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Thermodynamics Directory | Heat Transfer Directory
By combining the results of Charles' and
Boyle's experiments, the relationship
may be obtained. The constant in the above
equation is called the ideal gas constant and isdesignated
by R; thus the ideal gas equation becomes
where the pressure and temperature are
absolute values. The values of the ideal gas constant(R)
for several of the more common gases are given in Figure 39.
The individual gas constant (R) may be
obtained by dividing the universal gas constant (Ro) bythe molecular weight (MW) of the
gas, 
The units of R must always be consistent with
the units of pressure, temperature, and volume used in the
gas equation. No real gasesfollow
the ideal gas law or equation completely. At temperatures
near a gases boiling point,increases
in pressure will cause condensation to take place and drastic
decreases in volume. Atvery
high pressures, the intermolecular forces of a gas are
significant. However, most gases arein
approximate agreement at pressures and temperatures above
their boiling point.
The ideal gas law is utilized by engineers
working with gases because it is simple to use andapproximates
real gas behavior. Most physical conditions of gases used by
man fit the abovedescription.
Perhaps the most common use of gas behavior studied by
engineers is that of thecompression
process using ideal gas approximations. Such a compression
process may occurat
constant temperature (pV = constant), constant volume, or
adiabatic (no heat transfer).Whatever
the process, the amount of work that results from it depends
upon the process, asbrought
out in the discussion on the First Law of Thermodynamics. The
compression processusing
ideal gas considerations results in work performed on the
system and is essentially the areaunder
a P-V curve. As can be seen in Figure 40, different amounts
of work result from differentideal
gas processes such as constant temperature and constant
pressure.
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