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Talk:First Law of Thermodynamics (view source)
Revision as of 19:57, November 20, 2016
, 19:57, November 20, 2016no edit summary
:::Yesterday you changed the first paragraph of the article to something that wasn't true. Is what you've written here true? [[User:Karajou|Karajou]] ([[User talk:Karajou|talk]]) 13:38, 20 November 2016 (EST)
:::Yesterday you changed the first paragraph of the article to something that wasn't true. Is what you've written here true? [[User:Karajou|Karajou]] ([[User talk:Karajou|talk]]) 13:38, 20 November 2016 (EST)
::::I'm guessing you are talking about changing the sign of the work term from <math>-\delta W</math> to <math>\delta W</math>. The sign of <math>\delta W</math> and <math>\delta Q</math> depends on how you define those terms and is arbitrary. If we define <math>\delta W</math> as the work the system does on its surroundings, then the first law is:
::::<math>\,d U = \delta Q - \delta W</math>
::::If instead we define it as the work done on the system it is:
::::<math>\,d U = \delta Q + \delta W</math>
::::The magnitude of the work done is the same in both cases but the sign is different because we have defined it differently. Different textbooks use different conventions, but so long as you are consistent with your definitions of the terms, it doesn't matter and you will end up with the same answer to any problem.
::::I learnt the first law as <math>\,d U = \delta Q + \delta W</math>, so when I added it to the [[Thermodynamics]] page, that was the form I used. When I saw this page I edited it to change the differentials to be inexact for heat and work, and then to be consistent, changed the definition of work and its sign in the equation.
::::If you want you can change the [[Thermodynamics]] page to have the definition that was on here previously.
::::These are some sites that explain it. They use different conventions for the sign of work: [https://carnotcycle.wordpress.com/2013/06/01/on-exact-and-inexact-differentials/], [http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/State_Functions/Differential_Forms_of_Fundamental_Equations#First_Law_of_Thermodynamics]
::::The first explains what exact/inexact differentials are and the second shows an example of someone defining <math>\delta W</math> in the way I would. Note that inexact differentials are often denoted by a <math>\delta</math> or using a bar (I don't know the Latex for that)
::::Hope it helps [[User:PeterIceHockey|PeterIceHockey]] ([[User talk:PeterIceHockey|talk]]) 14:57, 20 November 2016 (EST)