Short Answer
J = −k∇T 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.
Fourier's law: heat flux J = −k∇T, where k is thermal conductivity — heat flows from hot to cold proportionally to the temperature gradient.
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. Q = mcΔT
- B. J = −k∇T
- C. P = σT⁴
- D. J = σE
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.