Lei de Charles (V1/T1=V2/T2)

Charles's law: V₁/T₁ = V₂/T₂

Keep the pressure fixed and the volume of a set amount of ideal gas tracks its absolute temperature directly: V/T = constant. Jacques Charles found this around 1787. One catch: the temperature has to be in kelvin. Plug in °C and you get nonsense, sometimes negative volumes. Heat 10 L sitting at 273 K (0 °C) up to 546 K (273 °C) and you land at V₂ = 10·(546/273) = 20 L. It is also why hot-air balloons fly. Heating the air inside makes it expand, some of it slips out the open bottom, the balloon's average density drops below the cooler air around it, and buoyancy does the rest.

Applications

Think Montgolfier hot-air balloons and sport ballooning, airships and dirigibles, gas thermometers, and refrigeration cycles. It also turns up in lung-volume estimates that correct for body temperature (BTPS in spirometry) and in forensic work that gauges how tyre pressure shifts between cold and hot weather.

FAQ

Why kelvin, not Celsius? Because V ∝ T breaks down on the Celsius scale. Volume does not hit zero at 0 °C. In the ideal-gas extrapolation it reaches zero at −273.15 °C, which is 0 K. So convert first: T(K) = T(°C) + 273.15.

What if pressure also changes? Reach for the combined gas law, P₁V₁/T₁ = P₂V₂/T₂. Charles's law is just what happens when the pressure holds (P₁ = P₂).

Why does a balloon shrink in the cold? Charles's law says that at fixed P a lower T means a lower V. Carry a balloon from 25 °C (298 K) out to −10 °C (263 K) and it shrinks to roughly 88% of where it started.