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Sign Convention for Work (In Thermodynamics)

  1. Jan 25, 2016 #1
    Okay so after reading various books on sign conventions for work
    I am told by my teachers that in chemistry,work done on the system is positive while work done by the system is negative while in physics it's the exact opposite
    I only use the former convention
    And modify the first law of thermodynamics so that i can work with physics problems using this convention
    But what i want to know is
    Which one is correct?
    The former or the latter?
    Some insight is much appreiciated!:)
  2. jcsd
  3. Jan 25, 2016 #2
    It is convention. There is no "correct" on this as far as I can see. What matters far more than the sign convention is that we are consistent in our employment of the convention so that we all agree about what is actually happening physically.
  4. Jan 25, 2016 #3
    umm okay
    So which convention is more readily accepted?
  5. Jan 25, 2016 #4
    That will depend upon who is doing the accepting. :-) If you are working in chemistry, you better use the "Work is positive when done on the system" convention. If you are working in mechanical engineering, you will see many confused looks, and you might even hear some snickering, if you say something like that! The convention to use is the convention of the field you are working in, generally speaking.
  6. Jan 26, 2016 #5
    Okay i get it
    But to be honest,it's weird,you know,to use two different conventions for the same concept in 2 different subjects
  7. Jan 26, 2016 #6
    Welcome to the real world. If this were the only example of such a thing, it would be great. But it is only the tip of the iceberg. In solid- and fluid mechanics, some people treat tensile stress as positive and others treat tensile stress as negative. If you think that applying the sign convention for work is confusing, it doesn't compare with the confusion created by the opposite sign conventions for stress (which involve tensors).

  8. Jan 26, 2016 #7
    Only if by "weird" you mean "quite common". :)
  9. Jan 27, 2016 #8
    Well looks like I am not entirely prepared to face the full wrath of the real world XD
    And let's not forget the cartesian sign-convention in geometric optics :)
  10. Jan 27, 2016 #9
    I agree with what has been said about 'convention' it is a mutually agreed system....nothing more nothing less.
    My experience is very very limited to teaching introductory thermodynamics to A level students. All of this is related to gases and in this case the convention is easy to justify.
    When a gas expands (capable of lifting a weight) the work done BY the gas is positive +. When a gas is compressed, work done ON the gas (a weight being lowered) the work done is negative -.
    Good luck with this
    I am fairly certain that someone will disagree with my A level course's convention !!
  11. Jan 27, 2016 #10
    Some A level courses will call the work done ON the gas as positive and the work BY the gas as negative. In that case, ##\Delta U=Q+W##. I learned this stuff using your convention, but chemists often use the other convention.
  12. Jan 27, 2016 #11
    Its a minefield ! If you approach it from emphasis on HEAT energy can do 2 things...increase INTERNAL energy and do EXTERNAL WORK then my syllabus approach is Q = U + W means that expansion gives +W (lifting a weight.)
    If you look at it from the emphasis on internal energy then
    U = Q + W it makes some sense to label compression as +W (increases internal energy.
    You have to feel sorry for students.
    Sorting out W is the problem, I dont think there is any disageement about +/ - Q or + / - U
  13. Jan 27, 2016 #12
    It just takes a little getting used to. The important thing is that, if you are reading a paper or someone's analysis of a specific problem, you need to determine from the get-go what what sign convention they are using.
  14. Jan 27, 2016 #13
    Absolutely right! there is no right/wrong answer, I think an earlier post highlights a similar problem with sign conventions in optics.
    you have to feel sorry for students !!

    and have some sympathy for teachers !!
  15. Jan 27, 2016 #14
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