# Second law of thermodynamics

1. Jun 7, 2006

### francisco

can the second law of thermodynamics be defied? in other words, can mother nature be fooled somehow? have we (homo sapiens) accomplished such a thing with religion? for example, the seventh day of the week is nothing but a positive entropy change from sunset to sunset. can we somehow use that positive entropy change to defy the positive entropy change of the first six days of the week?

"if you observe a system in which the entropy appears to decrease, you can be sure that somewhere there is a change in the entropy of the environment large enough to make the total entropy change positive." (physics by resnick, halliday, and krane, 4th ed. p 585)

can we somehow change this statement? for example,

if you observe the first six days of the week in which the entropy appears to decrease, you can be sure that there is a change in the entropy of the seventh day large enough to make the total entropy change positive.

i am searching for something, but it is like searching for a needle in a hay stack.

i need some feedback, criticism, and exchange of ideas regarding this new thread. thank you.

2. Jun 7, 2006

### Gokul43201

Staff Emeritus
The thing with science is that it can not address human misperceptions of nature, so it really doesn't have anything to say about entropy "appearing to decrease", other than "I'm sorry Francisco, but your measurement is probably wrong".

It's actually like searching for an elephant in a haystack. A very large elephant...in a very small haystack.

If the elephant were there, someone would have been bound to see it.

3. Jun 7, 2006

### rcgldr

Maybe someone can explain this. A lot of human activity, such as construction, seperation of chemicals, and reproduction ... seem to be decreasing entropy. Where's the "waste" product of these activities to compensate and result in an overall increase of entropy?

4. Jun 7, 2006

### vanesch

Staff Emeritus
In short, no you can't. If you observe the first six days of the week a decrease in entropy (AND YOU ARE SURE THAT YOU HAVEN'T FORGOTTEN A CONTRIBUTION SOMEWHERE), then you've falsified the second law of thermodynamics.

If you observe apples fall up from an apple tree, and you're sure it is gravity which does so (and not some joker with some nylon wire, say), then you've falsified Newton's theory of gravity.

The point is that nobody has ever witnessed a decrease of entropy during six days. That's why we still think that the second law of thermodynamics is valid. We have had (and have more and more ) people who THOUGHT that they witnessed a decrease in entropy during six days, but that's simply because they forgot to take all contributions into account.
A common example is "human constructive activity", or "life" or...

The second law doesn't say that entropy has to increase EVERYWHERE. You can have a decrease here, on the condition that you have SIMULTANEOUSLY an increase elsewhere. The most common increase is the photon flux: low-entropy photons (high-energy, such as visible light) from the sun IN, high-entropy photons (low energy, thermal infrared radiation) OUT. It's our main "absorber of entropy".
Anything that contributed in this transformation can have a local decrease in entropy (such as a growing plant) because it is set of by the increase in entropy in the photon flux.

5. Jun 7, 2006

### Gokul43201

Staff Emeritus
Vanesch, he does not conjecture that you observe an absolute decrease but only an apparent[/i] decrease.

6. Jun 7, 2006

### vanesch

Staff Emeritus
Ah. Well, then there is an apparent violation of the second law

7. Jun 7, 2006

### LURCH

My biology textbook in highschool defined life as the ability to reverse entropy (however innefficiently or temporarily). so in that sense, entropy can be reversed within a system, but only at the expanse of increased entropy outside (but somehow connected to) that system.

Jeff brings up another example of appearent reversal (using the construction sight as an example), but the food energy used up by the workers and the feul burned by their equipment constituted as much energy as was required to organise and assemble the structure they've built, (and then some). Entropy within the system was reversed only if we define "the system" as that part of the process wich built up the complex organised structure, and excluding the energy expediture that went into building it.

8. Jun 7, 2006

### vanesch

Staff Emeritus
Mmm, then my fridge's alive

This is a strange definition of life, no ?

9. Jun 7, 2006

### Andrew Mason

Ok. That would explain all green stuff at the bottom under the Christmas meatloaf.

To verify the hypothesis, try turning the fridge off for a few days to increase the entropy and see if it stops begin alive.

AM

10. Jun 7, 2006

### LURCH

Yeah, it's a bit oversimplified, but it's the best definition I know of for answering the question of why flame is not considered a living thing. You've heard it before, right?
"Flames eat, breathe, grow, move and reproduce, but they're not alive; why?"

Because fire is enegry being released as things break down. Life takes in energy and builds itself up into more and more complex and organised structures. Your fridge can delay the decay of a slab of beef inside it, but it cannot reverse that decay. Now, if your fridge were capable of taking raw materials from nature, seeking out its own power source without the need of an outside intelligence to plug it in, and use that power source to build those raw materials into beef, and continue doing so as older bits decay, so that the overall amount of beef it contained were constant, or even increased as time went by, and could also use that same manufactured beef to form and maintain its own operating parts, and use the surpluss to make other refridgerators out of beef that could carry on the process, then it would be alive.

Of course, it wouldn't be a fridge anymore, it'd be a cow. (Like that delightfull quote from the new Dr Who, where he defines life as it looks to medical nanobots as, "nature's way of keeping meat fresh").

11. Jun 7, 2006

### rcgldr

But most of the expediture of the energy used during contruction results in heat (from the machinery), much of which is captured by the Earth's atmoshpere, heating up the earth relative to the the rest of the universe. Since much of the heat energy is trapped, and takes a long time to dissipate via radiation, how is this an increase in entropy?

For a better example, what if alll of the constuction was done by robots and machinery powered from solar energy? Where is the increase and decrease in entropy under these circumstances?

What if the heat energy from the sun is simply captured? It's my understanding that entropy increases when the earth radiates heat, cooling off the earth, and eventually heating up other objects in the universe. If instead, the heat were captured, the earth would get hotter, and the rest the universe would end up cooler due to less radiated heat from the earth, and this would be a decrease in entropy.

Last edited: Jun 7, 2006
12. Jun 7, 2006

### 3trQN

A human is a complex chemical fire :P

13. Jun 7, 2006

### Rach3

Theological thermodynamics... :yuck:

14. Jun 7, 2006

### Ivan Seeking

Staff Emeritus
..heat....

15. Jun 7, 2006

### Data

As above, mainly heat probably. There'll be increases from other things too, of course. For complicated systems, obviously it gets hard to do real calculations. But in situations like this, physics is more useful as a predictive tool. If you can build a system like that and get perfect efficiency, then you'll be famous! If you don't have perfect efficiency then there's an entropy increase somewhere.

16. Jun 7, 2006

### rcgldr

In my last situation, I'm only talking about heat and temperature. Entropy increases when the temperature of everything in the universe starts approaching the same value. In the situation I described, the average temperature of the earth would be getting hotter, and everything else would be getting less hot than it would if the earth was radiating the heat instead of capturing it. Where's the increase in entropy in this situation?

If there was an earth bound process that converted this captured heat into another form of energy, like kinetic, such as changing the rotational rate of the earth, then the earth remains at is current temperature while radiating less heat to warm up the rest of the universe. Again, where's the increase in entropy in this case?

17. Jun 7, 2006

### Ivan Seeking

Staff Emeritus
Sorry, I had skipped right over this somehow. It has to do with useful energy. When the photons from the sun impact the earth and get used to do work, the energy becomes less accessible. We could make a heat engine that runs using the earths temp compared to the temp of deep space, but this would yield less work - it would be less efficient - than an engine [or in this case, a motor] designed to run directly from the solar panels discussed. This loss in "useful" energy is represented by the increase in entropy. See also Carnot.

Last edited: Jun 7, 2006
18. Jun 7, 2006

### Gokul43201

Staff Emeritus
All the heat from the Sun gets to the earth and the earth doesn't radiate to the rest of the universe, right?

That rate of change of the Sun's entropy is:

$$\frac{dS_{Sun}}{dt} = \frac{1}{T_{Sun}} \frac{dQ_{Sun}}{dt} ~~~~---(1)$$

And the rate of change of entropy of the earth is :

$$\frac{dS_{Earth}}{dt} = \frac{1}{T_{Earth}} \frac{dQ_{Earth}}{dt} ~~~~---(2)$$

Also, since all the heat from the Sun gets to the Earth,

$$dQ_{Sun} = -dQ_{Earth} \equiv dQ<0~~~~~~~~~~----(3)$$

So, the total rate of change of entropy is just the sum, given by:

$$\frac{dS_{total}}{dt} = \frac{dS_{Sun}}{dt} + \frac{dS_{Earth}}{dt} = \left(\frac{1}{T_{Sun}} - \frac{1}{T_{Earth}} \right) \frac{dQ} {dt} ~~----(4)$$

The assumption that the Sun radiates to the Earth (one we can make without loss of generality, since "Sun" and "Earth" are merely labels) and not vice versa establishes that $T_{Sun}~>~T_{Earth}$. This makes $1/T_{Sun}~-~1/T_{Earth}~<~0$ and hence, from (3) and (4), $dS_{total}/dt~>~0$.

19. Jun 7, 2006

### Cyrus

Simple, and elegant post Gokul. Nice job.

20. Jun 8, 2006

### rcgldr

Why is it that some chemical reaction produce a reduction in temperature while other produce an increase in temperature.

I've read a few web sites, but some issues like the creation of stars after the big bang, are confusing. Is matter in a higher or lower state of potential energy than actual energy itself? Some texts states that matter is a lower state, but noted that stars convert matter back into energy, this is my confusion.