Sign Convention for Work (In Thermodynamics)

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Discussion Overview

The discussion revolves around the differing sign conventions for work in thermodynamics as understood in chemistry versus physics. Participants explore the implications of these conventions, their acceptance in various fields, and the resulting confusion for students and educators.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Meta-discussion

Main Points Raised

  • Some participants note that in chemistry, work done on the system is considered positive, while work done by the system is negative, contrasting with the physics convention.
  • One participant emphasizes that the choice of convention is a matter of consistency rather than correctness.
  • Another participant suggests that the acceptance of a convention depends on the field of study, with chemistry and mechanical engineering having different norms.
  • Concerns are raised about the confusion caused by having multiple conventions for the same concept across different subjects.
  • A participant shares their experience teaching thermodynamics, explaining how the convention can be justified in the context of gas behavior during expansion and compression.
  • Some participants express uncertainty about the implications of different conventions on understanding internal energy and work.
  • There is a recognition that students may struggle with these conventions, and sympathy is expressed for both students and teachers navigating these complexities.

Areas of Agreement / Disagreement

Participants generally agree that the sign convention for work is a matter of convention and that different fields may adopt different standards. However, there is no consensus on which convention is superior or more widely accepted, and the discussion remains unresolved regarding the implications of these differences.

Contextual Notes

Participants highlight that the conventions can lead to confusion, especially when transitioning between fields such as chemistry and physics. The discussion also touches on related issues in solid and fluid mechanics, where sign conventions for stress can vary.

UchihaClan13
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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?
As,usual
Some insight is much appreiciated!:)
 
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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.
 
umm okay
So which convention is more readily accepted?
 
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.
 
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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
:)
 
UchihaClan13 said:
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
:)
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).

Chet
 
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UchihaClan13 said:
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
:)
Only if by "weird" you mean "quite common". :)
 
Chestermiller said:
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).
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 :)
 
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 !
 
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lychette said:
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 !
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.
 
  • #11
Chestermiller said:
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.
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 don't think there is any disageement about +/ - Q or + / - U
 
  • #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.
 
  • #13
Chestermiller said:
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.

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 !
 
  • #14
hahahaa
 

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