5 - Interactions between systems

Summary

Energy can be transferred between systems by the following three mechanisms

• the transfer of heat ΔQ;

• the transfer of work ΔW (i.e., one system does work on another);

• the transfer of particles ΔN.

These are called thermal, mechanical, and diffusive interactions, respectively (see Figure 5.1). The first three sections of this chapter introduce these interactions in a manner that is intuitive and qualitatively correct, although lacking in the mathematical rigor of the chapters that follow.

Heat transfer – the thermal interaction

In the preceding chapter we learned that thermal energy gets distributed equally among all available degrees of freedom, on average. So the energy of interacting systems tends to flow from hot to cold until it is equipartitioned among all degrees of freedom. The energy that is transferred due to such temperature differences is called heat, and it travels via three distinct mechanisms: conduction, radiation, and convection.

Conduction involves particle collisions (Figure 5.2a). On average, collisions transfer energy from more energetic particles to less energetic ones. Energy flows from hot to cold.

Energy transfer via radiation can be illustrated by toy boats in a tub. (Figure 5.2b). If one is jiggled up and down, it sends out waves. Other toy boats will oscillate up and down as these waves pass by. In a similar fashion (but at much higher speeds), electromagnetic waves are generated by accelerating electrical charges, and this energy is absorbed by other electrical charges that these waves encounter.