# First and second laws of thermodynamics

1. Jan 20, 2012

### ugalpha

I don't understand this. The Entropy described in Isaac Asimov's " The last Question " is what i always tought to be the 2nd law of Thermodynamics. Now if this is correct i just can't understand how such entropy is possible with the first law of thermodynamics. I obviously know that i am wrong somewhere so please enlighten me.

Thank you!

2. Jan 20, 2012

### Drakkith

Staff Emeritus
The first law is only concerned with the conservation of energy. Even with an increase in entropy, energy is still conserved between all the interactions and reactions.

3. Jan 24, 2012

### Telemachus

You are dealing with different concepts. Entropy reefers to the disorder on the system. In the other hand, the energy concerns to the energy contained in the system, momentum, potential energy, and all involving the microscopic scale, the momentum of every single molecule, rotating and colliding with each other. Every systems tends to a state of equilibrium, and that state is characterized by a maximum on entropy, or a minimum in the energy. That is, you can describe the state of the system using the entropy, or the energy. There are actually other potentials, but I think these two are the most important. You can describe the state of the system in the energy form, this could be over an equation of the form U=U(S,V,N), it's a function of the entropy S, the volume, and the mole number N. If you invert this function, you can get the state function with the entropy as the dependent variable, so you get S=S(U,V,N).

The first principle it's never violated. It is described in the first postulate of thermodynamics, which is the conservation of energy postulate. The entropy is treated on the second postulate.

4. Jan 24, 2012

### _DJ_british_?

Basically, the first law state that energy is conserved and the second law state that there exists a "potential function for work" ie. d(work)/temperature is an exact differential. That's a dumb guy explanation of the 1st and 2nd law...that might help.

The way I understand it, those laws are mutually excusive by experiments. That is, no experimental correlation where found between them. Hence we postulate, which is intuitive, that they are not related (see, for example, the kelvin statement of the second law).

5. Jan 26, 2012

### georgir

The second law is actually just a statistical observation and not a real physical law, and is not true in every case.

Most obviously, it directly contradicts the time-reversal invariance of all other laws (or, time-charge-parity reversal invariance if you want to be pedantic) which allows us to construct a system where it doesn't hold from every system where it does.

6. Jan 26, 2012

### D H

Staff Emeritus
But it is a very strong statistical law. You will not, for example, get a patent on the basis of demonstrated short-term violations of the second law of thermodynamics.

Consider a box that contains molecules of gas. Imagine there is a plane that splits the box into two halves. What are the probabilities that all of the molecules are in the same half of the box, and that the two halves of the box contain more or less the same number of molecules? If there is only one molecule of gas in the box, the probabilities are tautologically 100% and 0%. Add another molecule. The probabilities are now 50% and 50%. The probability that all molecules are in the same half drops precipitously as you add more and more molecules to the box while the probability of a nearly equal distribution rises. With a mole of molecules, the first probability is essentially zero, the latter, essentially 100%.