# Are the Laws of Thermodynamics Really laws?

1. May 7, 2014

### BiGyElLoWhAt

At least the first 2. The 3rd seems obvious to me.

Don't get me wrong, I'm not denying CoE, I'm just questioning the reason why we have a law called Conservation of Energy, and an identical law called The First Law of Thermodynamics.

As for the second law, I've seen it defined 2 ways: in terms of the entropy of an isolated system, and in terms of the entropy of the universe. If it's the entropy of an isolated system I have less of a problem than if it's the entropy of the universe. (I'm gonna get a little out there...) Assuming the Big Bang-Big Crunch theory is still at least plausible, upon analysis, the big bang portion is compliant with the 2nd law, but the big crunch is in complete contradiction... rather I think it implies a somewhat parabolic relationship between entropy and time. $S = k - m(t - a)^2$ or something along those lines... S is entropy, t is time, k m and a are constants.

Does anybody else get what I'm saying?
Didn't really know where to put this.

2. May 7, 2014

### BiGyElLoWhAt

I guess what I'm getting at, is I feel 'Law' is a strong term for these concepts, as the 2nd law is to the best of our knowledge a good approximation.

3. May 7, 2014

### Staff: Mentor

What we call a law and what we don't is largely historical accident - there's never been a clear and universally accepted standard for what might be called a "law of physics". Indeed, the entire concept went out of fashion around the start of the 20th century, which is why you don't see any of the discoveries since then graced with the word "law".

4. May 7, 2014

### BiGyElLoWhAt

Ah, I see. That makes sense. Thanks.

5. May 7, 2014

### Jano L.

They are not identical. The first law of thermodynamics says that for given system there is this quantity called internal energy which is a function of equilibrium state of this system and can change either by work or by heat supplied from outside, while given amount of heat is equivalent to definite amount of work. In special case the system is isolated, the internal energy remains constant. All this is part of the first law.

The law of conservation of energy is a general law believed to be valid for any isolated system even if the concept of heat does not apply, such as globular star cluster or quasistatic LC circuit. It states that the state of the system evolves in such a way that certain function of the state variables - energy - remains constant.

After EM radiation was discovered, the concept of "energy conservation law" shifted to include even situations where the system cannot be isolated and energy is being radiated away or comes from there. Such situations do not belong to thermodynamics but they are said to obey "energy conservation".

Eventually it may be possible to explain the first law based on models for which energy conservation is valid, but nevertheless these two laws apply to two different kinds of models.

Both these ways are misleading. Second law is not meant to apply to Universe in thermodynamics (there is long traditionof misstating it while invoking Universe, but this is not the original meaning and has no experimental support). And if the system was isolated, nothing could happen to it in equilibrium state (equilibrium state is stable in thermodynamics) so there would be nothing interesting to state about its entropy.

One good way to state the second law with entropy is this: when system goes from equilibrium state A to equilibrium state B, while the process is adiabatic (no heat is transferred), the entropy of the system cannot decrease. Notice that the initial and final state have to be equilibrium states. Second law talks about change of entropy of equilibrium states. For non-equilibrium states, there are possible generalizations, but again these are not the original meaning.

6. May 7, 2014

### Andrew Mason

It depends on what definition of "law" you are using. I would call a law a true law if it is one that is always obeyed.

The second law is a statistical law, but it is, nevertheless, always obeyed. So it is a true "law".

There are other physical "laws" that I would not really call laws but merely good approximations (such as Hooke's law or the laws of Friction).

AM

7. May 7, 2014

### dauto

The Big crunch is not incompatible with the 2nd law of Thermodynamics. entropy would continuously grow until everything was crunched into a singularity.

8. May 7, 2014

Staff Emeritus
Off course they aren't really laws. Nobody gets arrested for making a circuit element that doesn't follow Ohm's Law. It's an analogy, and like all analogies, works better in some respects than others.

9. May 7, 2014

### Andrew Mason

First of all, the big crunch has not occurred. There is no actual evidence that it will occur. It is just a theory.

The main point, however, is that the second law applies between states of thermodynamic equilibrium. It has no real meaning during a dynamic process. I don't think one can define the entropy of a singularity.

AM

10. May 8, 2014

### dauto

Yes, off course. The Big Crunch is a theory. A theory compatible with the 2nd LoT. I wouldn't be so fast at declaring entropy meaningless during a dynamical process. The entropy just counts the numbers of states compatible with a macroscopic description of a system. That may be hard to calculate - or even to define - for a dynamical system. Does that mean its meaningless?

11. May 9, 2014

### Andrew Mason

Provided the macroscopic description is of a system in equilibrium. After all, the definition of a change in entropy is ∫dqrev/T. Since you need a reversible process between the two states, the beginning and end states must be in equilibrium and they must have a temperature.

AM