Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles36Perpetual-motion machine of the second kind PMM2 is a device that violates the
second law of thermodynamics.
Phase diagram is the P-T diagram of a pure substance and shows all three phases
separated from each other by the sublimation line, vaporization line, and melting line.
Phase equilibrium is the condition that the two phases of a pure substance are in
equilibrium when each phase has the same value of specific Gibbs function. Also, at the
triple point (the state at which all three phases coexist in equilibrium), the specific Gibbs
function of each one of the three phases is equal.
Phase equilibrium for liquid water that is open to the atmosphere can be expressed as
follows: The vapor pressure of the water in the air must be equal to the saturation
pressure of water at the water temperature.
Piezoelectric effect is the emergence of an electric potential in a crystalline substance
when subjected to mechanical pressure, and this phenomenon forms the basis for the
widely used strain-gage pressure transducers.
Piezoelectric transducers, also called solid-state pressure transducers, work on the
principle that an electric potential is generated in a crystalline substance when it is
subjected to mechanical pressure. This phenomenon, first discovered by brothers Pierre
and Jacques Curie in 1880, is called the piezoelectric (or press-electric) effect.
Piezoelectric pressure transducers have a much faster frequency response compared to
the diaphragm units and are very suitable for high-pressure applications, but they are
generally not as sensitive as the diaphragm-type transducers.
Polytropic process is a process in which pressure and volume are often related by
PV
n
= C, where n and C are constants, during expansion and compression processes ofreal gases.
Potential energy PE is the energy that a system possesses as a result of its elevation in a
gravitational field and is expressed as PE = mgz.
Pound-force lbf, in the English system, is the force unit defined as the force required to
accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s^2.
Power is the work done per unit time is called and has the unit kJ/s, or kW.
Prandtl–Meyer expansion waves are the Mach waves that compose a continuous
expanding region called an expansion fan.