Glossary
to accompany
Thermodynamics: An Engineering Approach, 5th edition
by Yunus A. Çengel and Michael A. Boles29Kinetic energy KE is energy that a system possesses as a result of its motion relative to
some reference frame. When all parts of a system move with the same velocity, the
kinetic energy is expressed as KE = m V
2
/2.Kinetic theory treats molecules as tiny balls that are in motion and thus possess kinetic
energy. Heat is then defined as the energy associated with the random motion of atoms
and molecules.
Kirchhoff’s law is defined for radiation that the emissivity and the absorptivity of a
surface are equal at the same temperature and wavelength.
Knock, or engine knock, is the audible noise occurring in the engine because of
autoignition, the premature ignition of the fuel.
Latent energy is the internal energy associated with the phase of a system.
Latent heat is the amount of energy absorbed or released during a phase-change process.
Latent heat of fusion is the amount of energy absorbed during melting and is equivalent
to the amount of energy released during freezing.
Latent heat of vaporization is the amount of energy absorbed during vaporization and is
equivalent to the energy released during condensation.
Laval nozzles (see converging–diverging nozzles)
Lighting efficacy is defined as the ratio of the amount of light output by lighting devices
in lumens of light output to the electrical energy input in W.
Liquefied petroleum gas LPG is a byproduct of natural gas processing or crude oil
refining. It consists mainly of propane (over 90 percent), and thus LPG is usually referred
to as propane. However, it also contains varying amounts of butane, propylene, and
butylenes.
Liquid phase has a molecular spacing not much different from that of the solid phase,
except the molecules are no longer at fixed positions relative to each other. In a liquid,
chunks of molecules float about each other; however, the molecules maintain an orderly
structure within each chunk and retain their original positions with respect to one another.
The distances between molecules generally experience a slight increase as a solid turns
liquid, with water being a rare exception.
Liquid–vapor saturation curve is a plot of saturation temperature Tsat versus saturation
pressure Psat.