2. Energy in Thermal Processes

Thermal power

Entropy releases energy when it flows from a higher to a lower thermal level (Fig. 1). Sadi Carnot compared heat (he called it caloric) to water which, when falling down, can drive an engine. Taking this idea we postulate that the thermal power (the rate at which energy is released when entropy falls from a higher to a lower temperature) is given by

Entropy can be pumped like water, and as in the case of water the rate at which energy is needed to pump a current of entropy is also given by Eq. 3.2.

T is the absolute temperature (measured in terms of Kelvin temperatures) independent of any particular material (see Text 3). Therefore, T represents the thermal potential.

Entropy production and dissipation

A special case of pumping of entropy occurs when entropy is produced in an irreversible process (Fig. 2). If it is produced, the entropy appearing in a system has to be lifted from absolute zero to the temperature of the body. Therefore the relation between the rate at which energy is used to produce entropy (which is called dissipation rate), the entropy production rate and the temperature of the system is

Energy transfer in heating and cooling

As in all other physical processes, the energy released does not appear out of the blue. It is usually brought to the system by a flow of entropy (Fig. 3). In conductive entropy transport we have