What is Viscosity?
Viscosity is the tendency of a Fluid to resist any change in its shape or Motion. It is a measure of the internal forces of a Fluid. The internal forces or Friction of Fluid comes into play when one layer of Fluid is subjected to move over another layer. More is the Friction; more is the amount of force required to move the layers; this is called shear. Shearing occurs when a Fluid moves or is distributed, like pouring, spraying, spreading, mixing, etc. This is the reason that Fluids with high Viscosity need more force to move as compared to those of low Viscosity.
How to Measure Viscosity?
There are various methods to measure Viscosity, depending upon the type of materials used and the circumstances. Selecting the right viscometer that suits your experiment is quite a difficult task. Many different types are available for Viscosity measurements; simple ones involve counting of seconds as a Liquid drop from a stick, while complex instruments involve sophisticated automatic recordings. This makes it difficult for a user to decide the instrument type while experimenting with accurate measurement.
What is the Law of Viscosity?
Let us consider a Liquid in which the Fluid particles move in a specific direction in straight parallel lines. Sir Isaac Newton has postulated some assumptions regarding the Fluid Viscosity.
In Fluids, the flow velocity varies linearly at different points, being 0 at the bottom, and a velocity ‘u’ at the top. The force F acting on the Fluid particles is directly proportional to the Fluid velocity ‘u’ and the area ‘A’ of the layer and is inversely proportional to the distance ‘y’ between them.
According to Newton, the Viscosity formula is,
\[F=\mu A \frac {u} {y}\]
Where \[\mu^{i}\] is the Viscosity factor for a Fluid, and the unit is: Pa.su/y ratio is called shear deformation or the rate of Shear velocity.
This is a derivative property of Fluid velocity in a direction perpendicular to the plates.
What if the Fluid velocity doesn't vary linearly with y? In this case, the generalized equation is given as
\[r = \mu\frac{\partial u}{\partial y}\]
Where τ=F/A i.e. force per unit area, and ∂u/∂y is the local shear velocity. This equation denotes Newton's law of Viscosity. For planar symmetry surfaces, the shearing velocity is given by: \[\mu^{i}\], which is a particular case of Fluid Viscosity. The generalized expression in coordinate-free form is provided by.
Importance of Viscosity
Viscosity is an essential feature of Liquids that are used for lubrication, like lubricating oils and grease. Viscosity is the resistance produced in a Liquid to its flow. Fast-moving Liquids like water have low Viscosity and slow-moving Liquids like honey have high Viscosity.
If the Viscosity of a Fluid is too high at low temperatures, it cannot be pumped. If Viscosity is too low at high temperatures, it can't be used for Lubrication.
A moderately viscous Fluid is required for lubrication purposes. Oil is not always the right option for lubrication. The quality of lubricant matters and it should have a proper Viscosity and be a multi-grade like API and SN, supplied by some reputed firm. This helps in better lubrication preventing any wear and tear.
Principle of Viscosity
The principle of Viscosity is stated here:
When a layer of Liquid is subjected to move upon a surface or another layer of the same Liquid, the Fluid particles tend to oppose such movement; this resisting force developed by a Liquid is called Viscosity.
Tips to Study Viscosity
The introduction to Viscosity, its importance, laws, principles and ways to measure it, was explained in the above paragraphs.
An important discipline of the Physics quotient as discussed above gives you a clear insight into which formulas are to be used when.
To learn and revise the concepts, you shall attempt some of the quizzes or test papers available on the website or App.
Let us now try and understand some of the useful techniques that students shall follow to prepare well for the upcoming Exams.
WWHH Techniques
W- What to do
W- When to do
W- Where to do
H- How to do
W- WHAT TO DO: Use retrieval practice
Students must try this technique as it makes them memorize easily and gives their confidence a major push which is required at difficult times such as Exam days. Retrieval practice is the technique that uses graphics, diagrams and flowcharts for learning, it gives you a lot of clarity and most importantly, it doesn't support cramming which is a great thing. As you all must be knowing that cramming may help you pass the test but it will not help in future because everything you crammed in the night before the Exam, you may not remember it a month later. All the students may have different study patterns but when we generalize it, it should suit everybody. And this makes learning effective, fun and efficient.
W- WHERE TO DO: Use spaced learning
All the students have different learning methods and it is understandable. But an important technique that everyone should adopt is spaced learning. This technique comprises of cleaning your study place on regular basis and keeping the place spic and span. It not only consists of cleaning the surroundings but also eliminating the things that distract you such as mobiles, games, devices etc. You may also include some stuff that motivates you to take action such as pictures of your idols or some motivational quotes etc. This practice might sound funny to a lot of you but the results would surprise you!
W: WHEN TO DO: Revise repeatedly
The key to succeeding in the Examination is repetitive revision. You may take revisions very seriously. As soon as you are done with your syllabus, you shall start revising. You should always do the revisions at least twice before the Exam and opt for the right revision technique. Not only oral but written as well because it's a fact that when you write more you learn more and memorize it well and that too for longer-term which is a great thing. Once you revise things well, you gain confidence and that is the thing that is a must-have attribute, especially during Exam days. Losing confidence in yourself is the worst thing that can happen with a student but when you decide to take control of your mindset, things start falling in place.
H- HOW TO DO: Maximize learning
Another important skill that you must have is to maximize learning. You should always focus on long term learning and not cramming. The most important thing that you should pay heed to is conceptual learning and give your learning process a particular strategy because that will surely give the learning process a boost and it will help you scale heights. There is a very thin line difference between learning and cramming. You should always try to maintain it and give your 100% while learning the topics given in the syllabus. As students, you shall understand the importance of time management and always follow a time-based schedule.
If you’re someone who is crazy about achieving some wild goals, it is natural for you to be low on motivation. However, successful are only those who invest their energies in pushing these limiting thoughts away and use practices that can help them fuel to take action and move ahead.
FAQs on Viscosity
1. What is viscosity in Physics and how is it measured as per the CBSE 2025–26 syllabus?
Viscosity is defined as a fluid's resistance to flow, caused by internal friction between its layers. It is most commonly measured using devices called viscometers, such as capillary viscometers or rotational viscometers, depending on accuracy and fluid type, as required by the syllabus.
2. Explain the difference between dynamic and kinematic viscosity with examples.
Dynamic viscosity measures a fluid's internal resistance to flow under an applied force (unit: Pa·s), while kinematic viscosity is the ratio of dynamic viscosity to fluid density (unit: Stokes or m2/s). For example, oil has higher dynamic and kinematic viscosity compared to water, meaning it flows more slowly and resists motion more strongly.
3. Why does temperature affect the viscosity of liquids and gases differently?
For most liquids, viscosity decreases as temperature increases because the increased kinetic energy allows molecules to move more freely. In contrast, the viscosity of gases increases with temperature since faster-moving gas molecules collide more frequently, increasing resistance to flow.
4. State and explain Newton's law of viscosity. Why are some fluids termed Newtonian?
Newton's law of viscosity states that the shear stress (τ) between adjacent fluid layers is proportional to the velocity gradient (du/dy) perpendicular to the direction of flow: τ = μ (du/dy), where μ is the viscosity. Newtonian fluids are those whose viscosity remains constant regardless of the applied shear rate, like water or air.
5. List major applications of viscosity in daily life and industry.
- Lubricants: Oils with suitable viscosity are used in engines to reduce wear and friction.
- Paints and varnishes: Correct viscosity ensures smooth application and even coverage.
- Food preparation: Controlling viscosity in syrups, batters, and sauces affects texture and pourability.
- Medicine: Cough syrups use high viscosity to coat and soothe the throat.
- Adhesives: Highly viscous fluids like glue provide strong sticking power.
6. How is viscosity important in the design of lubricants for machines?
Proper viscosity ensures that lubricants form a protective film between moving parts, minimizing friction and wear. Too high viscosity may hinder flow at low temperatures, while too low viscosity can lead to inadequate lubrication at high temperatures. Multi-grade oils are often used to adapt to varying conditions.
7. What are non-Newtonian fluids? Give two examples from daily life.
Non-Newtonian fluids have viscosity that changes with the applied shear rate or stress. Examples include ketchup (which becomes runnier under force) and toothpaste (which holds its shape until squeezed).
8. How can students avoid the common misconception that viscosity and density are the same?
While viscosity refers to a fluid's resistance to flow due to internal friction, density is the mass per unit volume. High density does not always mean high viscosity; for example, honey is more viscous than water but not necessarily denser than mercury.
9. If a fluid has a very low viscosity, how does it affect the flow and mixing process?
Fluids with low viscosity (like water) flow easily and mix quickly, as there is little internal resistance. This property is utilized in applications where rapid spreading or mixing is required, such as in solvents or paints.
10. Describe a practical experiment that demonstrates the measurement of viscosity in a laboratory setting.
A common experiment is the falling sphere method: a small ball is dropped into a fluid, and the time taken to fall is measured. Viscosity is calculated using Stokes' law, considering the sphere's radius, fluid density, and fall velocity, as outlined in CBSE practical physics.
