Modes of heat transfer — microscopic mechanism Heat transfer that occurs primarily through molecular collisions within a material (without bulk motion) is called:

Introduction / Context:
Heat transfer occurs via three fundamental modes: conduction, convection, and radiation. Recognizing the mechanism at play is critical for modeling and design.

Given Data / Assumptions:

• No bulk fluid motion is implied.
• Energy migrates because of temperature gradients within solids or stationary fluids.
• Microscopic interactions (molecules, electrons, phonons) dominate.

Concept / Approach:
Conduction is the transfer of thermal energy due to molecular activity and lattice vibrations in solids (and molecule collisions in stationary fluids). It is described macroscopically by Fourier’s law: q″ = −k * dT/dx, relating heat flux to the temperature gradient and thermal conductivity k.

Step-by-Step Solution:
Identify absence of bulk motion → rules out convection.No requirement for an intervening medium → radiation not the primary mode here.Hence, the heat transfer mechanism due to molecular collisions is conduction.

Verification / Alternative check:
In metals, free electrons carry a significant part of the conductive heat; in nonmetals, phonon transport dominates; in gases at rest, intermolecular collisions account for conduction.

Why Other Options Are Wrong:
Convection requires bulk fluid motion; radiation is electromagnetic emission and requires no medium; advection is transport by bulk flow, not molecular collisions; “thermal dispersion” is not a fundamental mode in classical heat transfer.

Common Pitfalls:
Confusing conduction within a boundary layer with convection of the overall boundary layer process; locally at the surface, heat still crosses the fluid film by conduction, but the macro mode is convection.

Final Answer:
Conduction