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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
- IIT JAM
- Physics (PH)
Sign convention in thermodynamics
explain me about the sign conventions like when work done is positive or negative similarly when heat is positive or negative (related to absorbed and released) & WHY SIGN CONVENTION IN PHYSICS IS OPPOSITE to CHEMISTRY
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Somnath
I understand your problem. It is little bit confusing for the beginners. Actually in physics Heat supplied to a system is taken as positive and workdone by the system is posive. Heat emitted the system and workdone on the sytem is taken as negative. The first law is written as dQ=du+dw which means if heat is supplied to the system a part of it will increae the internal energy of the system and remaining part will help the system to perform some work. But in chemistry Heat supplied the system is taken as positive and workdone on the system is posive. Heat emitted the system and workdone by the system is negative. The first law is written as dU=dQ+dW dQ=dU-dW Which also mean heat supplied to the system(+dQ) is equal to increase in internal energy(+dU) plus workdone by the system(-dW). Thus in both subject statement is same only convention is different. You may use any one of these. It depends on your choice but you have to mention the sign convention which you are using.
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Ruby negi
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dQ=+ve for heat absorbed by the system, dQ=-ve when heat is released.. dW=+ve when work is done by the system and dW=-ve when work is done on the system.. this is sign convention in Physics and this convention is opposite to convention in chemistry
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Ruby negi
physics is more concerned with obtaining work from the system, thus it assumes a positive sign for work done by the system. On the other hand chemistry deals more with changes in the composition of the working fluid or composition of the system-surroundings more in general. Thus, work done on the system which brings about changes in the system is considered positive.
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Dhairya sharma
when work done by the system it is supposed to be +ve because in this case volume increases while work done on the system is -ve because in this case volume decreases.
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Abhishek singh Best Answer
sign convention for heat exchange and work on/by the system is explained by 3 people. I just want to add an important thing, work done, W=integral(-PdV) . here this negative sign. is very important. Do make a note of it. One more thing about difference in physics and chemistry is, In physics our concern is on physical system(not on the gas/working substance), while in chemistry thier concern is on gas. Now think yourself that if you compress the piston you have to do work(+ve work for physicists), on the other hand, in compressing piston gas particles inside will feel work done on themselves (according to chemistry people). Give it a thought. Best regards.