2.3-Properties Involving Therma...pdf

Properties Involving Thermal Energy

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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/ρ) remains constant while the temperature changes, then

the specific heat is identified as c v . However, if the pressure is held constant during the change in state, then the

specific heat is identified as c p . The ratio c p /c v is given the symbol k. Values for c p 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 /ρ 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 /ρ are functions of temperature alone.

Consequently their sum, specific enthalpy, is also a function solely of temperature.

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