The discussion clarifies the distinctions between three types of gas expansions: Joule's expansion (free expansion), adiabatic expansion, and Joule-Thomson expansion. Joule's expansion occurs when a gas expands into a vacuum in a thermally isolated container, while Joule-Thomson expansion involves gas passing through a valve or porous plug under similar conditions. Both Joule's and Joule-Thomson expansions are irreversible processes, whereas adiabatic expansions can be reversible, such as when gas expands against a piston in a cylinder. The key takeaway is that types 1 and 3 are specific instances of adiabatic expansion.
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An adiabatic expansion involving a piston and cylinder is not necessarily reversible.andrewkirk said:I think that 1 and 3 are both specific types of 2, as both are defined to be adiabatic - which just means thermally isolated.
Type 1 is when a thermally-isolated container is divided into two parts separated by an openable divider, with one part containing gas and the other a vacuum, and the divider is then opened, to allow the gas to expand into the vacuum part.
Type 3 is a more complicated process (explained here) where a gas is forced through a valve or porous plug, and the system where this happens is thermally isolated.
Both 1 and 3 are irreversible processes.
An example of an adiabatic expansion that is neither type 1 or 2 would be where the gas in a cylinder expands by pushing back a piston, while remaining thermally isolated. I think (but am not sure) that that would be a reversible process.