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1. The efficiency of an irreversible heat engine is always less than the efficiency of a
reversible one operating between the same two reservoirs.
2. The efficiencies of all reversible heat engines operating between the same two
reservoirs are the same.

Celsius scale (formerly called the centigrade scale; in 1948 it was renamed after the
Swedish astronomer A. Celsius, 1701–1744, who devised it) is the temperature scale
used in the SI system. On the Celsius scale, the ice and steam points are assigned the
values of 0 and 100 °C, respectively.

Chemical energy is the internal energy associated with the atomic bonds in a molecule.

Classical thermodynamics is the macroscopic approach to the study of thermodynamics
that does not require knowledge of the behavior of individual particles.

Clausius statement of the second law is expressed as follows: It is impossible to
construct a device that operates in a cycle and produces no effect other than the transfer
of heat from a lower-temperature body to a higher-temperature body.

Closed system consists of a fixed amount of mass (control mass), and no mass can cross
its boundary. But energy, in the form of heat or work, can cross the boundary.

Coefficient of performance COP is the measure of performance of refrigerators and
heat pumps. It is expressed in terms of the desired result for each device (heat absorbed
from the refrigerated space for the refrigerator or heat added to the hot space by the heat
pump) divided by the input, the energy expended to accomplish the energy transfer
(usually work input).

Compression ratio ε of an engine is the ratio of the maximum volume formed in the
cylinder to the minimum (clearance) volume. Notice that the compression ratio is a
volume ratio and should not be confused with the pressure ratio.

Compressor is a device that increases the pressure of a gas to very high pressures
(typical pressure ratios are greater than 3).

Condenser is a heat exchanger in which the working fluid condenses as it rejects heat to
the surroundings. For example, in the condenser of a steam power plant steam leaving
the turbine as a vapor condenses to the saturated liquid state as the result of heat transfer
to a cooling medium such as the atmosphere or water from a lake or river.

Conduction is the transfer of energy from the more energetic particles of a substance to
the adjacent less energetic ones as a result of interaction between particles.

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