Sign Convention for Work (In Thermodynamics)

  • #1
145
12

Main Question or Discussion Point

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!:)
 

Answers and Replies

  • #2
275
95
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.
 
  • #3
145
12
umm okay
So which convention is more readily accepted?
 
  • #4
275
95
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.
 
  • Like
Likes winx
  • #5
145
12
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
:)
 
  • #6
20,205
4,248
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
 
  • Like
Likes JC2000
  • #7
3,741
418
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". :)
 
  • #8
145
12
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 :)
 
  • #9
413
72
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 !!
 
  • #10
20,205
4,248
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
413
72
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 dont think there is any disageement about +/ - Q or + / - U
 
  • #12
20,205
4,248
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
413
72
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
145
12
hahahaa
 

Related Threads on Sign Convention for Work (In Thermodynamics)

  • Last Post
Replies
2
Views
836
  • Last Post
Replies
3
Views
4K
Replies
9
Views
8K
  • Last Post
Replies
3
Views
931
  • Last Post
Replies
4
Views
1K
Replies
8
Views
9K
  • Last Post
Replies
20
Views
2K
  • Last Post
Replies
6
Views
10K
  • Last Post
Replies
6
Views
854
  • Last Post
Replies
2
Views
4K
Top