How to Convert Viscosity Units: SI, CGS, Poise, and Pa·s
Viscosity is a fundamental property of fluids that describes the resistance offered by a liquid or gas to flow or a change in shape. When a fluid flows, its different layers move at different velocities. The internal friction arising due to this velocity difference is called viscosity. The concept of viscosity is essential in understanding how fluids behave in various real-world processes, from flowing water to lubricants in machines.
What is Viscosity?
In simple terms, viscosity measures how much a fluid resists moving or flowing when a force is applied. A high-viscosity fluid, like honey, moves slowly because its internal friction is significant. In contrast, a fluid with low viscosity, such as water, flows easily. Viscosity affects both liquids and gases, but is especially important in applications involving flow, motion, and material processing.
Physical Meaning and Types of Viscosity
When a fluid is forced to flow, the molecules layers slide past each other. The friction between these layers leads to resistance, known as viscosity. In Physics, viscosity is often discussed as either:
- Dynamic (or Absolute) Viscosity
- Kinematic Viscosity
Kinematic viscosity is the ratio of dynamic viscosity to the density of the fluid.
Viscosity Formula and Explanation
The force required to move one layer of fluid over another is proportional to the area of contact, the velocity gradient between layers, and the fluid’s viscosity. The relationship is given by:
F = η × A × (dv/dy)
F = force required (N)
η = dynamic viscosity of fluid
A = area (m2)
dv/dy = rate of change of velocity with respect to distance (velocity gradient)
The coefficient of viscosity (η) can thus be calculated using this relation.
Units of Viscosity
Understanding and using correct units is important when solving problems involving viscosity. Dynamic and kinematic viscosities have different units, and it is vital not to confuse them.
| Type | Symbol | SI Unit | Other Units |
|---|---|---|---|
| Dynamic Viscosity | η | pascal-second (Pa·s) | poise (P, CGS) |
| Kinematic Viscosity | ν | m2/s | stokes (St, CGS) |
Pascal-second (Pa·s) is used for dynamic viscosity in the SI system. In the CGS system, poise (P) is used, where 1 Pa·s = 10 P.
Examples: Applying Viscosity in Problems
Let’s see a practical example. Suppose you have a fluid with a dynamic viscosity of 0.5 Pa·s. To convert this to the CGS unit:
- 1 Pa·s = 10 P
- So, 0.5 Pa·s = 5 P
If you need to convert to centipoise (cP), remember: 1 P = 100 cP, so 5 P = 500 cP.
Step-by-Step Approach to Viscosity Problems
- Identify what is given – Is it dynamic or kinematic viscosity?
- Check the unit. If required, convert to the desired SI or CGS unit.
- Use the appropriate formula, such as F = ηA(dv/dy), to relate force, area, and velocity gradient.
- Carry out the calculation with correct units for consistency.
Maintaining correct units throughout ensures your answer is physically meaningful.
Key Formulas
| Formula | Meaning |
|---|---|
| F = ηA (dv/dy) | Force required to move fluid layer |
| ν = η / ρ | Kinematic viscosity (ν) as ratio of dynamic viscosity to density |
Visual Comparison: Viscosity of Common Substances
| Substance | Approximate Viscosity (Pa·s) | Flows Easily? |
|---|---|---|
| Water (25°C) | 0.001 | Yes |
| Air (room temp.) | 0.000018 | Very Easily |
| Honey | 10 | No |
Applications of Viscosity
Understanding viscosity is important in areas like lubrication, fluid flow in pipes, and determining how liquids and gases move in daily and industrial processes. Viscosity plays a role in mechanics, thermodynamics, and more.
Deepen Your Understanding
- Explore more: Viscosity
- Advanced topics: Difference Between Kinematic and Dynamic Viscosity
Next Steps for Learners
- Summarize and memorize key formulas for quick revision.
- Work through practice numericals with unit conversions.
- Connect viscosity concepts to real-life examples you observe daily.
To gain more experience in problem-solving and a deeper grasp of the concept, make use of Vedantu’s guided practice resources and topic explanations.
FAQs on Units of Viscosity in Physics: SI, CGS, and More
1. What is the SI unit of viscosity?
The SI unit of dynamic viscosity is the Pascal-second (Pa·s). This can also be expressed as kg·m-1·s-1 in base units. It measures the internal resistance of a fluid to flow under an applied force.
2. What is the CGS unit of viscosity?
The CGS unit of dynamic viscosity is the poise (P). This unit is often used in laboratory and academic settings. 1 poise = 0.1 Pa·s. Centipoise (cP) is a more commonly used subunit, where 1 cP = 0.01 poise = 0.001 Pa·s.
3. How do you convert the unit of viscosity from SI to CGS and vice versa?
To convert between SI and CGS units of viscosity:
- 1 Pa·s = 10 poise
- 1 poise = 0.1 Pa·s
Multiply or divide by 10 depending on which direction you are converting.
4. What is the unit of kinematic viscosity in SI and CGS systems?
The SI unit of kinematic viscosity is square metre per second (m2/s).
The CGS unit is stokes (St) where 1 m2/s = 104 stokes. Kinematic viscosity is calculated as the ratio of dynamic viscosity to fluid density.
5. Is poise an SI unit of viscosity?
No, poise (P) is not an SI unit. It is the unit of viscosity in the CGS system. The SI unit is Pascal-second (Pa·s).
6. What is the meaning of centipoise (cP) in viscosity measurement?
Centipoise (cP) is a common subunit of viscosity in the CGS system.
1 cP = 0.01 poise = 0.001 Pa·s. It is often used for measuring the viscosity of liquids like water and oils for convenience.
7. What is the formula for dynamic viscosity?
The formula for dynamic viscosity (η) is:
η = (F × d) / (A × v)
Where:
• F = force applied (N)
• d = separation between layers (m)
• A = area of the layer (m2)
• v = relative velocity (m/s)
8. How is kinematic viscosity different from dynamic viscosity?
Dynamic viscosity (η) measures a fluid's internal resistance to flow, whereas kinematic viscosity (ν) is the ratio of dynamic viscosity to density (ν = η / ρ). Their SI units are Pa·s and m2/s respectively.
9. What is the viscosity of water at 20°C in SI and CGS units?
The dynamic viscosity of water at 20°C is approximately:
• 0.001002 Pa·s (SI)
• 0.01002 poise (CGS)
This value may be approximated to 1 mPa·s or 1 cP for many calculations.
10. Why is understanding the unit of viscosity important for exams?
Knowing the correct units of viscosity is crucial:
• Direct unit-based questions are asked in physics exams like NEET, JEE, and board tests.
• Proper unit conversion prevents calculation errors.
• Applying correct units demonstrates conceptual clarity for scoring full marks.
11. Can you list all common units of viscosity used in physics?
Common units of dynamic viscosity:
• Pascal-second (Pa·s) – SI unit
• Poise (P), Centipoise (cP) – CGS units
• mPa·s (1 mPa·s = 1 cP)
Common units of kinematic viscosity:
• Square metre per second (m2/s)
• Stokes (St), Centistokes (cSt)
12. How do you solve numerical problems involving unit conversion in viscosity?
To solve these numericals:
1. Write the given value and its unit.
2. Use the conversion factor (e.g., 1 Pa·s = 10 P).
3. Multiply or divide as required.
4. Write the answer with the correct unit and significant figures.
