In thermodynamics work done by the system is negative but in case of expansion of gas it is positive because force and displacement are in same direction, why?
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asked Sep 13, 2015 at 1:58
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$\begingroup$ Because force and displacement are in same direction, just like you said. Really, what a strange kind of science the thermodynamics would be if the work were always negative? $\endgroup$– Ivan NeretinCommented Sep 13, 2015 at 2:03
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$\begingroup$ But it should be negative as per convention adopted $\endgroup$– ashwini abhishekCommented Sep 13, 2015 at 2:16
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2 Answers
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You are correct. The system does positive work. Textbooks introduce the negative to show that the system loses energy as it does expansion work. As the gas expands, it does positive work but it loses this energy in the process. Work, like heat, is basically a transfer of energy.
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I think it is due to pressure which is negative in the pressure volume work equation actually here two types of pressure act at a time one is external other is internal so net effect of external is less then internal which is taken negative.
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1$\begingroup$ Your very long answer including just one sentence is not easy to read. Could you split it up into shorter sentences? $\endgroup$– JanCommented Nov 12, 2015 at 15:39