h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

nice entry - but counter to what I thought of as the laws of thermodynammics... my backup is at http://www.cus.cam.ac.uk/~jae1001/teaching/mphil/MP3_2_handouts.pdf

Zeroth: "If body A is in thermal equilibrium with body B, and body B is in thermal equilibrium with body C, then body A is in thermal equilibrium with body C." (not mentioned)

First: "Energy is conserved when heat is taken into account" (this is correctly stated and explained)

Second: "The net entropy change in any closed system is always positive" (this is correctly stated and explained)

Third: "The net entropy change of any process tends to zero as the absolute temperature tends to zero." (vastly different)

Humm. Good entry, though, even if it does focus on the ever-popular second law...

I think I would have to agree with you there. From my hazy recollections of thermodynamics (9 AM lectures not really being good for the brain) I recall that Entropy has no definite zero point and so it is only changes in entropy that can be measured, not total entropy. I can, however, assure you that cheese DOES exist, unless my lunch was a figment of my imagination.

Wick

The third law in the guide entry was "The entropy of a perfect crystal is zero. A perfect crystal is one where there is no entropy (as mentioned) as the temperature in absolute zero. Zero entropy requires absolute zero, and absolute zero requires zero entropy, and both are impossible to attain.

But we can get close. As we approach absolute zero, entropy (which can only ever increase) change becomes smaller. So, "the net entropy change of any process tends to zero as the absolute temperature tends to zero." The definition given in the article and the one you gave are one and the same, just approach from different angles.

I've never heard that Zeroth one though.

Merlin the Time Traveller

Here's a statement that's definately in error:

"If you put a cold object next to a hot one, heat energy will flow out of the warmer object and into the cold one. Once the two objects have the same amount of heat energy, the flow stops."

Actually, heat flow stops when the two objects reach the same TEMPERATURE. This can be demonstrated fairly easily. Suppose you take a kilogram of iron, heat it up to 500 degrees Celsius and then toss it into a 50,000 liter tank of water at 20 degrees Celsius. The two objects will reach thermal equilibrium very quickly at about 20.00000000000000001 degrees Celsius. And at that point, the heat flow will stop.

However, the amount of heat contained in that 50,000 liter tank of water at 20 degrees Celsius is orders of magnitude greater than the amount of heat contained in the 1 kilogram lump of iron at 500 degrees Celsius. If you could apply all the heat available in that tank of water to that lump of iron, you'd almost certainly vaporize the iron. That's because water has a relatively huge specific heat. Actually, it's unity. Iron has a very low specific heat. In other words, it takes an awful lot of heat to make water hot, but it doesn't take very much heat to make iron hot. Heat is energy, temperature is power.

What I'm trying to say, Merlin, is that the third law is wrong... and the version I gave is correct...
{ok - I could have made it clearer, eh}

Agreed, you can't talk about thermodynamics without referring to the zeroth law. But I thought zeroth was the principle of conservation of energy, whilst the first was the Work-Energy Equivalence.

(what do I know? I can never remember what the third law is)

And I said that the version you gave, and the one given in the article, were one and the same, just approached from different angles.

Merlin the Time Traveller

You're right!

I always got the concepts of heat and temperature confused in high school. My statement is only accurate if the two objects are made of the same material... which obviously isn't always true.

Does anybody out there know precisely how to talk to the Editors about a correction to an entry? This should be fixed ASAP...

(Incidentally, I thought about including the zeroth law, but decided that it wasn't really one of the "three laws". It was added on much later, after all. Maybe I should try to add a link to an entry about it... Anyone interested in writing an entry on the zeroth law of thermodynamics?)

[bows respectfully]

--Caledonian

There is another bit too. Prigogine got a Nobel Prize for his work in 'Dissipative Structures': he pointed out that, while entropy increases overall, the flow of entropy can power a REDUCTION in entropy locally. For example, water flowing downhill can power a waterwheel, which produces electricity, which pumps water into a house on the hill. More radically, it produces life, intelligence, and other anti-entropic events. The cost is that the entropy is EXPORTED out of the system at an even greater rate. That is, the development of life and intelligence is not debarred by the laws of thermodynamics, but it also has an unavoidable ecological cost in the surroundings, where the entropy increases faster.

Peter

Taking into account that it is a very complicated issue, the article is informative and fine. But some statements are indeed inaccurate.

At one point the article suggests that little entropy means much energy (the adjective 'useful' is missing). The friction example is quite weak in that context.

I may have been worth mentioning that the Second Law is one of the very few physical laws that isn't always true. (In contrast to the First, for example). That's rather important, otherwise we wouldn't exist probably.

There are many ways to formulate the laws. I don't know, however, whether the two versions of the article and Lucinda are the same. I learnt an easy-to-remember equivalent of this Third Law, by the way also known as Nernst's Law: 'You can't reach absolute zero'. In the article, this is included as an assumption. But it is the Third Law itself.

I regret that I don't see these articles on the coming-up page. (Not old enough on h2g2 probably.) Now it's actually too late.

But, by and large, it's a good article. After all, I don't know one person who understands entropy or the Second Law completely.

My lecturer, some time ago, suggested the following approach to understand the 2nd law:

"Get a cup of hot coffee. As you sit and drink it, consider that as it cools, the entropy of the universe is increasing."

Although taken somewhat out of context, and it better implemented with a German accent, it has made me think, with no definite conclusions so far, apart from a cold cup of coffee! (....just like this bloody "one hand clapping problem".....)

Caledonian, you said: "My statement is only accurate if the two objects are made of the same material... which obviously isn't always true."

That's not exactly true either. Let me revisit my example. Suppose you heat a 1 kilogram chunk of iron to 500 degrees Celsius and then toss it onto structure containing 50,000 liters of iron at 20 degrees Celsius. Again the heat flow will stop when the two objects reach the same TEMPERATURE, which again would be approximately 20.000000000001 degrees Celsius. And again, the latent heat available in that 50,000 liter iron structure at 20 degrees Celsius is orders of magnitude greater than the latent heat in the 1 kg of iron at 500 degrees Celsius. The amount of heat required to change the temperature of the 50,000 liter iron structure from 20 degrees C. to 21 degrees C. would vaporize the 1 kg chunk of iron. Your statement would be accurate if and only if the two objects are made of the same material or have the same specific heat AND have the same thermal mass.

Hope this helps.

I agree with your revisions, although for the uninitiated, I think the "ever popular" second law can be simplified by saying "the entropy of the universe is increasing". This makes all three laws slightly easier to understand for those who do not make thermodynamics a major part of their lives. Basically it means that the universe proceeds towards higher and higher levels of energy dispersion. A good analogy is that of a bucket of marbles dumped on the ground. They bounce and roll around for a while, but eventually settle in a widely dispesed manner. If the marbles are analogous to the molecules of the universe, we can assume that eventually all energy in the universe will be dispersed widely enough that nothing moves, and then it's GAME OVER as they say.

Well, yes. You are, of course, correct.

I go into that much detail when I was writing the article itself, which is probably a good thing as it would have been dozens of pages longer if I had.

Thank you for correcting me...

Any chance you'll write an entry on the Zeroth Law, or possibly corrections to this one?

[bows respectfully]

--Caledonian

OKay .. Im sick and tired of people saying the the existence of life disproves the second law.

Ill use an example stolen from another thread becasue I like it ... the second law does not preclude localised decreases in entropy (ie .. us, life cheese etc) just as long as the total entropy in the system is increasing. Insert clever analogy about water rolling downhill and passing over rocks as it goes .. when the water swells over the rocks it isnt contravening the law of gravity, it is just a bump in the process. As are we. The falacy is in thinking that life is a closed system, which it obviously isnt.

~Beeblefish

Sorry Merlin, but I think the two versions are fundamentally different...

Caledonian's would also appear to be wrong. A crystal at near absolute zero would have a near zero entropy change, but it would still have entropy. For starters, one could convert the whole thing into energy via E=mc², and use it to do work - that alone implies that it is not at the lowest possible entropy.

Caledonian's also seems less general - it says nothing about the entropy at any point except absolute zero - only at absolute zero itself.

But what do I know - I gave all this stuff up ages ago...

Do you really think so or is your 'biggrin' smiley suggesting that you don't want to be taken too seriously? If your perspective was right, we could explain every loss in entropy just by defining our system big enough.

Life doesn't contravene the Second Law, only the very very first life form did (if it wasn't created by aliens). But as I said, it's a mathematical fact that the Second Law isn't always true. The smaller the system, the greater the chances to see it being invalid.

By the way, the law of gravity doesn't say that everything has to fall downwards. It just says that there is a force. The Second Law is stricter: Entropy must not be decreased.

btw - what exactly is thermal mass (and how does it differ from normal mass?)

No, Lucinda, you are wrong. The zero point of entropy is arbitrary and it's possible and convenient to set entropy to zero at zero temperature. Thermodynamics doesn't know Einstein's equivalence, so you can't use it here. (Sounds strange, but this is part of how physics works.)

By the way, at the moment I don't understand why a system at the lowest possible entropy couldn't do work. Not that I thought it's wrong -- I really don't understand it.

Despite that, I find your definition of the Third Law better than that in the article. (Would definitely have better chances in an oral examination.)

Hmm - I /am/ wrong. Dang.

A system at zero entropy would indeed be able to do work - by increasing its entropy - which would be the whole point () - I'd got my thinking the wrong way round...

There are many things about entropy which are arbitrary, and the zero-point is one of them... off the top of my head the ability to do addition on entropy, such that entropy of A plus entrropy of B = entropy of A+B, is another (which is where all the logs come from). However, I don't /think/ that these arbitrary decisions are part of the laws of thermodynamics.

Generally, the zero-point is arbitrarilly set such that entropy is zero when the number of possible ways in which a system could be organised (while still being in the same observed state) is one. The example of a crystal at absolute zero is not at this state - it will have (due to it's mass) some finite mass, which is equivalent to energy, and there are loads of ways you could organise a system with that energy. A vaccum at absolute zero, on the other hand, would have zero entropy according to this standard arbitrary zero point.

BTW, I have a recc'd entry on 'Entropy versus Information' at http://www.h2g2.com/A452387 which may be of interest to some here. Comments and criticisms are, as ever, very welcome. Especially if I'm wrong.