Talk:First law of thermodynamics

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Contents 1 Separate article 2 Succinctly 2.1 Negative work: on or by the system? 3 A note on notation 4 Bank account analogy 5 Article is a total mess (regarding the sign of work) 5.1 Cut parts (temporally) 6 Sign convention in relation to Gibbs free energy 7 What were the experiments from Joule that contributed to this law?

[edit] Separate article

I agree with such reunification, on the basis that statistical mechanics defines internal energy as the mean of energy. The averaging is done on the set of microstates of the systemThorinMuglindir 21:51, 4 November 2005 (UTC)

Agree - I was going to do this myself, but Karol was quicker. PAR 00:23, 5 November 2005 (UTC)

Agree - I was tempted to do this myself but feared that there would be some resistance. Cutler 12:26, 6 November 2005 (UTC)

[edit] Succinctly

Hello - your recent edit states:

• The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process; or,
• The sum of heat flowing into a system and work done by the system is zero.

Mathematically, the first law is δQ=dU+δW. Mathematically the above two statements amount to the following

• If δQ=0 then δW is an exact differential or,
• δQ+δW=0

The first is true, but nowhere near a complete statement of the first law, the second is simply wrong. The sign of δW is wrong, and the dU is missing. PAR 23:06, 7 November 2005 (UTC)

A succinct statement would help this article. I am not particular to the math and a english version of the math would be apreciable to the non-mathemtician reading Wikipedia. I gotta go right now ... but it seem to be related to this article Thermodynamic potentials (from a search) ... I just got your msg, but have not the time to respond in full. Mabey tomarrow .... sincerel,y JDR 22:46, 8 November 2005 (UTC)

Here is the statement from the previous page:

The first law states that the amount of energy added to a thermodynamic system
by heating is equal to the increase in the internal or "stored" energy of the
system plus the amount of energy lost by the system as a result of work done
by the system on its surroundings.

What about this do you think needs improvement so we can fix it. PAR 01:46, 9 November 2005 (UTC)

It's a lil wordy ... a good start and doesn't really need improvement .... I'm not sure if this would be any better, but how about "The amount of energy added to a system is equal to the increase of internal system energy plus energy lost in its surroundings as a result of work done". But if not the prior is good with me ... hopefully someone else can suggest a briefer one. Sincerely, JDR 21:57, 9 November 2005 (UTC) (PS., even this one I did seems a bit word to me ... oh well)

Using your statement, I took a cut at it. PAR 22:09, 9 November 2005 (UTC)

Thanks for the note on my talk ... but I think either the version is good (and why I just indented it a few days ago and didn't change it) ... I don't have any objection to the sentence (just wish I was better @ words to 'condense' it) .... I just wanted something that got the concept out without the math or complicated word ... the main thermodynamic laws page need the statement too (I'll copy it over to the main page; which seems to need to be cleaned up ... mabey something I could take a stab at ...). I saw that you removed "thermodynamic", "or stored", and "work done by the system". I was just thinking that a nice short statement would be good for the article. The sentence is hard to cut down without losing needed parts. As I said before, mabey someone else will suggest a better one (though I think this doesn't get much attention; eg., trying to give laymen tersm to technical articles) ... but it may be "good enough". Sincerely, JDR

[edit] Negative work: on or by the system?

The statement in this section that the common convention in Physics is ΔU = Q − W is simple wrong. To quote authority Feynman, Landau and Liftshitz and Callen all use ΔU = Q + W. This is by far the most common convention, and since this is a physics article I propose changing the convention of the article to match the mainstream of physics. Nonsuch 22:46, 24 February 2006 (UTC)

As far as I know, Callen is the best authority, with Landau and Lifshitz high on the list. If they use the plus sign, then I am in favor of using it. Lets be sure and keep it as dU = δQ + δW using "d" and "δ" to differentiate between the exact differential and inexact differential. PAR 00:19, 25 February 2006 (UTC)

In the IUPAC pages, one convention is used here: http://www.iupac.org/didac/Didac%20Eng/Didac01/Content/T02.htm when it explicitly states dU = δQ + δW and then the other convention is used here: http://www.iupac.org/didac/Didac%20Eng/Didac01/Content/T12%20-%20T13%20-%20T14.htm in an actual problem that concludes with the common statement that W=Q (not -Q) for motors. The wikipedia article right now is confusing. One convention is stated once as "the usual", then the other convention is stated twice and applied to open systems. Flying Jazz 17:04, 26 February 2006 (UTC)

To me this section could be deleted entirely. There is already too much fluff in this article which obscures the physical basis of the first law completely. I would actually delete everything from the first equation to the math section but maybe it can be cleaned up.152.1.22.105 20:38, 3 March 2006 (UTC)

[edit] A note on notation

Many modern textbooks use upper case for internal energy and lower case for heat and work. This leads to equations such as dU = δq + δw. An instructor can use this notation (with care!) to emphasize that two different types of properties are being represented (state and path). -- Astrochemist 03:34, 15 September 2006 (UTC)

[edit] Bank account analogy

"As an analogy, if heat were money, then we could say that any change in our savings (dU) is equal to the money we put in (δQ) minus the money we spend (δW)."

Problems with this:

• We could just as easily switched the money put in and the money spent, which renders our choice of concepts (spending and spending) vacuous.

• We are using the definition utilizing a plus sign for δW and a minus in the analogy.

The analogy doesn't have to be perfect, but this will only confuse the laypersons which the analogy is supposed to make the article more accessible to. Maybe, we could change δW to interest, but then again that can't be negative. If there no objections, I'm removing this.Loodog 20:41, 17 July 2006 (UTC)

I think an analogy is a nice idea - shame the current one is appalling! LeBofSportif 20:51, 17 July 2006 (UTC)

You could also try this for the dU = δQ + δW sign convention: There are two ways for your bank account's balance (U) to be changed, legal methods (δQ) and illegal methods (δW). Whenever money comes in, you regard the change as positive, reflecting an increase in the balance. Whenever money is removed from your account, either by writing a check (legal, therefore δQ) or bank robbery (illegal, therefore δW) then it's a reduction to your account. Not perfect, I admit. There's no particular reason for associating "legal" with either δQ or δW, so take your pick as to which is used. -- Astrochemist 03:28, 15 September 2006 (UTC)

[edit] Article is a total mess (regarding the sign of work)

Yes, I know this is a valid argument and each field uses its own peculiar variation of work depending upon the system it is studying; however, this is the first law of thermodynamics, not the first law of physics. According to Amazon.com book sales, the top three thermodynamics books used are (#1) Cengel (2006), (#2) Van Ness (1969), and (#3) Fermi (1936); they all use ∆U = Q – W. According to Van Ness: “The equation is usually written in this form. The reason is that it is just an historical accident of history. The first applications of thermodynamics were made on heat engines, devices which take in heat and put out work.” I am going to give this article a quick clean and please simply use side notes in the article if you have an issue regarding how signs are used in each respective field or book. Later: --Sadi Carnot 00:25, 26 July 2006 (UTC)

[edit] Cut parts (temporally)

Essentially, the First Law of Thermodynamics declares that energy is conserved for a closed system, with heat and work being the forms of energy transfer. Heat is a process which transfers energy as a result of a temperature difference between a system and its surroundings. Mechanical work is the product of the force acting on a system and the distance moved in the direction of the force. However, work input can produce a temperature rise that is a mechanical equivalent of heat.

As an analogy, if heat were money, then we could say that any change in our savings (dU) is equal to the money we put in (δQ) minus the money we spend (δW).

The δ's before the heat and work terms are used to indicate that they describe an increment of energy which is to be interpreted somewhat differently than the dU increment of internal energy. In mathematical terms, heat and work are not exact differentials. Work and heat are processes which add or subtract energy, while the internal energy U is a particular form of energy associated with the system. Thus the term "heat energy" for δQ means "that amount of energy added as the result of heating" rather than referring to a particular form of energy. Likewise, the term "work energy" for δW means "that amount of energy added as the result of work". The most significant result of this distinction is the fact that one can clearly state the amount of internal energy possessed by a thermodynamic system, but one cannot tell how much energy has flowed into or out of the system as a result of its being heated or cooled, nor as the result of work being performed on or by the system.

After cleaning up and re-organizing the article these are the parts cut-out; if someone wants to re-insert parts of these, without duplicating anything, please do so.--Sadi Carnot 00:56, 26 July 2006 (UTC)

[edit] Sign convention in relation to Gibbs free energy

Moreover, the historical sign convention is the standard form used to define and derive the Gibbs free energy equation. Starting from the first law, and neglecting differential details:

From the second law we have:

Hence:

By substituting this into the first law, we have:

Rearranging we have:

Letting dW be pressure-volume work, we have:

By assigning the quantity to the left of the equals sign the symbol G, as Willard Gibbs did in 1876, this reduces to the following at thermodynamic equilibrium:

Or for a spontaneous process:

Thus, this expression is referred to by many as the combined law of thermodynamics; Gibbs showed that deviations of this quantity could be used to predict the direction of various natural chemical processes. The other sign convention doesn't facilitate this derivation.--Sadi Carnot 18:41, 26 July 2006 (UTC)

[edit] What were the experiments from Joule that contributed to this law?

"It was Jame Joule, sho first laid down the foundation of The First Law Of Thermodynamics, saying that The Heat and Work are mutually convertible through his extraordinary series of experiments."

The word "extraordinary" would entice readers to question what were these "experiments". The article should at least include one or two experiments into this sentence such that it would say:

"...through his extraordinary series of experiments such as [experiment 1] and [experiment 2]".

Joule Paddle Wheel experiment is certainly a famous one, but I can't be bothered to do sufficient research to make a worthy edit. Perhaps someone wants to look it up. It may even have a wiki article already. LeBofSportif 16:32, 7 November 2006 (UTC)

As far as I know, Julius_Robert_von_Mayer was the first (1842) who postulated the conservation of energy principle. Beeing a physican and not a physicist, he couldn't express and publish his idea according to the scientific standards of his time. Five years later Hermann_von_Helmholtz refined the first law of thermodynamic and disseminated the news. --Gunnar.Kaestle 11:04, 4 January 2007 (UTC)