Reference 8-51
Ideal Gas Law
The Ideal Gas Law can be written in a slightly different manner from the
Generalized Gas Law: PV/T = nR, or
PV = nRT. When written this way, it is called the Ideal-Gas Law. R is the gas
constant, and n is the number of moles of gas. The gas constant can be examined
experimentally as R = 0.082 liter atm/Kelvin moles. Knowing R, the fourth
variable can be evaluated if any three are known.
The gas Laws are valid for most gases at moderate temperatures and pressures. At
low temperatures and high pressures, gases deviate from the above Laws because
the molecules are moving slowly at low temperatures and they are closer together
on the average at higher pressures.
Ideal vs. Real Gas
Gases are typified as ideal or real. The ideal gas follows certain gas Laws exactly,
whereas a real gas closely follows these Laws only at low density. Ideal behavior
can be ascribed to a real gas if its molecules are separated by very large distances,
so that intermolecular attraction is negligible.
Adiabatic Process
(ad-ee-uh-bat-ik)
Adiabatic compression and expansion are thermodynamic processes in which the
pressure of a gas is increased or decreased without any exchange of heat energy
with the surroundings. Any process that occurs without heat transfer is called an
adiabatic process.
The adiabatic compression or expansion of a gas can occur if the gas is insulated
from its surroundings or if the process takes place quickly enough to prevent any
significant heat transfer. This is essentially the case in a number of important
devices, including air compressors.
An adiabatic expansion is usually accompanied by a decrease in the gas
temperature. This can be observed in a common aerosol can, which becomes cold
after some compressed gas is released. The reason for the temperature drop is that
the gas is released too quickly to absorb any significant heat energy from its
surroundings. Work performed in expanding the released gas drains some internal
energy of the gas still in the can, making it colder. After the can metal becomes
cold, however, the process is no longer adiabatic.
In a similar fashion, adiabatic compression usually increases the temperature of a
gas, since work is done on the system by the surroundings. For example, when air
is pumped into an automobile tire, the air temperature rises as a result of adiabatic
compression.
To Next Page
Loading...
