When the temperature of a confined gas is changed, the gas will change in volume if the pressure upon it is kept constant. A careful distinction should be drawn between a linear relation and a direct proportion in the present and many similar cases. The pressure here varies linearly with the temperature. It is also true that the pressure is directly proportional to the temperature in degrees Kelvin (absolute), but the pressure is not directly proportional to the temperature. This experiment measures the volume of an air sample at two temperatures, a high temperature and a low temperature. The volume of the air sample at the high temperature decreases when the sample is cooled to the low temperature. All of these measurements are made directly.
Gases expand when heated, and contract when cooled. At the end of the 18th century, this behaviour was investigated in detail by Jacques Charles, a French physicist with a passion for hot-air ballooning. Charles determined that the volume occupied by any sample of gas is directly proportional to its temperature, as long as the pressure is held constant. Charles’s law is summarized as:
VP = k T or, VP / T = k
where T is the absolute temperature (in Kelvin, K) VP is the volume (at constant pressure) and k is a constant of proportionality. Charles’s law implies that the volume a sample of gas occupies will approach zero as the temperature approaches zero on the absolute scale. Absolute zero is the temperature at which everything virtually stops moving, including molecules (although the atoms within the molecules still vibrate a tiny bit). Purposely cooling things to extremely low temperatures might therefore seem uninteresting, but in fact, as the temperature approaches absolute zero, the properties of some materials become very interesting indeed.
1. Prepare a set-up as described...