Charles's Law: formula, Kelvin, and worked examples
Understand V1/T1 = V2/T2, why absolute temperature matters, how to rearrange for any variable, and worked numbers you can check.
The law and where it comes from
Charles's Law describes how a gas held at constant pressure changes volume as its temperature changes. Named after Jacques Charles, who studied hot-air balloons in the 1780s, it says the volume and the absolute temperature stay in a fixed ratio. Comparing an initial state and a final state gives the working form V1/T1 = V2/T2. It is one of the simple gas laws that combine into the ideal gas law, alongside Boyle's Law for pressure and volume and Gay-Lussac's Law for pressure and temperature.
Why Kelvin, not Celsius
The proportional relationship only works when temperature is measured from absolute zero, the point where molecular motion is at its minimum. On the Celsius scale zero is just the freezing point of water, so dividing two Celsius readings gives a meaningless ratio. Converting to Kelvin fixes this: add 273.15 to any Celsius value. That is why a change from 27 C to 327 C, which looks like a factor of twelve in Celsius, is really a change from 300.15 K to 600.15 K, a factor very close to two.
Rearranging for any unknown
Because V1/T1 = V2/T2 is a single equation with four terms, knowing any three lets you solve for the fourth by cross-multiplying. Final volume is V2 = V1 times T2 divided by T1. Final temperature is T2 = T1 times V2 divided by V1. Initial volume is V1 = V2 times T1 divided by T2, and initial temperature is T1 = T2 times V1 divided by V2. The calculator picks the right rearrangement automatically based on the 'Solve for' choice, but knowing the algebra helps you sanity-check the output.
A worked example you can verify
Suppose a balloon holds 2 litres at 300 K and you warm it to 600 K at the same pressure. Plugging into V2 = V1 times T2 divided by T1 gives 2 times 600 divided by 300, which is 4 litres, exactly what the tool returns. If instead you know the balloon grew from 2 to 3 litres while starting at 300 K, solving for the final temperature gives T2 = 300 times 3 divided by 2, or 450 K. These clean numbers make Charles's Law a favourite for teaching the link between heat and expansion.