Surface tension for an ideal fluid is:
Try it first: pick an option, then read the explanation.
Correct answer: D. zero
Asked in SSC JE 25-01-2018 (Morning Shift)
Detailed Explanation
An ideal (or perfect) fluid is an analytical idealization used to simplify mathematical equations in fluid mechanics. Real fluids possess intermolecular cohesion, which ideal fluids neglect.
Why “zero” Is Correct
An ideal fluid is defined as frictionless (zero viscosity), incompressible (infinite bulk modulus), and devoid of intermolecular cohesive forces, resulting in zero surface tension.
| Fluid type | Viscosity changes with rate of shear? | Examples |
|---|---|---|
| Ideal fluid | No (μ = 0) | Theoretical only |
| Newtonian fluid | No (μ = const) | Water, air, oil, mercury |
| Non-Newtonian | Yes | Blood, toothpaste, ketchup, paint |
| Ideal plastic (Bingham) | No, but needs yield stress τ₀ first | Fresh concrete, drilling mud |
Why the Other Options Are Wrong
- A. depends on temperature: While real fluids have temperature-dependent surface tension, an ideal fluid has zero surface tension at all temperatures.
- B. one: One is an arbitrary non-zero value.
- C. infinite: Infinite surface tension would prevent droplet formation and fluid deformation entirely.
Key Point
An ideal fluid has zero viscosity and zero surface tension, and is incompressible.
Key Concepts for Students
- Ideal fluid assumptions: μ = 0, σ = 0, ρ = constant.
- No real fluid is completely ideal, but water and air approximate ideal behavior outside boundary layers.
- Potential flow theory applies exclusively to ideal fluids.
Practice more: this is question 40 in Properties of Fluid MCQs – Part 3. Try the whole set and check every answer instantly.