## Viscosity – Class 11 | Chapter – 10 | Physics Short Notes Series PDF for NEET & JEE

Viscosity: Viscosity is a measure of a fluid’s resistance to flow. It is a fundamental property of fluids and is determined by the internal friction between adjacent layers of fluid as they move relative to each other. The term “viscosity” comes from the Latin word “viscum,” which means “sticky substance.”

Viscosity is commonly measured in units of Pa·s (Pascal seconds) or cP (centipoise). Fluids with high viscosity, such as honey or molasses, flow slowly and are said to be “thick” or “viscous,” while fluids with low viscosity, such as water or gasoline, flow more easily and are said to be “thin” or “low-viscosity.”

Viscosity is influenced by several factors, including the molecular structure of the fluid, the temperature, and the pressure. For example, as the temperature of a fluid increases, the viscosity generally decreases, because the increased thermal energy causes the fluid molecules to move more quickly and more easily past one another. Similarly, as pressure increases, the viscosity of a fluid can also increase due to the compression of the fluid molecules.

Viscosity is an important property of fluids in many areas of science and engineering, including fluid mechanics, chemical engineering, and materials science. It plays a critical role in the design and analysis of many systems, such as pipelines, lubrication systems, and polymer processing operations.

## Examples of Viscosity

Viscosity is a property of fluids and is exhibited by all types of liquids and gases. Some examples of viscosity in different fluids include:

• Honey – Honey is a highly viscous fluid, with a viscosity of around 2000 to 10,000 centipoise (cP). This high viscosity causes honey to flow slowly, making it difficult to pour quickly.
• Water – Water has a low viscosity, typically around 1 cP at room temperature. This low viscosity allows water to flow easily and quickly, making it a common choice for many industrial and household applications.
• Blood – Blood is a non-Newtonian fluid, meaning that its viscosity changes with the rate of flow. At low shear rates, blood is highly viscous, with a viscosity of around 4 to 5 cP, but at high shear rates, such as in the arteries, the viscosity drops to around 0.04 cP.
• Engine oil – Engine oil is a type of lubricant that is used to reduce friction between moving parts in an engine. It typically has a viscosity of around 5 to 30 cP, depending on the grade and temperature.
• Air – Air is a low-viscosity gas, with a viscosity of around 0.02 cP at room temperature and atmospheric pressure. This low viscosity allows air to flow easily and quickly, making it an important component of many industrial and natural processes, such as combustion and respiration.

These are just a few examples of viscosity in different fluids, and many other substances exhibit different levels of viscosity depending on their composition and conditions.

## Measurement of Viscosity

Viscosity can be measured using a variety of techniques, depending on the properties of the fluid and the precision and accuracy required for the measurement. Some common methods for measuring viscosity include:

• Viscometers – A viscometer is a device used to measure the viscosity of a fluid. There are many types of viscometers available, including capillary viscometers, rotational viscometers, and falling ball viscometers. These devices typically measure the time required for a fluid to flow through a narrow orifice or along a surface, or the torque required to rotate a spindle or rotor immersed in the fluid.
• Rheometers – A rheometer is a specialized type of viscometer that is used to measure the properties of non-Newtonian fluids, such as blood or polymers. These devices typically apply a controlled stress or strain to the fluid and measure the resulting deformation or flow behavior.
• Flow visualization – In some cases, the viscosity of a fluid can be observed directly by visualizing its flow behavior. For example, the behavior of a fluid flowing through a pipe or over a surface can indicate its viscosity, with more viscous fluids exhibiting slower, more laminar flow and less viscous fluids exhibiting faster, more turbulent flow.
• Computational methods – In some cases, the viscosity of a fluid can be estimated using computational methods, such as computational fluid dynamics (CFD). These methods use mathematical models and simulations to predict the behavior of fluids under different conditions, and can provide detailed information about viscosity, flow rates, and other properties.

These are just a few examples of the many techniques used to measure viscosity in different fluids and under different conditions. The choice of method depends on the specific properties of the fluid and the requirements of the application.

## Types of Viscosity

There are several types of viscosity that are commonly used to describe the behavior of fluids under different conditions. These include:

• Dynamic viscosity – Also known as absolute viscosity, dynamic viscosity is a measure of a fluid’s resistance to flow under a given stress or shear rate. It is typically measured in units of Pa·s (Pascal seconds) or cP (centipoise).
• Kinematic viscosity – Kinematic viscosity is a measure of a fluid’s resistance to flow under the influence of gravity. It is calculated by dividing the dynamic viscosity by the density of the fluid, and is typically measured in units of m2/s or cSt (centistokes).
• Apparent viscosity – Apparent viscosity is a measure of a fluid’s resistance to flow under a specific set of conditions, such as temperature, pressure, or shear rate. It can vary depending on the conditions of the measurement and the type of instrument used.
• Shear viscosity – Shear viscosity is a measure of a fluid’s resistance to flow under a constant shear rate or stress. It is commonly used to describe the behavior of non-Newtonian fluids, which can exhibit different viscosity values at different shear rates.
• #### Bulk viscosity – Bulk viscosity is a measure of a fluid’s resistance to changes in volume or compression. It is typically used to describe the behavior of gases and other compressible fluids, and is often measured in units of Pa·s.

These different types of viscosity can be used to describe the behavior of fluids under different conditions, and are important for a wide range of scientific and engineering applications.

## Formula of Viscosity

The formula for viscosity depends on the type of viscosity being considered. Here are the formulas for some common types of viscosity:

• Dynamic viscosity (µ): This is the ratio of shear stress (τ) to shear rate (du/dy) in a fluid, and is given by: µ = τ / (du/dy)The SI unit of dynamic viscosity is Pascal-second (Pa·s).
• Kinematic viscosity (ν): This is the ratio of dynamic viscosity (µ) to the density (ρ) of the fluid, and is given by: ν = µ / ρThe SI unit of kinematic viscosity is square meter per second (m2/s).
• Apparent viscosity (η): This is the ratio of shear stress (τ) to shear rate (du/dy) under a specific set of conditions, such as temperature or pressure. The formula for apparent viscosity depends on the specific conditions of the measurement.

These are just a few examples of the formulas used to calculate different types of viscosity. The choice of formula depends on the specific properties of the fluid and the conditions under which it is being measured.

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