I have found a statement in a book saying that "In anyone non flow process there will be either heat supplied or hear rejected but not both similarly there will be either work input or work output but not both, Hence $$U_2 -U_1 = Q + W $$

Now what is meant by either one and not both?

  • $\begingroup$ can you state the question more clearly? $\endgroup$
    – Bruce Lee
    Commented Dec 27, 2015 at 2:28
  • $\begingroup$ I suppose it means there can only be one direction of heat flow / work. Either the engine is doing (net) work, or you are doing (net) work on the engine. But I am speculating without having further context. $\endgroup$
    – Floris
    Commented Dec 27, 2015 at 2:47
  • $\begingroup$ Floris is probably up to something there, but it's a bit more complicated, depending on how they define "flow process". The statement seems to make much more sense for homogeneous processes. In practice there always has to be some form of heat flow, it is simply assumed to be of no importance. $\endgroup$
    – CuriousOne
    Commented Dec 27, 2015 at 3:13

1 Answer 1


The statement seems incorrect. Just consider the case of a reversible isothermal expansion of an ideal gas in which the heat input is equal to the work output, and there is no change in internal energy.,


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