At constant pressure a gas expands in step with its absolute temperature, so V ÷ T stays constant. Drag the sliders below to set the initial volume and the two temperatures, and watch V2 = V1·T2÷T1 trace a straight line through the origin.
Warm a fixed mass of gas while letting a weighted piston keep the pressure constant, and its volume climbs in direct proportion to temperature. Slide the Initial temperature and Final temperature controls and the piston stretches or shrinks; slide the Initial volume and the whole response scales with it. The relationship is Charles's law, written as a ratio: V1/T1 = V2/T2, which rearranges to V2 = V1·T2/T1.
The panel computes the constant k = V/T for both states, and its central point is that the two values are identical at every slider position. That is what makes the volume-versus-temperature plot a straight line passing through the origin: double the kelvin temperature and the volume doubles exactly. The one rule you cannot break is the unit — temperature must be in kelvin, because the ratio only holds when it is measured from absolute zero.
That is why a jump from 20 °C to 40 °C raises the volume by only about 7%, not 100%: in kelvin it is 293 K to 313 K. To put exact numbers on any change, open the Charles's law calculator, then bring pressure into the picture with the ideal gas law simulator or browse the rest of our interactive physics labs.
At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature: V/T stays constant, so V1/T1 = V2/T2. Doubling the kelvin temperature doubles the volume.
Because the V/T ratio is only constant when temperature is measured from absolute zero. Celsius breaks it: 0 °C is 273.15 K, not zero, so a Celsius ratio gives the wrong volume change.
The pressure is held fixed, and the ratio k = V/T is constant. The simulator shows k for both states, and it reads identically at every temperature — that is the whole content of the law.
A hot-air balloon: heating the air makes it expand, lowering its density so the balloon rises. Gas cylinders and car tyres also show higher pressure or volume on hot days for the related reason.