At mention of the word conduction, we should immediately conjure up concepts of atomic
and molecular activity because processes at these levels sustain this mode of heat transfer.
Conduction may be viewed as the transfer of energy from the more energetic to the less
energetic particles of a substance due to interactions between the particles.
The physical mechanism of conduction is most easily explained by considering a gas and
using ideas familiar from your thermodynamics background. Consider a gas in which a temperature
gradient exists, and assume that there is no bulk, or macroscopic, motion. The gas
may occupy the space between two surfaces that are maintained at different temperatures, as
shown in Figure 1.2. We associate the temperature at any point with the energy of gas molecules
in proximity to the point. This energy is related to the random translational motion, as
well as to the internal rotational and vibrational motions, of the molecules.
Higher temperatures are associated with higher molecular energies. When neighboring
molecules collide, as they are constantly doing, a transfer of energy from the more energetic
to the less energetic molecules must occur. In the presence of a temperature gradient, energy
transfer by conduction must then occur in the direction of decreasing temperature. This would
be true even in the absence of collisions, as is evident from Figure 1.2. The hypothetical plane
at is constantly being crossed by molecules from above and below due to their random
motion. However, molecules from above are associated with a higher temperature than those
from below, in which case there must be a net transfer of energy in the positive x-direction.
Collisions between molecules enhance this energy transfer. We may speak of the net transfer
of energy by random molecular motion as a diffusion of energy