : The correct option is George Gabriel Stokes.

In 1851, George Gabriel Stokes, an Irish mathematician and physicist, founded the science of hydrodynamics with his law of viscosity. This seminal work described the motion of a small sphere through a viscous fluid, providing fundamental insights into fluid dynamics and paving the way for significant advancements in fluid mechanics. He also made significant contributions to fluid dynamics, optics, and the behaviour of waves, coining the term Stokes' Law for viscous drag in fluids.

View All
Back to top