Laws of Thermodynamics

Four laws form the basis of thermodynamics. It defines the structure of heat and work transfers in thermodynamic processes.

The laws of thermodynamics have very general validity and do not change depending on the details of the interactions or the properties of the system under study. In other words, even if only matter or energy input and output of a system is known, it can be applied to this system.

0th Law of Thermodynamics

This law, which was defined by Ralph H. Fowler in 1931, entered the literature after the 1st and 2nd laws were defined. It is called the “zero law” because it appears as a basic physics principle and naturally comes before the 1st and 2nd laws of thermodynamics.

According to this simplest law of thermodynamics, if two systems do not exchange heat or matter when interacting with each other, these systems are in thermodynamic equilibrium. The zeroth law states: If systems A and B are in thermodynamic equilibrium and systems B and C are in thermodynamic equilibrium; Systems A and C are also in thermodynamic equilibrium.

In simpler terms, if there is a contact based on heat exchange between two objects with different temperatures, the hot object cools and the cold object heats up. The basis of the work lies in the fact that the heat flow between two objects with two different temperatures occurs from the hot object to the cold object. It is possible to perceive some cold objects as hot and some hot objects as cold. For example, -30° temperature can be considered categorically in the cold class, but it is warmer than –50°. The basis of the heat flow not being from cold to hot is: Temperature; It is a factor that affects the kinetic energy of material atoms – more precisely, their electrons. Electrons behave in such a way that they are always at the fundamental energy level. The desire to transfer excess kinetic energies and return to the basic energy level is dominant. Heat is transmitted by vibration of atoms in the material. For this reason, the heat flow takes place from the hot object to the cold object.

1st Law of Thermodynamics

The change in the internal energy of a system is the sum of the heat supplied to the system and the work applied to the system by the environment. U₂ – U₁ = Q + W

This law is also known as “conservation of energy“. Energy cannot be created out of nothing; existing energy cannot be destroyed either; it simply transforms from one form to another. For any cycle of a system, heat exchange and work exchange during the cycle must be equal to each other in the same unit system and proportional to each other in different unit systems. The accuracy of these statements has been observed through experiments, but cannot be proven. All these expressions can be expressed mathematically much more easily.

In the formulas below

• Q = net heat exchange throughout the cycle
• W = net business exchange throughout the cycle

let it show. But we also need a cycle, now let’s draw it simply,

Now, any two states of the system can be seen in this way, namely points 1 and 2. Let the state changes be provided by the lines A, B, and C. Arrow directions are also directions in which phase changes will occur. Now if the states are 1A2 and 1B2, then 2C1 is the initial state. Now let’s set up the cycles, we have 1A2C1 and 1B2C1 cycles:

• _1A∫².δ.Q + _2C∫¹.δ.Q = _1A∫².δ.W + _2C∫¹.δ.W ( 1A2C1 Cycle ) (a equation)
• _1B∫².δ.Q + _2C∫¹.δ.Q = _1B∫².δ.W + _2C∫¹.δ.W ( 1B2C1 Cycle ) (b aquation)

1A2C1 and 1B2C1 cycles are equal to each other. When the 1st law of thermodynamics is applied, a and b equations emerge. If we subtract b equation from equation a, we find equation c.

• _1A∫² ( δ.Q – δ.W ) = _1B∫²( δ.Q – δ.W ) (c equation)

Since 1A2 and 1B2 are any two processes between the same states, it can be said that the expression δQ – δW is independent for all state changes between points 1-2. Their difference is the point function and is perfectly differential. This is a system-specific feature and it is the energy of the system and it is denoted by E (E=δQ-δW).

• Q_1-2 : Heat exchange in the state of the system
• W_1-2 : Exchange of work in the state of the system
• E₁ : The initial energy of the system and
• E₂ : The final energy of the system

including; Q_1-2 – W_1-2 = E₂ – E₁

In thermodynamics, energy can be divided into internal energy depending on the structure of the substance and kinetic energy (E_K) and potential energy (E_P) depending on the coordinate axes; E = U + E_K + E_P

The energy of the system in any state change is;

Q_1-2 – W_1-2 = E₂ – E₁ = (U₂ – U₁) + (1/2) m (V₂² – V₁²) + m g (z₂ – z₁)

• U: internal energy
• m: mass
• V: velocity
• g: acceleration due to gravity
• z: height

2nd Law of Thermodynamics

The second law, which can be applied in many fields, can be defined as follows: It is impossible to obtain a cycle that draws heat from a heat source and does an equal amount of work and has no other consequences. (Kelvin-Planck Declaration) or It is impossible to obtain a process that has no effect other than the flow of heat from a cold body to a hot body. (Clausius Statement)

The entropy of a large thermally isolated system never decreases (see Maxwell’s Demon). However, a microscopic system can experience fluctuations in entropy contrary to what the law says (see: The Fluctuation Theorem). In fact, the mathematical proof of the ripple theorem derived from the time-reversible dynamics and causality principle constitutes a proof of the second law. Logically speaking, the second law thus becomes a theorem valid for relatively large systems and long times rather than actually a law of physics. It was defined by Ludwig Boltzmann. It tells that as long as the system is not energized from outside, order will turn into disorder and disorder will turn into chaos. Classically, an example is given that a broken glass cannot be restored by using less than the energy expended while standing or breaking it. Likewise, in order to fix a book that has been knocked down, it is necessary to use more than the energy expended while tilting it, some of the potential energy has been converted into heat and cannot be recovered. It also describes the tendency towards disorder in the universe. It uses the word entropy when describing the disorder tendency. In Greek, en = like ‘in’ in English, it gives the suffix -de, -da to the word it precedes, and tropos = from the word ‘tropoi’ (pronounced tropy), which is the plural of the word road. Well; “on the way”).

• Irregularity either does not change or increases. Diffusion can be given as an example. Separated substances are more ordered than together, and it is impossible to separate warm water, which consists of hot and cold water mixed by itself, again as hot and cold.
• It is the cause of actions such as aging, aging and aging.
• The most disordered energy is heat, and one day all energy will be heat, and that means the end of the universe.
• The theories to be put forward should not contradict the 2nd law of thermodynamics.
• Entropy is also defined as energy that is not capable of doing work. If gas at different temperatures is placed in two glass balloons and a propeller is placed between the glass balloons, it will be seen that the propeller is spinning at first. But then the propeller will stop spinning as the entropy increases.
• Considering that a 100-meter run is done in a park to do sports, that at the end of 100 meters one gets tired and cannot run, and one sits down, the energy that is spent while running and cannot be regained is called entropy.
• Any function that increases as the disorder of the system increases can easily be an entropy function. For example, let’s imagine that we have a glass of water and we drop a drop of ink into it and observe it, and try to imagine what’s going on inside. The ink molecules will start to disperse into the water after they stay together for a short time at first. Because they will be pushed in different directions by the water molecules hitting them (because the chemical bonds of water and ink materials are suitable for repelling each other). Now suppose an extraordinary computer can count all possible states of the system. When we say a state of the system, what we need to understand is that a molecule has a certain coordinate and a certain speed; is the configuration in which another molecule has another specific coordinate and velocity. In the example of ink on the glass, it is clear that the number of such cases is very large. Because the vast majority of them correspond to irregular, that is, high-entropy states, where the molecules of the ink are randomly dispersed here and there in the glass. At the level we perceive, these are all homogeneous states. Because when we look at the mixture, we can say that the ink is dispersed homogeneously, regardless of whether that molecule is here or someone else is there. That is, an extraordinary number of different microscopic states correspond to a single macroscopic state, that is, the homogeneous state.
• In fact, systems do not deteriorate, they try to get the most stable state on the basis of energy change. This is the meaning of life, life is one of the ways of entropy, it is like a spoon that allows sugar to mix into tea much more quickly.
• In a closed system, entropy always increases. The closed system part is very important. By energizing the system, its entropy can be reduced. The world is not a closed system. Energy is constantly flowing from the sun to the world, and this provides order.
• “The most likely thing happens when the number of particles goes to infinity”: If some coin is tossed into the air, the probability of all of them landing heads is only one. All but one are more likely to land heads. It is even more likely that half will land heads and half will land heads. This last one is the system with the maximum entropy. As a result, an increase in entropy means that the system moves from the improbable state to the more probable state. Although it is possible for all the molecules in the room to be collected at a point in the right corner of the room, there is only one configuration that satisfies this condition. However, there are more configurations where the atoms are evenly distributed throughout the room.

3rd Law of Thermodynamics

This law states why it is impossible to cool a substance to absolute zero:

As the temperature approaches absolute zero, all motions approach zero.

As the temperature approaches absolute zero, the entropy of a system approaches a constant. The reason why this number is not zero but a constant is that although all motions have ceased and the associated uncertainties have disappeared, there is still an uncertainty due to the different molecular arrangement of non-crystalline materials. In addition, thanks to the third law, absolute entropy can be defined, which is very useful in the study of chemical reactions, with reference to the absolute zero entropies of substances.

A machine that violates one of these laws is called a circulation machine of the number type of that law (for example, of the first type if it creates energy out of nothing).

Ginsberg’s theorem: (1) you cannot win, (2) you cannot draw, and (3) you cannot leave the game.

Or: (1) you can’t get anything without working, (2) the most you can get by working is just being harmless, and (3) you can get it only at absolute zero.

Or, (1) you can neither win nor quit the game, (2) you can’t draw unless it’s too cold, (3) the weather won’t get that cold.

In summary; All laws can be explained as follows.

About Author