Properties Involving Thermal Energy http://edugen.wiley.com/edugen/courses/crs2436/crowe9771/crowe9771...
2.3 Properties Involving Thermal Energy
Specific Heat, c
The property that describes the capacity of a substance to store thermal energy is called specific heat. By
definition, it is the amount of thermal energy that must be transferred to a unit mass of substance to raise its
temperature by one degree. The specific heat of a gas depends on the process accompanying the change in
temperature. If the specific volume v of the gas (v = 1/Á) remains constant while the temperature changes, then
the specific heat is identified as cv. However, if the pressure is held constant during the change in state, then the
specific heat is identified as cp. The ratio cp/cv is given the symbol k. Values for cp and k for various gases are
given in Table A.2.
Internal Energy
The energy that a substance possesses because of the state of the molecular activity in the substance is termed
internal energy. Internal energy is usually expressed as a specific quantity that is, internal energy per unit
mass. In the SI system, the specific internal energy, u, is given in joules per kilogram; in Traditional Units it is
given in Btu/lbm. The internal energy is generally a function of temperature and pressure. However, for an ideal
gas, it is a function of temperature alone.
Enthalpy
The combination u + p/Á is encountered frequently in equations for thermodynamics and compressible flow; it
has been given the name specific enthalpy. For an ideal gas, u and p/Á are functions of temperature alone.
Consequently their sum, specific enthalpy, is also a function solely of temperature.
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