The gas laws describe the results of experiments investigating the relation between the pressure, volume and temperature of a fixed mass of gas.

Consider a quantity of gas in a container of constant volume. If the pressure exerted by the gas is measured at different temperatures, the results are as shown by the graph below. (See experiment 5TP.)

If we continue the graph beyond the pressure axis we can find the temperature at which the pressure exerted by the gas should be zero.

This temperature is (about) -273°C. The pressure of a gas is due to the motion of its molecules so we must assume that at this temperature, the molecules have stopped moving. We therefore suggest that -273°C is the lowest temperature possible; it is the absolute zero of temperature.

These results do not depend on the type of gas.

This gives us the Kelvin or absolute scale of temperature. Temperatures on this scale are written TK, without any "degree" symbol. Thus 0°C becomes 273K, 100°C becomes 373K etc.

Note that the absolute zero of temperature has not been attained in practice; it is a theoretical prediction found by extrapolation of experimental results.

• Charles’ Law
• The Boyle/Marriotte Law
• Thermal Physics Chapters Index
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