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How do molecular collisions transfer heat through matter?

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Engraved portrait of Rudolf Julius Emanuel Clausius, a bearded man in formal attire with sideburns, facing forward. — thermodynamics Rudolf Clausius (1822 - 1888) is considered one of the central founders of the science of thermodynamics.

In the context of thermodynamics, heat transfer through matter via molecular collisions is primarily described as thermal conduction.12

At the microscopic level, heat is understood as the kinetic energy of the random motion of molecules. When a temperature difference exists within a substance, molecules in a region of higher energy (higher temperature) collide with adjacent molecules in a region of lower energy (lower temperature). Through these collisions, energy is transferred from the more energetic molecules to the less energetic ones, effectively conveying heat through the material.341

In addition to translational kinetic energy, internal molecular motions—such as rotation and vibration—also contribute to this energy transport. While this process is generally a slow mechanism compared to convection, it is a fundamental way that energy and entropy are conveyed through solids or fluids at rest. In gases, this process is described by the kinetic theory of gases, where molecules carry net energy across a mean free path as they move and interact.3412