The formula
P₁ · V₁ / T₁ = P₂ · V₂ / T₂
| Symbol | Quantity | Unit |
|---|---|---|
| P₁ | Initial pressure | Pa |
| V₁ | Initial volume | l |
| T₁ | Initial temperature | K |
| P₂ | Final pressure | Pa |
| V₂ | Final volume | l |
| T₂ | Final temperature | K |
What it means
The combined gas law says PV/T stays constant for a fixed amount of gas. Hold the temperature and you get Boyle's law; hold the pressure and you get Charles's.
How to work it out by hand
- Convert both temperatures to kelvin
- Work out P₁V₁/T₁
- Rearrange to find whichever final value you need
What is worth knowing
Kelvin is not optional here either, and for the same reason as the ideal gas law: the relationship is proportional to absolute temperature. This is the physics behind why tyre pressure drops in cold weather — the volume is roughly fixed, so pressure falls with temperature, and a 20-degree drop costs about 7 % of the pressure. It also explains why a sealed bag of crisps swells on a plane and why a balloon shrinks in the fridge. All three are the same law with a different quantity held constant.