Modes of heat transfer — precise definition of conduction Which statement best defines heat transfer by conduction within a body or between bodies in contact, in classical thermal engineering terms?

Introduction / Context:
Engineering heat transfer recognizes three fundamental modes: conduction, convection, and radiation. Correctly distinguishing them is essential for selecting correlations, sizing insulation, and designing heat exchangers.

Given Data / Assumptions:

• No phase change in the described mode.
• Continuum assumption applies; microscopic details are abstracted.
• Steady or unsteady processes both permitted.

Concept / Approach:
Conduction is microscopic energy transfer due to temperature gradients, arising from molecular agitation and electron transport. It does not require net bulk motion of the material. In contrast, convection is energy transfer augmented by macroscopic fluid motion. Radiation is electromagnetic energy exchange that requires no medium and travels in straight lines in free space.

Step-by-Step Solution:
Identify key phrase “without bulk motion” → characteristic of conduction.Option (a) matches: particle-to-particle energy migration via collisions/electrons across a temperature gradient.Option (b) describes convection (forced or natural).Option (c) describes thermal radiation traveling through space or transparent media.

Verification / Alternative check:
Fourier’s law q_x = -k * dT/dx formalizes conduction: heat flux proportional to temperature gradient and thermal conductivity, independent of bulk flow.

Why Other Options Are Wrong:
(b) Bulk fluid motion is convection, not conduction. (c) Ray-like transfer without medium interaction is radiation. (d) Incorrect as (a) is correct. (e) Conduction occurs in solids, liquids, and gases; not “only in fluids at rest.”

Common Pitfalls:
Confusing “conduction in fluids” with “convection”; both occur simultaneously in fluids, but convection requires bulk motion.

Final Answer:
Transfer from one particle to another without bulk motion of the particles