A heat engine is a cyclic device that takes heat in from a hot reservoir, converts some of it to work , and rejects the rest of it to a cold reservoir so that at the end of a cycle it is in the same state (and has the same internal energy) with which it began. The net work done per cycle is (recall) the area inside the curve.
The efficiency of a heat engine is defined to be
It is impossible to construct a cyclic heat engine that produces no other effect but the absorption of energy from a hot reservoir and the production of an equal amount of work.
A refrigerator is basically a cyclic heat engine run backwards. In a cycle it takes heat in from a cold reservoir, does work on it, and rejects a heat to a hot reservoir. Its net effect is thus to make the cold reservoir colder (refrigeration) by removing heat from inside it to the warmer warm reservoir (warming it still further, e.g. as a heat pump). Both of these functions have practical applications - cooling our homes in summer, heating our homes in winter.
The coefficient of performance of a refrigerator is defined to be
It is impossible to construct a cyclic refrigerator whose sole effect is the transfer of energy from a cold reservoir to a warm reservoir without the input of energy by work.
The Carnot Cycle is the archetypical reversible cycle, and a Carnot Cycle-based heat engine is one that does not dissipate any energy internally and uses only reversible steps. Carnot's Theorem states that no real heat engine operating between a hot reservoir at temperature and a cold reservoir at temperature can be more efficient than a Carnot engine operating between those two reservoirs.
The Carnot efficiency is easy to compute (see text and lecture example). A Carnot Cycle consists of four steps:
Entropy is a measure of disorder. The change in entropy of a system
can be evaluated by integrating:
We extend our definition of reversible processes. A reversible process is one where the entropy of the system does not change. An irreversible process increases the entropy of the system and its surroundings.
The entropy of the Universe never decreases. It either increases (for irreversible processes) or remains the same (for reversible processes).