Thermodynamics as an independent branch of physical science arose in the first half of the 19th century. The age of machines has dawned. The industrial revolution demanded to study and comprehend the processes associated with the functioning of heat engines. At the dawn of the machine era, lone inventors could afford to use only intuition and the “poke method”. There was no public order for discoveries and inventions, it could not even occur to anyone that they could be useful. But when thermal (and a little later, electric) machines became the basis of production, the situation changed. Scientists finally gradually sorted out the terminological confusion that prevailed until the middle of the 19th century, deciding what to call energy, what force, what impulse.
What thermodynamics postulates
Let's start with common knowledge. Classical thermodynamics is based on several postulates (principles) that were successively introduced throughout the 19th century. That is, these provisions are notprovable within it. They were formulated as a result of generalization of empirical data.
The first law is the application of the law of conservation of energy to the description of the behavior of macroscopic systems (consisting of a large number of particles). Briefly, it can be formulated as follows: the stock of internal energy of an isolated thermodynamic system always remains constant.
The meaning of the second law of thermodynamics is to determine the direction in which processes proceed in such systems.
The third law allows you to accurately determine such a quantity as entropy. Consider it in more detail.
The concept of entropy
The formulation of the second law of thermodynamics was proposed in 1850 by Rudolf Clausius: "It is impossible to spontaneously transfer heat from a less heated body to a hotter one." At the same time, Clausius emphasized the merit of Sadi Carnot, who as early as 1824 established that the proportion of energy that can be converted into the work of a heat engine depends only on the temperature difference between the heater and the refrigerator.
In further development of the second law of thermodynamics, Clausius introduces the concept of entropy - a measure of the amount of energy that irreversibly transforms into a form unsuitable for conversion into work. Clausius expressed this value by the formula dS=dQ/T, where dS determines the change in entropy. Here:
dQ - heat change;
T - absolute temperature (the one measured in Kelvin).
A simple example: touch the hood of your car with the engine running. He is clearlywarmer than the environment. But the car engine is not designed to heat the hood or the water in the radiator. By converting the chemical energy of gasoline into thermal energy, and then into mechanical energy, it does useful work - it rotates the shaft. But most of the heat produced is wasted, since no useful work can be extracted from it, and what flies out of the exhaust pipe is by no means gasoline. In this case, thermal energy is lost, but does not disappear, but dissipates (dissipates). A hot hood, of course, cools down, and each cycle of cylinders in the engine adds heat to it again. Thus, the system tends to reach thermodynamic equilibrium.
Features of entropy
Clausius derived the general principle for the second law of thermodynamics in the formula dS ≧ 0. Its physical meaning can be defined as the "non-decreasing" of entropy: in reversible processes it does not change, in irreversible processes it increases.
It should be noted that all real processes are irreversible. The term "non-decreasing" reflects only the fact that a theoretically possible idealized version is also included in the consideration of the phenomenon. That is, the amount of unavailable energy in any spontaneous process increases.
Possibility of reaching absolute zero
Max Planck made a serious contribution to the development of thermodynamics. In addition to working on the statistical interpretation of the second law, he took an active part in postulating the third law of thermodynamics. The first formulation belongs to W alter Nernst and refers to 1906. Nernst's theorem considersbehavior of an equilibrium system at a temperature tending to absolute zero. The first and second laws of thermodynamics make it impossible to find out what the entropy will be under given conditions.
When T=0 K, the energy is zero, the particles of the system stop chaotic thermal motion and form an ordered structure, a crystal with a thermodynamic probability equal to one. This means that entropy also vanishes (below we will find out why this happens). In reality, it even does this a little earlier, which means that the cooling of any thermodynamic system, any body to absolute zero is impossible. The temperature will arbitrarily approach this point, but will not reach it.
Perpetuum mobile: no, even if you really want to
Clausius generalized and formulated the first and second laws of thermodynamics in this way: the total energy of any closed system always remains constant, and the total entropy increases with time.
The first part of this statement imposes a ban on the perpetual motion machine of the first kind - a device that does work without an influx of energy from an external source. The second part also forbids the perpetual motion machine of the second kind. Such a machine would transfer the energy of the system into work without entropy compensation, without violating the conservation law. It would be possible to pump out heat from an equilibrium system, for example, fry fried eggs or pour steel due to the energy of the thermal movement of water molecules, while cooling it.
The second and third laws of thermodynamics forbid a perpetual motion machine of the second kind.
Alas, nothing can be obtained from nature, not only for free, you also have to pay a commission.
Heat Death
There are few concepts in science that caused so many ambiguous emotions not only among the general public, but also among the scientists themselves, as much as entropy. Physicists, and first of all Clausius himself, almost immediately extrapolated the law of non-decreasing, first to the Earth, and then to the entire Universe (why not, because it can also be considered a thermodynamic system). As a result, a physical quantity, an important element of calculations in many technical applications, began to be perceived as the embodiment of some kind of universal Evil that destroys a bright and kind world.
There are also such opinions among scientists: since, according to the second law of thermodynamics, entropy grows irreversibly, sooner or later all the energy of the Universe degrades into a diffuse form, and “heat death” will come. What is there to be happy about? Clausius, for example, hesitated for several years to publish his findings. Of course, the "heat death" hypothesis immediately aroused many objections. There are serious doubts about its correctness even now.
Sorter Daemon
In 1867, James Maxwell, one of the authors of the molecular-kinetic theory of gases, in a very visual (albeit fictional) experiment demonstrated the seeming paradox of the second law of thermodynamics. The experience can be summarized as follows.
Let there be a vessel with gas. The molecules in it move randomly, their speeds are severaldiffer, but the average kinetic energy is the same throughout the vessel. Now we divide the vessel with a partition into two isolated parts. The average velocity of the molecules in both halves of the vessel will remain the same. The partition is guarded by a tiny demon that allows faster, "hot" molecules to penetrate one part, and slower "cold" molecules to another. As a result, the gas will heat up in the first half and cool down in the second half, that is, the system will move from the state of thermodynamic equilibrium to a temperature potential difference, which means a decrease in entropy.
The whole problem is that in the experiment the system does not make this transition spontaneously. It receives energy from outside, due to which the partition opens and closes, or the system necessarily includes a demon that expends its energy on the duties of a gatekeeper. The increase in the entropy of the demon will more than cover the decrease in its gas.
Unruly Molecules
Take a glass of water and leave it on the table. It is not necessary to watch the glass, it is enough to return after a while and check the condition of the water in it. We will see that its number has decreased. If you leave the glass for a long time, no water will be found in it at all, since all of it will evaporate. At the very beginning of the process, all water molecules were in a certain region of space limited by the walls of the glass. At the end of the experiment, they scattered throughout the room. In the volume of a room, molecules have much more opportunity to change their location without anyconsequences for the state of the system. There is no way we can collect them into a soldered "collective" and drive them back into a glass in order to drink water with he alth benefits.
This means that the system has evolved to a higher entropy state. Based on the second law of thermodynamics, entropy, or the process of dispersion of the particles of the system (in this case, water molecules) is irreversible. Why is that?
Clausius did not answer this question, and no one else could do it before Ludwig Boltzmann.
Macro and microstates
In 1872, this scientist introduced the statistical interpretation of the second law of thermodynamics into science. After all, the macroscopic systems that thermodynamics deals with are formed by a large number of elements whose behavior obeys statistical laws.
Let's get back to water molecules. Flying randomly around the room, they can take different positions, have some differences in speeds (molecules constantly collide with each other and with other particles in the air). Each variant of the state of a system of molecules is called a microstate, and there are a huge number of such variants. When implementing the vast majority of options, the macrostate of the system will not change in any way.
Nothing is off limits, but something is highly unlikely
The famous relation S=k lnW connects the number of possible ways in which a certain macrostate of a thermodynamic system (W) can be expressed with its entropy S. The value of W is called the thermodynamic probability. The final form of this formula was given by Max Planck. The coefficient k, an extremely small value (1.38×10−23 J/K) that characterizes the relationship between energy and temperature, Planck called the Boltzmann constant in honor of the scientist who was the first to propose a statistical interpretation of the second the beginning of thermodynamics.
It is clear that W is always a natural number 1, 2, 3, …N (there is no fractional number of ways). Then the logarithm W, and hence the entropy, cannot be negative. With the only possible microstate for the system, the entropy becomes equal to zero. If we return to our glass, this postulate can be represented as follows: the water molecules, randomly scurrying around the room, returned back to the glass. At the same time, each exactly repeated its path and took the same place in the glass in which it was before departure. Nothing forbids the implementation of this option, in which the entropy is equal to zero. Just wait for the implementation of such a vanishingly small probability is not worth it. This is one example of what can only be done theoretically.
Everything is mixed up in the house…
So the molecules are randomly flying around the room in different ways. There is no regularity in their arrangement, there is no order in the system, no matter how you change the options for microstates, no intelligible structure can be traced. It was the same in the glass, but due to the limited space, the molecules did not change their position so actively.
The chaotic, disordered state of the system as the mostthe probable corresponds to its maximum entropy. Water in a glass is an example of a lower entropy state. The transition to it from the chaos evenly distributed throughout the room is almost impossible.
Let's give a more understandable example for all of us - cleaning up the mess in the house. To put everything in its place, we also have to expend energy. In the process of this work, we become hot (that is, we do not freeze). It turns out that entropy can be useful. This is the case. We can say even more: entropy, and through it the second law of thermodynamics (along with energy) govern the Universe. Let's take another look at reversible processes. This is how the world would look if there were no entropy: no development, no galaxies, stars, planets. No life…
A little more information about "heat death". There is good news. Since, according to the statistical theory, "forbidden" processes are in fact unlikely, fluctuations arise in a thermodynamically equilibrium system - spontaneous violations of the second law of thermodynamics. They can be arbitrarily large. When gravity is included in the thermodynamic system, the distribution of particles will no longer be chaotically uniform, and the state of maximum entropy will not be reached. In addition, the Universe is not immutable, constant, stationary. Therefore, the very formulation of the question of "heat death" is meaningless.
