3.2b Gas laws

By using the ideal gas kinetic model described above we can derive relationships between the macroscopic properties of gases (i.e. pressure, volume and temperature). By (rather artificially) keeping one of these properties constant we can derive a set of simple relationships between the variable properties. These are summarised in the table above, and described as:

• Boyle's Law: pressure and volume and inversely proportional (fixed temperature)

• Charles's Law: volume and temperature are directly proportional (fixed pressure)

• Pressure Law (or Gay-Lussac's law): pressure is directly proportional to temperature (fixed volume)

In practical situations, such as internal or external combustion engines, gases are heated and cooled, expanded and compressed. There are four common processes that are used either alone or in combination (these aren't required for the course - but I think they're kind of cool):

• Isothermal expansion and compression takes place (slowly) at a constant temperature

• Isochoric heating and cooling takes place in a constant volume

• Isobaric expansion and contraction takes place at a constant temperature

• Adiabatic expansion and compression takes place quickly, not allowing for thermal transfer between the gas and the surroundings