1.9 The Gas Laws

Whilst air is not an ideal gas, it does conform, within close limits, to the results of Boyle's and Charles' laws.

Boyle's Law 

The volume (V) of a given mass of gas at constant temperature is inversely proportional to pressure (P):

            V = 1/P or PV = constant

 This can be expressed in the form:

              P₁ V₁ = P₂V₂

Charles' Law 

Charles' Law states that at a constant pressure the volume of an ideal gas is directly proportional to its absolute temperature.

The volume of a given mass of gas at constant pressure, increases by 1/273 of its volume at 0°C for every 1°C rise in temperature:

                   V α T 

                     or 

               V/T = k (Constant)

Charles' law also be expressed as:

                   V₁/T₁ = V₂/T₂

Combined Boyle's and Charles' Law Equation 

The results of both laws may be combined in one equation, expressing the behaviour of a gas under varying conditions of pressure, volume and temperature: 

                          P₁V₁/T₁ = P₂V₂/T₂

        Where 

            P₁ = Initial Pressure

            V₁ = initial Volume

            T₁ = Initial absolute temperature

            P₂ = Final pressure

            V₂ = Final volume 

Note: Temperatures are absolute temperatures measured only in Kelvin.