Short Answer
Ratio of momentum diffusivity to thermal diffusivity is the best answer.
Thermodynamics questions usually test sign conventions, state variables, or what is being held constant. Before calculating, decide whether the system is exchanging heat, doing work, or both.
Pr = ν/α = (μCₚ/k) — comparing viscous to thermal diffusion; Pr ≫ 1 means momentum diffuses faster than heat.
Why This Answer Is Correct
This is a Medium-level question in Thermodynamics. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Write the system boundary first. Many thermodynamics mistakes disappear once you know what counts as heat, work, and internal-energy change.
Choices At A Glance
- A. Ratio of viscosity to density
- B. Ratio of momentum diffusivity to thermal diffusivity
- C. Ratio of thermal to pressure force
- D. Ratio of heat flux to mass flux
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Topic Snapshot
Topic: Thermodynamics
Difficulty: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.