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Ideal Gas Law
The Generalized Gas Law can be written in a slightly different manner from the Generalized
Gas Law:
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. However, after the metal of the can becomes cold 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.
