Charles's law – constant-pressure gas experiment

What it shows

Charles's law states that for a fixed mass of gas at constant pressure, the volume is directly proportional to the absolute temperature: V/T = constant. Here the pressure cannot change because the mercury pellet slides freely, so the trapped air always supports atmospheric pressure plus the weight of the pellet. A graph of V against Celsius temperature is a straight line; extended back to zero volume, it meets the temperature axis close to −273 °C. This is where the Kelvin scale and the idea of absolute zero come from.

How to use

Set Gas volume at the initial temperature and Initial water temperature, then press Heat (or Cool to add ice). Each time the temperature changes by about 10 °C, press Record data; the table converts every reading to kelvin and calculates V/T. After five or six readings, press Heater off and study the graph: tick Extend the graph to 0 K to see where the line meets the temperature axis. Use Clear table to repeat with a different initial volume.

Parameters you can change

• Gas volume at the initial temperature 2–20 cm³
• Initial water temperature 0–40 °C
• Heating or cooling rate 1–10 °C/s
• Extend the graph to 0 K

Questions to explore

1. Why does the pressure of the trapped air stay the same while the mercury pellet moves up or down?
2. At what temperature does the extended V–t line meet the temperature axis, and what does that point mean?
3. If you start with a larger initial gas volume, how does the slope of the V–T graph change?