Talk:Internal energy

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[edit] External energy

Does anyone know what "external energy" is then? Is that enthalpy? Or am I making words up? -- postglock 07:41, 24 October 2005 (UTC)

I would make a good bet that "external energy" would mean the "internal energy" of the area outside the "well defined boundary. 68.6.112.70 00:34, 6 April 2006 (UTC)

External energy is the kinetic and potential energy of a system due to their common motion and position. So the total energy of a flying test-tube is its external KE and PE analysed classically as a projectile, plus its internal energy (due to the molecular motions and reactions).

[edit] "Body or system with well defined boundaries"

Can this be changed to "closed system" which is much more often used. I'm not sure if they're the same thing, but it looks like they are - and the definition now is sorta cumbersome. 68.6.112.70 00:34, 6 April 2006 (UTC)

Cleaned per request.--Sadi Carnot 19:28, 25 April 2006 (UTC)

[edit] Major Changes

I have made major changes to the sections following the introduction. The main problems were:

  • PΔV only represents mechanical work when pressure is held constant. Using infinitesimals is just as simple, as well as being correct.
  • The whole discussion about the sign of PdV belongs in the first law article, it does not help the reader to understand internal energy.
  • There was no extended intuitive explanation of the internal energy. I'm not sure the one I added is the best.
  • There is no expression for the internal energy in the entire article, only the change.
  • The other thermodynamic stuff seemed to have no point. I tried to rewrite the other thermodynamic material with the motivation of providing a number of expressions for the internal energy U (not the differential dU or the change ΔU).

PAR 01:49, 26 April 2006 (UTC)

[edit] U=TS-PV?

It is written here that

U=TS-PV\,

I tried to check it for a monoatomic ideal gas in 3D, and saw that it is not true. IT is true only for

U=TS-PV+\mu N \,

so:

  1. It should be cleared that the μN term is mandatory.
  2. This erise the question of how much this expression is usefull (and indeed, I don't recall ever using it).

eman 13:30, 2 June 2006 (UTC)

You're right. Strictly speaking, there are many types of "U"s in thermodynamics. At constant values for all intensive variables, U=TS-PV+\mu N \,, but often the chemical potential is incorporated into internal energy and the new thermodynamic potential is still called potential energy but it's represented as U[\mu]=U-\mu N \, where the chemical potential is a natural variable. So for this internal energy, U[\mu]=TS-PV \,. And then there's the multi-component, multi-phase systems. See [1] for the whole business. Maybe chemists, engineers, and physicsts have emphasized slightly different things in the past when it comes to nomenclature and a lot of the rigorous nomenclature is unfamiliar (at least to me), but I guess it's good that rigorous nomenclature is out there and hopefully it will be taught to undergrads soon. Whether the exact distinctions are too esoteric for this encyclopedia is debateable. Flying Jazz 04:01, 29 June 2006 (UTC)

[edit] Why is the rest mass not part of the internal energy?

Much of the mass of an atom lies in the nucleus, and that mass is not entirely due to the individual protons and neutrons, but also to their motions; similarly the mass of a proton is not just the mass of its three quarks but mostly due to their motions. For all we know all masses are due to internal motions, and it is the latter that makes up internal energy.

If one includes the rest mass as internal energy, then it is possible to know the internal energy of a system - measure its mass, convert to energy as mc^2 and subtract the external energy. Of course, this would include the nuclear energy which is usually excluded from thermodynamics texts. Chrystomath 2006.10.11

[edit] Does potential energy of fields count or not?

In the first section, it says that internal energy does not include potential energy due to gravitational or electrostatic field,s but later on it says that for the distribution of internal energy in a gas, some of it can come from gravitational, electric, or magnetic fields. Was this just a couple of edits that weren't checked, or do the fields in the gas come from other gas molecules and not outside? It seems like the second one would make more sense, but some clarification would be useful.

edit: looked up how to sign. 12.182.100.224 17:42, 24 October 2006 (UTC)

[edit] Possible error in the "The first law of thermodynamics section"

"Q is heat added to a system" "W is the mechanical work done on a system"

according to "Thermal Physics" By C. B. P. Finn (Page 27) this is just one convention, and some text books define positive Q as heat traveling from the system to the surroundings and possative w as mechanial work done by the system. As long as your consistant with your definition both conventions can work.

Obviously we should stick with the more widely used convention, but it might be worth mentioning that the alturnative convention exists. Otherwise a reader could get very confused if they come across the other convention. 81.137.148.225 16:09, 5 March 2007 (UTC) Melissa

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