Viscosity - Definition, Formula, Unit, Principle, FAQs

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.

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1. What is viscosity? Or define viscosity
2. Formula of viscosity-
3. How to measure viscosity?
4. Unit of viscosity-
5. Principle of viscosity-

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.

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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:

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:

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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-

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NCERT Physics Notes:

• NCERT Notes Class 11th Physics
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