Lead
The "janitor" is not an understandable role that may not be so real in life, but it takes on a very important role when it is used to prove some scientific laws.
This "janitor" has a very bright name - "Maxwell the Demon", which comes from James Maxwell.
In fact, Maxwell's function is to divide the heat into two parts, one part of the heat is high and the other part is low, and then only the low part of the heat remains.
However, this process of "stealing points" can make the entropy of heat smaller, and the entropy reduction contradicts the thermodynamic irreversibility of the second law of thermodynamics.
So can Maxwell's "stealing points" become a perpetual motion machine?
1. Maxwell's demon.
There are many formulations of the second law, but the most direct and simple formulation is that the entropy of an isolated system never decreases.
This law may seem a bit complicated and obscure, but it actually makes it very clear that the disorder of an isolated system will never decrease, and it may become more chaotic because of certain processes.
Of course, the concept of entropy was not first proposed by Maxwell, it came from the second law of thermodynamics of Carnot, the father of thermodynamics, and in fact, Maxwell's demon proposed at that time was not really to "steal" entropy, but hoped to express some doubts about the second law through the virtual doorman of Maxwell's demon.
We can understand Maxwell as a very intelligent doorman, this doorman has a very magical superpower, that is, when a car is about to drive in, he can drive the people in the car down according to his own ideas when that car enters the door.
Of course, in this way, this doorman will be able to control who comes and who goes well, and he will also be able to bring some benefits to himself, so the question arises, can the doorman mess with the rules infinitely for his own benefit?
Maxwell's model is actually a metaphor, he designed it like this, if the doorman comes, then he will break the traffic rules, and the traffic will be a mess in the future, but if the doorman does not break the rules, he can get more benefits, then all the cars will be stopped by him, or there will be no cars coming in.
In this way, a very good strange phenomenon is formed, either the doorman will mess with the rules, or the doorman will stop the car casually, so the doorman becomes a perpetual motion machine.
This strange phenomenon was later explained by Maxwell, of course, Maxwell wanted to refute the irreversibility of the second law through Maxwell's demon, but in fact his model was meaningless, because this model must have some flaws.
Second, the "stealing points" of calories.
So how does Maxwell do it?
Maxwell's demon is very simple, we can understand Maxwell's demon as a container, and there are two kinds of gases in this container, one is a very slow gas A, which is in a very slow motion, and the other is a very fast gas B, which is in a very fast motion.
When two gases are mixed together, their velocity will change, according to the second law of thermodynamics, entropy can only increase, so the entropy of gas B can only decrease, that is, the temperature of gas B will decrease, which is actually Maxwell's demon "stealing points".
But what is all this "stealing" all about?
The average kinetic energy of gas molecules has a very direct relationship with temperature, when the temperature rises, the molecular velocity of gas will become faster, we all know that the velocity of gas molecules has a very important physical quantity, that is, the root mean velocity.
Because the quadratic velocity of a gas molecule is the kinetic energy of the gas molecule, when we make the velocity of the gas molecule cubic open, we can get the average kinetic energy of the gas molecule, which is the temperature of the gas.
When we turn the velocity of the gas molecules in the gas to the third power, we can get a formula V=sqrt(3RT/M), which is the average velocity of the gas molecules, where R is the gas constant, R=8.31J/(mol· K), T is the temperature of the gas, and M is the mass of the gas molecule.
When we calculate the velocity of gas A and gas B respectively, we can get that the velocity of gas A is about 500 m/s, and the velocity of gas B is about 1000 m/s.
So, we can see that the velocity of gas B is obviously faster than the velocity of gas A, which is the heat of gas B, and the velocity of gas A is slower than the velocity of gas B, which is the cold of gas A.
Because these two gases do not have any kinetic energy and move randomly in the container, this process is irreversible, so Maxwell can only separate the two gases, but cannot mix the two gases together.
So how does Maxwell do it?
This process is achieved through Maxwell's gatekeeper, and when gas molecules come in from Maxwell, Maxwell can determine whether the molecule is hot or cold based on the speed of the molecule's movement.
When we give all the hot molecules to Maxwell, then Maxwell can't separate the two gases without the help of any hot molecules, so Maxwell can only separate these hot molecules.
3. Calorie consumption.
In this way, we will form a very clever structure, when a gas molecule comes in from Maxwell, Maxwell can judge whether it is hot or cold based on the speed of the molecule, so that as long as Maxwell keeps letting the hot gas molecules escape, then all that is left is hot gas molecules.
In this way, we can separate the hot gas molecules from the cold gas molecules through a very simple process, and finally pile the hot gas molecules in one corner and the cold gas molecules in the other corner, so that we create a temperature difference for the gas molecules, and the temperature difference can do work.
Therefore, Maxwell's demon can divide gas molecules into hot and cold through a very simple process, and then work can be done through this temperature difference, so as to achieve a perpetual motion machine.
Then, Maxwell's demon can do work through the movement of gas molecules, so as to do work on the outside, so can Maxwell's demon become a perpetual motion machine?
Obviously, we can do work through Maxwell, but in the process of doing work, heat will be released, and the heat released will eventually cause Maxwell to become an unpopular guard.
Because Maxwell divides gas molecules into hot and cold through the movement of gas molecules, and the process of separating gas molecules requires heat consumption, when Maxwell has escaped all the hot gas molecules, then Maxwell cannot separate gas molecules without the help of hot gas molecules.
Therefore, if Maxwell keeps separating gas molecules, then eventually his heat will be consumed and he will become an unpopular guard, so Maxwell can only divide gas molecules into hot and cold in one process, but cannot mix hot and cold gas molecules together.
Therefore, Maxwell's model is unrealizable from the beginning, because the movement of gas molecules in this model is irreversible, so Maxwell can only divide gas molecules into hot and cold, but cannot mix hot and cold gas molecules.
Eventually, the model of Maxwell was gradually forgotten, so what does Maxwell's failure teach us?
epilogue
Obviously, Maxwell is a very ingenious model, but this ingenuity does not mean that it is a real model, to be precise, Maxwell is not a model, more precisely, Maxwell is actually just a fictional character.
However, this fictional character is very helpful for Maxwell to explain the irreversibility of heat in the second law of thermodynamics, so we can often see some unreal characters in academic research, but these characters are very good at helping scientists with some complex theories or concepts.
However, just because these characters exist, we must not pursue the existence of this character, we cannot go against the laws of nature, we cannot pursue unrealistic things.
We must respect the laws of nature, but we must also follow the rules of precision and practicality of scientific research, so Maxwell's failure also gave me some inspiration.
So, what exactly is the thermodynamic irreversibility of the second law of thermodynamics?
The thermodynamic irreversibility of the second law of thermodynamics is due to the increase in entropy, when we connect one system to another, if this system can only transfer heat to another system, but cannot obtain heat from another system, then the process is irreversible.
This irreversible process is that the heat is transferred from the high temperature to the low temperature, and not the other way around, so the heat can never be transferred from the low temperature to the high temperature through such a process, so such a process is impossible, so there can be no perpetual motion machine.