Gay-Lussac's Law: pressure and temperature at constant volume
The physics behind P1/T1 = P2/T2, why the volume must stay fixed, the Kelvin requirement, and where the relationship shows up in daily life.
What the law says
Gay-Lussac's Law describes how the pressure of a sealed, fixed amount of gas responds to temperature when the volume cannot change. Heat the gas and its molecules move faster, striking the container walls harder and more often, which raises the pressure in exact proportion to the absolute temperature. Cool it and the pressure falls the same way. The compact statement P1/T1 = P2/T2 captures that: the ratio of pressure to Kelvin temperature stays constant through the change.
Why the volume has to stay constant
The clean proportion only holds when the gas is trapped in a rigid container so its volume is locked. Let the volume change and pressure now depends on two things at once, which pushes you toward Boyle's Law or the combined gas law. That is why the tool prints constant volume next to the equation: it is the assumption that makes the single-ratio relationship valid. A sealed metal can or a rigid gas cylinder is the classic physical setup this law is built for.
The absolute-temperature rule
Temperature must be measured from absolute zero for the proportion to work, because the law compares how much thermal energy the gas has, not an arbitrary point on a thermometer. Celsius places its zero at the freezing point of water, which is not the zero the physics cares about, so a Celsius ratio gives the wrong answer. Converting to Kelvin by adding 273.15 fixes that, and this calculator does the conversion for you in both directions. Forgetting this step is the single most common mistake students make with the gas laws.
Where you meet it in real life
The pressure rise in a heated sealed container is Gay-Lussac's Law at work. It is why an aerosol can carries a warning against heat, why tyre pressure reads higher after a long drive warms the air inside, and why a pressure cooker builds force as it heats. Engineers lean on the same relationship when they size relief valves and rate vessels for temperature swings. Recognising it turns a scary sounding warning label into a predictable, calculable effect.