Which of the following statements regarding the "Viscosity" of fluids are correct? 1. The dynamic viscosity of liquids decreases with an increase in temperature. 2. The dynamic viscosity of gases increases with an increase in temperature. 3. For a Newtonian fluid, the shear stress is directly proportional to the rate of shear strain.
- A1 and 2 only
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3Correct
Solution
Concept
Viscosity of Fluids and Newtonian Behavior
Formula
τ= μ (du/dy)
Explanation
All three statements are correct:1. As liquids heat up, their molecules move further apart, weakening intermolecular forces and making the fluid flow more easily, which reduces viscosity. 2. As gases heat up, gas molecules move faster and collide more frequently, increasing momentum transfer and raising viscosity. 3. By definition, a Newtonian fluid obeys Newton's law of viscosity, meaning shear stress changes in direct proportion to the rate of shear strain.
Shortcut
To easily remember viscosity trends and Newtonian behavior, use these simple rules: - Liquids Thin When Warm: Heating a liquid makes it flow easier (viscosity decreases). - Gases Thicken When Warm: Heating a gas increases molecular collisions (viscosity increases). - Newtonian = Straight Line: Shear stress is directly proportional to shear rate.
Common mistake
A common mistake is assuming that gases behave like liquids when heated, leading students to incorrectly believe that gas viscosity decreases with temperature. This causes them to rule out statement 2 and choose option C (1 and 3 only) instead of recognizing that gas viscosity increases with temperature due to higher molecular momentum transfer.
Correct answer: D) 1, 2 and 3