Fundamental principle of heat transfer: Heat flows from one body to another only when there is a temperature difference between them. State whether this statement is correct.

Introduction / Context:
Understanding when heat transfer occurs is foundational in thermodynamics and heat transfer. Heat is energy in transit due to a temperature difference. This question checks whether the learner recognizes that a temperature gradient is the driving potential for conduction, convection, and radiation.

Given Data / Assumptions:

• Two bodies or control regions at different temperatures.
• No work interaction is being considered, only thermal energy transfer.
• Transport may occur by conduction, convection, or radiation.

Concept / Approach:
Heat transfer requires a nonzero temperature difference. If temperatures are equal, there is thermal equilibrium and net heat transfer is zero. This holds across all three modes: Fourier's law for conduction, Newton's law of cooling for convection, and the Stefan–Boltzmann relation for radiation all imply a driving potential tied to temperature.

Step-by-Step Solution:
Identify thermal states: T_hot and T_cold.Recognize driving potential: Delta T = T_hot - T_cold.If Delta T ≠ 0 → heat flows from higher to lower temperature.If Delta T = 0 → no net heat transfer (thermal equilibrium).

Verification / Alternative check:
At equal temperatures, thermodynamic equilibrium implies no spontaneous energy flow. Any microscopic exchanges cancel out to zero net transfer.

Why Other Options Are Wrong:

• No: contradicts the definition of heat as energy in transit due to temperature difference.
• Only if solids: convection and radiation also require Delta T.
• Only when radiation is absent: radiation also ceases with no temperature difference between surfaces facing each other.
• Only in a vacuum: conduction and convection need matter; radiation needs a temperature difference regardless of medium.

Common Pitfalls:
Confusing heat (in transit) with internal energy (a stored property). Equal temperatures mean no net heat transfer even though internal energies may differ.

Final Answer:
Yes