Viscosity
“Viscosity is the property of a fluid that resists the force tending to cause the fluid to flow.”
What is Viscosity?
Viscosity is the internal resistance of a fluid to flow. It arises due to the cohesive forces between fluid molecules. A fluid with high viscosity resists motion because its molecular makeup offers a lot of internal resistance.
Example: Honey is more viscous than water.
Formula
η (eta) = Shearing Stress / Velocity Gradient
SI Unit: Pascal-second (Pa·s) or N·s/m²
CGS Unit: Poise (1 Pa·s = 10 Poise)
Types of Viscosity
- Dynamic Viscosity: Resistance to shearing motion.
- Kinematic Viscosity: Ratio of dynamic viscosity to density of fluid.
Factors Affecting Viscosity
- Temperature: Viscosity of liquids decreases with increase in temperature, while that of gases increases.
- Pressure: Has little effect on liquid viscosity but affects gas viscosity significantly.
Examples
- Water has low viscosity.
- Honey and glycerin have high viscosity.
- Lubricants are chosen based on their viscosity for machinery.
Previous Year Questions
- RRB JE 2019: What is the unit of viscosity in SI system?
- Pascal-second (Pa·s)
- SSC JE 2018: Which fluid has the highest viscosity among the following?
- Honey
- HSSC JE 2016: With increase in temperature, what happens to the viscosity of a liquid?
- Decreases
Applications of Viscosity
- Used in designing lubricants for machinery.
- Essential in blood flow studies (biofluid mechanics).
- Important in oil and gas industry for pipeline transport.
Did you know? The viscosity of pitch is so high that it flows extremely slowly—it took over 10 years to form one drop in the famous pitch drop experiment!
Conclusion
Viscosity is a fundamental concept in fluid mechanics. Understanding it helps in many real-life applications like machinery maintenance, medical science, and transportation systems. It is also frequently asked in competitive exams.