Let's consider a ballon, such that it has an adiabatic wall, and there is no exchange of heat with the surroundings. Now, let the balloon wall be completely impermeable to the movement of matter across the boundary. If so, then the ballon is theoretically an isolated system. However, if we stretch the balloon, to increase its volume, then the temperature of the air inside the balloon will decrease. Will that mean work is done by the system on the surroundings, despite it being isolated?

Here I thus feel that the concept of isolation is actually a consequence and highly bound by definition. The balloon continues to remain isolated unless, some work is done on it, leading to it losing its property of isolation if some work is done on it.

Is my argument correct?

  • 1
    $\begingroup$ It could be said it is "heat isolated", not "work isolated". There are implied rigid walls for isolated systems. $\endgroup$
    – Poutnik
    Jul 9 at 6:38
  • $\begingroup$ You're trying to split a hair here. If the system is thermally isolated, you should say "thermally isolated", not something else. Be specific! $\endgroup$
    – Karl
    Jul 9 at 14:17

1 Answer 1


For a nice definition of an isolated system in thermodynamics see the Wikipedia article on isolated systems. The headline is:

In physical science, an isolated system is either of the following:

  • a physical system so far removed from other systems that it does not interact with them.
  • a thermodynamic system enclosed by rigid immovable walls through which neither mass nor energy can pass.

Your balloon is not an isolated system if it can do work on its surrounding or have work done on it by its surroundings.


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