Laws of thermodynamics

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hey guys, we covered the laws of thermodynamic in physics, but I don't understand why they are laws. What's the proof for them?

Repeatably verifiable experimental evidence.

Did you take the course without even one laboratory session?

Is it in an under-funded public school or state university?

Thermodynamics is a part of how everything works. All correct ways to analyze any system will be consistent with the laws of thermodynamics. One is welcome to try and find a counterexample, but so far none have been found. There are rules against patenting anything that is inconsistent with the laws of thermodynamics because it saves the examiners' time by keeping them off of things that won't work.

I'm a bit rusty on the subject, I did physics 30 years ago. I seem to remember that Maxwell came up the core of thermodynamics and that they are deductible from the other laws of physics. However was it the direction of entropy that was not deductible but based upon empirical evidence? Somebody remind me.

How can you prove that something applies universally through an experiment? Also, I don't understand why a Carnot machine is the most efficient machine possible

I'm a bit rusty on the physics, and thermodynamics was my least favourite subject, but these are roughly the definitions as I understand them:

The first law says: You can move an aeroplane with propellor 1 and you can turn propellor 2 with the motion of the plane. But you can't run propellor 1 off propellor 2 and expect the plane to move.

The second law says: If you leave a hop cup of tea in a sealed room, the tea and the room will eventually become the same temperature as the energy moves out of the tea and into the room. But you can't get it back in the tea again without dissipating the same amount (in real life a greater amount) of energy somewhere else, by running a pump or something.

The third law says: There is a minimum temperature at which no more heat may be extracted from a substance, even if it still has a bit of energy left.

There's another way of putting these:

First law: You can't win
Second law: You can't even draw
Third law: You can't get out of the game.

I think that's meant to be a joke.

I don't think you can prove any of these in a school laboratory - the quantities (temperature aside) are too hard to measure. If I can give you a tip though; if you don't understand something in physics, try to prove it wrong. Try to think of reasons why the law can't be true and then, when you're proved wrong, you'll understand what you missed. And maybe one day you'll be right about something.

Maxwell came up with the critical equations for describing ELECTROMAGNETIC theory.

There are three laws of thermodynamics. There is also the Zeroth law of thermodynamics which has to do with measuring temperature.

He also came up with laws for thermodynamics as well, for which he is perhaps lesser known. His most memorable, to me anyway, was the principle which became known as Maxwell's monkey. As a physics student I came up with a thought experiment that could in principle bypass some of the principles of thermodynamics to pick background heat up and convert it into useful work contrary to the laws of thermodynamics. I confounded my physics lecturer at the time who could not fault my theory but it turned out Maxwell had already thought about this "loophole" and decreed that it would never be possible to exploit it. It is also called Maxwell's Demon.

I'm still a little suspicious that my thought experiment would have worked despite Maxwell. For it not to work would mean some pretty strange things would need to happen at the microscopic scale of matter. If there are any physics experts here I'll share the thought experiment.

Last edited by TallyMan on 08 Jan 2009, 5:24 pm, edited 1 time in total.

By repeating the same experiment - with different equipment and experimenters each time - and verifying that the results are the same and/or match the same theoretical foundations. This has been done since before the Laws of Thermodynamics were codified, and their validity in all reference frames has been consistently verified.


Well, there you go! A lack of understanding of physical laws often leads to their disbelief.

The thing I used to enjoy most about physics was looking for loopholes. Many physics laws are based on assumptions about the circumstances in which the experiments are done. While this works in practice, things can sometimes go a little haywire if you break some of the assumptions, very much as Einstein did with Newtonian mechanics.

You then find that the original theory was just a special case in a more grandiose and encompassing theory. Physics is fun. I wish I'd made a career of it.

Oh yeah, Maxwell's demon! That's a little creature who sits at the dividing wall between two chambers, each filled with gas at the same temperature. He has a little door, which he can open and shut, to let the gas molecules through. Everytime he sees a faster (hotter) than average molecule, he opens the door and everytime he sees a slower one he shuts it. This way heat flows from a cold object to a hot object, contrary to the second law of TDs.
The catch is he has to move energy to actually open the door, and also to identify which molecules are quick and which are slow (maybe he shines a light on them.) The entropy gain in these actions is always larger than the entropy loss in letting the molecule through the door, even for an imaginary creature.

Well, that's a fundamental problem in all science, not just thermodynamics. Generally, we assume that we can use induction in order to reason about things in general from a few instances. There are considerable problems with this manner of thinking (see: The Problem of Induction), but without it we could do pretty much nothing except mathematics. We simply can't use deduction to reason about the universe, so experimentation &c is what we've got to use (unless, of course, we have divine revelation at our disposal...).

I'd say that the issue is the cultural climate that these things were discovered in. During that time, there was a strong belief in "laws of nature", and this was across many fields, it is even found in earlier economics works. Economics for the most part has softened up on these matters due to a lot of questioning of their mechanics, but physics retains these "laws" as sort of a sign of a past scientific culture, and because there is no real reason to doubt them.

But right, a big issue in science is that it hasn't always held to the same conventions and ideas throughout the years, and this is one of the holdovers.

The big issue in Science is that it is self-correcting. Any violation of the Laws of Thermodynamics would result in their revision.

Wikipedia: Physical Law

They are laws because they are based on empirical observation, and verifiable through experiments. Its not possible for us to discover (at least now) if something is completely universal and will work on the other end of the universe, but no evidence on earth suggests otherwise.

Not every physics equation is 100% exact, most just give a rough but usable estimation as not all variables can be taken into account. Its near impossible to create a single gas law that is based on the movement of EVERY single atom, but you can make a rough estimation using multiplicities and such to understand whats going on.



Carnot machines are not real machines. Due to the fact that a real engine is mechanical, there must be a loss of energy somewhere and an increase in entropy. A theoretically perfect Carnot engine operates at a maximum thermal efficiency of n=1-(Tc/Th) where Tc is the temperature of the cold reservoir and Th is the hot. Carnot engines can be made, but they seldom come close to this efficiency and as a matter of fact, they produce very little work in the process. Contrary to this, most internal combustion engines run at an efficiency of much less than 0.40.

Much of thermodynamics is experimentally based, especially given its enormous practical application. However it is entirely possible to place it on firm theoretical foundation. This can be observed right away by noting that Maxwell was not, by any stretch of the imagination, an experimentalist, yet laid much of modern thermo down.

The key is that it is not typically referred to as thermodynamics when brought up in a theoretical context, instead the theory is called statistical mechanics, i.e. the application of probability and statistical methods to classical mechanics.

Statistical mechanics only requires the laws from classical mechanics (i.e. Newton's laws, F=MA, etc), the theorems of probability theory (which is mathematics), and an assumption of so-called "equiprobable microstates", which without explaining what that is, is fairly acceptable as a general physical principle.

From these laws and principles, it is possible to derive the above laws of thermodynamics (in a statistical/probabilistic formulation)