# Energy conservation and thermodynamics

1. Dec 7, 2007

### Demystifier

I've seen many times people argue as follows:
"Energy must be conserved, because otherwise the first law of thermodynamics would be violated."
Clearly, such arguing is logically incorrect. The energy conservation law is much more general and fundamental, while the first law of thermodynamics is just a special case of the energy conservation law.
What I cannot understand is why such a conceptual mistake is so frequent? How it may appear to somebody that the first law of thermodynamics is more fundamental than the energy conservation law?

BTW, have you also noticed people make such a mistake?

Last edited: Dec 7, 2007
2. Dec 7, 2007

### Staff: Mentor

I think you are mixing the first and second laws. But I have never personally heard anyone say "Energy must be conserved, because otherwise the first law of thermodynamics would be violated" either.

Last edited: Dec 7, 2007
3. Dec 7, 2007

### Staff: Mentor

Did you mean first law of thermodynamics?

4. Dec 7, 2007

### Demystifier

Thanks, I have corrected the mistake.

5. Dec 7, 2007

Yes, thanks!

6. Dec 7, 2007

### dst

Actually, energy has to be conserved for anthropic reasoning & the anthropic principle to work. Simple as that.

7. Dec 7, 2007

### genneth

Always be careful to distinguish between "laws" which are actually postulates backed by direct empirical evidence, or "laws" which are a consequence of some other postulates. For example, I would say that the First Law of Thermodynamics is an empirically backed postulate -- to do with the observed equivalence between heat and work, and understanding them as two sides of the quantity we call energy. Energy conservation itself can be derived for a variety of systems, in a multitude of mathematical formalisms. The usual ones for classical and quantum mechanics yield as a consequence of time translation invariance of the system. The question of which is more "fundamental" requires a definition of fundamental. The First Law is more directly observed, but time translation invariance seems very neat mathematically.

8. Dec 7, 2007

### Demystifier

I would say, for example, that energy conservation in a collision of billiard balls is even more directly observed.

9. Dec 7, 2007

### rbj

i thought that the 1st law of thermodynamics is one and the same as the conservation of energy.

10. Dec 10, 2007

### Demystifier

But it is not! The first law of THERMODYNAMICS is a law of thermodynamics. Thermodynamics is just one branch of physics. Energy conservation is valid even out of the context of thermodynamics.
What I want to know, why do you think that this is the same? Can you point out a place where it is written so?

11. Dec 10, 2007

### Staff: Mentor

The 1st law is the thermodynamics version of energy conservation. It's the same law in perhaps slightly different wording than in other subjects.
http://en.wikipedia.org/wiki/First_law_of_thermodynamics

You basically already know this, so it looks like you are making something out of nothing here. Can you cite an example where you see it being used incorrectly?

Last edited: Dec 10, 2007
12. Dec 10, 2007

### Staff: Mentor

Sure, they are not "the same", since the energy conservation is more general. But clearly one is a subset of the other (and led us to the more general statement). And the applicability of thermodynamics itself is rather broad.

I don't have a textbook handy at the moment, but it's commonly taken that the first law is consequence or application of the more general law of energy conservation.

From hyperphysics: (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html" [Broken])
The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: ​

From wiki: (http://en.wikipedia.org/wiki/First_law_of_thermodynamics" [Broken])
In thermodynamics, the first law of thermodynamics is an expression of the more universal physical law of the conservation of energy.​

(Looks like Russ was writing up the same thing. )

Last edited by a moderator: May 3, 2017
13. Dec 10, 2007

### Count Iblis

First Law: Energy is conserved.

Second Law: Despite the first law, we still need to conserve energy. :rofl:

14. Dec 10, 2007

### Staff: Mentor

Not quite the same - I like the way you said it better. Saying one is a subset of the other is better than just saying that it is a different wording. But close enough that it isn't a hair I thought should be split in the first place!

I guess I've never really thought hard about it, but if I was debunking a PMM, I'd say:
"Lisa, in this house we obey the laws of THERMODYNAMICS!"
[-Homer]

But if someone was asking a pendulum motion question, I'd probably say it was a conservation of energy question.

Last edited: Dec 10, 2007
15. Dec 10, 2007

### cesiumfrog

Huh? I've always understood that "conservation of energy" is "the first first law of thermodynamics". What is this "out[side ..] of thermodynamics" of which you speak? Does entropy not increase in quantum physics, general relativity, nuclear processes, fluid mechanics, and electrodynamics? Is absolute zero temperature reachable in chemistry? Is the vacuum speed of light variable in entomology?

There was* a time when "thermodynamics" referred only to the study of steam engines. That time is over.

*Well, I imagine so. Or rather, "There may have been..."

Last edited: Dec 10, 2007
16. Dec 10, 2007

### wysard

No. But different colours of light have different speeds. All of them really, really fast, and within a close order of magnitude mathematically speaking, but not identical. So I guess that the speed of light is actually variable. Just within an very, very tight margin.

I wonder sometimes if the reason that red light is marginally faster than blue light might have some odd relationship with an unforseen or inherent doppler effect, but ~sigh~ I have work to do and no time for a PHD thesis...

Last edited by a moderator: Dec 11, 2007
17. Dec 11, 2007

### rbj

the fact is that heat is not some completely different kind of energy than the other forms of energy we know of. whether it's heat or electricity or a speeding automobile, all of this energy is essentially the same mechanical energy. with electricity, it's the mechanical energy associated with the movement of charged particles. with heat, the mechanical energy is associated with the random movement of the molecules of the substance, whether it's solid, liquid, or gas.

thermodynamics is another branch of physics, and it makes use of a lot of probabilistic or statistical (or "stochastic", if you like a fancy word) analysis. but it still just boils down to the same mechanics that precedes it. temperature is not a unique physical quantity like time, length, mass, or electric charge. it's a measure of average energy per particle per degree of freedom.

when the 1st law of thermodynamics says that thermal energy is conserved, it is simply submitting to the general Conservation of Energy law of classical physics.

18. Dec 11, 2007

### rbj

from what i read, it was the higher frequency photons that were thought to have gone faster.

is this variable speed of light (as a function of frequency) widely accepted physics or a hypothesis used to explain the early inflation of the universe (that appeared to have inflated faster than it could have if speeds were limited to c)?

19. Dec 11, 2007

### Staff: Mentor

Are you saying that different frequencies of light have difference speeds in vacuum? (It's certainly true in many materials, such as glass, but not in vacuum.) That's not accepted physics so far as I know. If I'm wrong, please give a reference.

20. Dec 11, 2007

### rbj

i don't think you're wrong, Doc, but i started reading Smolin's Trouble with Physics book. in it somewhere he writes of a VSL theory he developed with someone else that i can't remember (João Magueijo, perhaps?) in which, when the Universe was young and very very hot, that the high energy (and high frequency) photons moved faster back then than they do now, thus explaining how the inflationary period of the expansion of the Universe did not violate the speed limit of c.

i still have the same reservation i had after first reading the response that Michael Duff had to VSL. (the operationally meaninglessness of VSL.)

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