The second law of thermodynamics was practical

Not reversibility as an everyday experience

It is a constant everyday experience that many of the processes we experience are tied to a certain time sequence. A temporal reversal of such processes seems absurd.

Example: A roof tile that falls from the roof onto the street and shatters there. Nobody would seriously claim that the time reversal of this process has any relation to earthly reality. In a court of law, a witness who alleges such occurrences would simply be considered implausible.

The reversal process would obey the laws of microscopic mechanics at any point in time. The law of conservation of energy or “1. Main clause "is fulfilled at all times for the reversal process. The process is not impossible - it is just so extraordinarily improbable that it does not occur “practically” after all.

The experience of irreversibility (in the technical term "irreversible“Called) certain processes is so evident that it is included as a basic statement (an axiom) in the theoretical structure of the theory of heat: This basic statement forms the 2nd law of thermodynamics. That actually means nothing other than that it excludes processes like the one in the "wrong film" in the upper right corner.

The second law of thermodynamics is one of the foundations of thermodynamics and it is used within the framework of this theory not provenRather, its evidence arises from the everyday experiences listed above and from a large number of experimental observations. Despite various efforts, no one has succeeded to date in reproducing macroscopic processes that contradict the second law. Rather, all experiences support the second law - and that is the reason why it was formulated in the first place. Here, too, a long discussion about truth, evidence and provability could follow - see the earlier comment on the nature of scientific knowledge.

In this cup there are 22 marbles, which are all made the same, only the color is different. You shake it up vigorously. What is the result?

just like that?

or rather like that?

A somewhat deeper understanding is possible within the framework of statistical mechanics: There it is shown that the second law corresponds to the striving of a system made up of a large number of small, mixed individual parts to achieve an "equal distribution" - maximum "chaos" is, so to speak, the natural distribution, as long as no serious causes specifically counteract this. This can be seen as the probabilistic justification of the second law - the axiomatics then shifts to even more elementary axioms of probability theory.



Formulations of the 2nd law

The following formulations of the second main clause are equivalent to one another.

Note For those who are less familiar with physical quantities and equations: The main features of the second law are contained in each of the following formulations. So you do not have to struggle with understanding the other formulations, which are not very meaningful for you.

Note For those interested in a deeper understanding: Following the formulations, there is even a (shortened) proof that proves the equivalence of all of these formulations.

(a) In the overall balance, heat never flows by itself from the colder to the warmer medium.Sure, right?
(b) There is no such thing as a "perpetuum mobile" of the second kind.
This is a machine that does nothing but constantly convert heat from a single reservoir into work.

(c) A "power station" can consist of heat Q, which is stored in a reservoir of temperature TO is removed, only gain usable work if at the same time some of the heat is transferred to a second reservoir with a lower temperature Tu (often the "environment" is used) can deliver. The maximum work W that can be obtained from the heat extracted from the upper reservoir is η Carnot ⋅ Q. Where η Carnot the so-called Carnot efficiency:


A power plant is a machine whose purpose is to convert heat into work and which makes use of a heat source for this purpose.

A reservoir is a thermodynamic equilibrium system that maintains its state of constant temperature even when heat is withdrawn and supplied.

Carnot is a famous French physicist. He was one of the first to clarify the relationships described here.

(d) The exergy B in a closed system cannot increase over time.The exergy B is the maximum amount of work that can be obtained from one system through ideal process management if a reservoir of temperature Tu is available.
(e) The entropy S cannot decrease over time in a closed system.The entropy S is a measure of the energy contained in a system that can only be transferred as heat. It can also be viewed as a measure of the "disorder" in the system.



One final note:


The passive house does not work against the laws of physics - that would also be pointless, how should it work reliably then? Rather, the passive house uses knowledge of physics and uses it wisely. Making physics usable for practice in an amazing new form does not always require high-energy accelerators in the GeV area. Classical thermodynamics also has many innovative and beneficial applications.

The passive house does not contradict the second law - but it uses all the "tricks" known today to get as close as possible to reversibility with a building. These are roughly the same tricks that a good experimental physicist has to use if he wants to perform classic mechanical experiments or demonstrations on traditional energy conservation "without dirty effects".

The house system would be completely reversible if there were no more heat losses - but this is neither necessary (because a warming sun still shines on us) nor is it possible in the strict sense. In our technical environment, the passive house means that the entropy decreases compared to the usual solutions - but it still increases (a very little bit) over time. The entropy only remains lower compared to other scenarios - the second law does not forbid that. It only makes a statement about the sequence of the development over time.


Further articles in this series will appear in loose succession.

Articles already published:

Climate protection today - episode 1

Climate protection today - episode 2

Climate protection today - episode 3