In 1787, Jacques Charles discovered that in a closed system with a fixed mass of gas, the volume is directly proportional to the absolute temperature provided the pressure remains constant, as shown:

• Mathematically (Fig 1)
• Graphically (Fig 2)

Take the example of gas in a sealed cylinder, but with a moveable piston loaded to apply a constant pressure. If the cylinder is then heated, the pressure remains constant but the volume increases as the gas expands (Fig 3).

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