Hello Why ds=0 only in a steady rev. process? Can't it be zore for a rev. and not steady process? And if the process isn't rev. but still circular. since the entropy is still a state function it should still be zero if we come back to the starting point ,right? if it isn't true then dU also shouldn't be zero in non rev. circular process. In a rev. process , we don't have to "complete the circle" in order to call the process a rev. process, right? please help me understand it.