The Joule experiment

The Joule experiment was an attempt by James Joule in 1843 to determine for a gas expanding into a vacuum.

The experiment involves a system that consists of a gas-filled compartment (A) and an evacuated compartment (B), both of which are immersed in a water bath (see diagram above). When the valve between the compartments is opened, the gas expands irreversibly from A to B, doing no work in the process. Assuming that no heat transfer occurs between the system and its surroundings, the first law of thermodynamics states that . If there is a change in temperature of the water bath, the experiment measures the change in gas temperature with the change in gas volume at constant internal energy, i.e. , which is defined as the Joule coefficient :

To determine , we refer to eq16, where . Using eq15, . Substituting eq44 and eq88 in this expression,

For an ideal gas is independent of the volume of the gas, i.e. , because there is no intermolecular forces between the molecules (see eq43a). We would therefore expect a non-zero  for a real gas. However, Joule’s experiment setup was too crude to measure a non-zero . An improved version of Joule’s experiment is the Joule-Thomson experiment.


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