The first law of thermodynamics is the law of conservation of energy as applied to thermodynamic processes: energy does not disappear into nowhere and does not arise from nothing, but only passes from one type to another in equivalent quantities. An example would be the transfer of heat (thermal energy) into mechanical energy, and vice versa.

If a certain amount of heat dQ is added to M kg of gas occupying a volume V (m 3) at a temperature T at a constant pressure, then as a result the gas temperature will increase by dT and the volume by dV. An increase in temperature is associated with an increase in the kinetic energy of molecular motion dK.
An increase in volume is accompanied by an increase in the distance between molecules and, as a consequence, a decrease potential energy dH interactions between them. In addition, by increasing its volume, the gas does work dA to overcome external forces.
If, besides those indicated, no other processes occur in the working fluid, then, based on the law of conservation of energy, we can write:

dQ = dK + dH + dA.

The sum dK + dH represents the change internal energy dU of the molecules of the system as a result of the supply of heat.
Then the energy conservation formula for a thermodynamic process can be written as:

dQ = dU + dA or dQ = dU + pdV.

This equation is a mathematical expression first law of thermodynamics: the amount of heat dQ supplied to the gas system is spent on changing its internal energy dU and performing external work dA.

Conventionally, it is believed that when dQ > 0 heat is transferred to the working fluid, and when dQ< 0 теплота отнимается от тела. При dA >0 the system does work (gas expands), and at dA< 0 работа совершается над системой (газ сжимается) .

For an ideal gas, between the molecules of which there is no interaction, the change in internal energy dU is completely determined by the change in the kinetic energy of motion (i.e. increasing the speed of molecules), and the change in volume characterizes the work of the gas to overcome external forces.

The first law of thermodynamics has another formulation: the energy of an isolated thermodynamic system remains unchanged regardless of what processes occur in it.
It is impossible to build a perpetual motion machine of the first kind, that is, a periodically operating machine that would do work without expending energy.



Second law of thermodynamics

The first law of thermodynamics describes the quantitative relationships between the parameters of a thermodynamic system that take place in the processes of converting thermal energy into mechanical energy and vice versa, but does not establish the conditions under which these processes are possible. These conditions necessary for the transformation of one type of energy into another are revealed by the second law of thermodynamics.

There are several formulations of this law, and each of them has the same semantic content. Here are the most frequently cited formulations of the second law of thermodynamics.

1. To convert heat into mechanical work it is necessary to have a source of heat and a refrigerator, the temperature of which is lower than the temperature of the source, i.e. a temperature difference is necessary.

2. It is impossible to implement a heat engine, the only result of which would be the conversion of the heat of any body into work without some of the heat being transferred to other bodies.
From this formulation we can conclude that it is impossible to build a perpetual motion machine that does work thanks to only one source of heat, since any, even the most colossal source of heat in the form material body is not able to give off more thermal energy than enthalpy allows it (part of the total energy of a body that can be converted into heat by cooling the body to absolute zero temperature).

3. Heat cannot by itself move from a less heated body to a more heated one without the expenditure of external work.

As you can see, the second law of thermodynamics is not based on formulaic content, but only describes the conditions under which certain thermodynamic phenomena and processes are possible, confirming, in essence, common law energy conservation.

Spontaneous (spontaneous) processes are described by the following characteristics:

1. All natural spontaneous processes proceed in one direction, that is, they have a one-way direction. For example, heat transfers from a hot body to a cold one; gases tend to occupy the largest volume.

2. Part of the energy turns into heat, i.e., the system goes from an ordered state to a state with random thermal movement of particles.

3. Spontaneous processes can be used to produce useful work. As it transforms, the system loses its ability to produce work. In the final state of equilibrium it has the least amount of energy.

4. The system cannot be returned to its original state without making any changes in itself or in the environment. All spontaneous processes are thermodynamically irreversible.

5. In a spontaneous process, the initial state is less probable compared to each subsequent one and least probable compared to the final one.

Non-spontaneous processes occur with the expenditure of work; in this case, the system moves away from the equilibrium state (for example, gas compression, electrolysis).

Second law of thermodynamics- this is a postulate. It is statistical in nature and is applicable to systems from large number particles.

The second law of thermodynamics has the following formulations:

1. Heat cannot transfer spontaneously from a less heated body to a more heated one.

2. A process is impossible whose only result is the conversion of heat into work.

3. A perpetual motion machine of the second kind is impossible. The heat of the coldest of the bodies involved in the process cannot serve as a source of work.

Analytical expression of the second law of thermodynamics and its justification using the Carnot cycle. The essence of the expression of the second law of thermodynamics is the connection between the spontaneity of the process and the growth of entropy. This expression follows from consideration of the issue of the theoretical completeness of the conversion of heat into work in the reversible Carnot cycle.

The cycle consists of four processes:

AB- isothermal expansion due to heat Q 1, supplied to the gas at a temperature T 1;

Sun- adiabatic expansion;

SD- isothermal compression at temperature T 2, in this process the gas loses heat Q 2;

YES- adiabatic compression to the initial state.

The heat absorbed (or released) during the isothermal expansion (or compression) of one mole of an ideal gas is equal to the work

During adiabatic expansion (or compression)

Application of these equations to the corresponding cycle processes leads to the expression for the thermodynamic coefficient useful action(efficiency): . (4.3)

Equation (4.3) is the mathematical expression of the second law of thermodynamics.

Because T 1‹ T 2, That η ‹ 1.

According to Carnot's theory, replacing an ideal gas with any other substance will not change the efficiency. Carnot cycle. Replacing the Carnot cycle with any other cycle will result in lower efficiency. (Clasius-Carnot theorem). Thus, even in the case of an ideal heat engine conversion of heat into work cannot be complete.

The expression of the second law of thermodynamics allows us to introduce the concept of entropy, with the help of which the essence of the law is revealed in a convenient and general form.

Let's change expression (4.3):

on . (4.4)

The ratio is called reduced heat. Equation (4.4) shows that algebraic sum reduced heats according to the reversible Carnot cycle is equal to zero.

For an infinitesimal reversible Carnot cycle

where is the elementary reduced heat.

Any cycle can be replaced by a set of infinitesimal Carnot cycles: .

In the limit, this amount will turn into .

In the theory of integrals, it is proven that if the integral over a closed loop is equal to zero, then the integrand expression is the complete differential of some function of the parameters that determine the state of the system.

Where S- This entropy, such a function of the state of the system, the total differential of which in a reversible process is equal to the ratio of an infinitesimal amount of heat to temperature.

The concept of “entropy” was introduced by Clausius (1850) . This expression is the mathematical expression of the second law of thermodynamics for reversible processes.

The change in entropy in a reversible process is equal to the change in entropy in an irreversible process, i.e. . Let's compare the heats of reversible and irreversible processes. According to the first law of thermodynamics . Internal energy U is a function of the state of the system, so . Maximum work is done during a reversible process, therefore

In the general case for reversible and irreversible processes The second law of thermodynamics has the following mathematical expression:

Here dS = const, and only the right side of the equation changes, i.e. heat value. Entropy units: [ S] = J/mol K.

The combined equation of the first and second laws of thermodynamics is:

Calculation of the change in entropy of an ideal gas.

Let us express the change in internal energy

Dividing equation (4.6) by T, we determine the change in entropy:

(4.7)

From the ideal gas equation: it follows that . Then, after substituting this relation into (4.7):

(4.8)

Let us integrate expression (4.8) at and obtain The equation for calculating the change in entropy of an ideal gas is:

(4.9)

Isothermal process: , (4.10)

since then . (4.11)

Isochoric process: . (4.12)

Isobaric process: . (4.13)

Adiabatic process: . (4.14)

Planck's postulate has the following formulation: at absolute zero, the entropy of properly formed crystals of pure substances is zero. The postulate allows one to calculate the absolute value of entropy if the heats of phase transitions are known and if the heat capacities of the substance in various states of aggregation are known.

Second law of thermodynamics

Historically, the second law of thermodynamics arose from the analysis of the operation of heat engines (S. Carnot, 1824). There are several equivalent formulations. The very name “second law of thermodynamics” and historically its first formulation (1850) belong to R. Clausius.

The first law of thermodynamics, expressing the law of conservation and transformation of energy, does not allow us to establish the direction of thermodynamic processes. In addition, one can imagine many processes that do not contradict the first principle, in which energy is conserved, but in nature they do not occur.

Experience shows that different types energies are unequal in their ability to be converted into other types of energy. Mechanical energy can be completely converted into internal energy of any body. There are certain restrictions for the reverse transformation of internal energy into other types: the supply of internal energy, under no circumstances, can be completely converted into other types of energy. The noted features of energy transformations are associated with the direction of processes in nature.

The second law of thermodynamics is a principle that establishes the irreversibility of macroscopic processes occurring at a finite speed.

In contrast to purely mechanical (without friction) or electrodynamic (without release of Joule heat) reversible processes, processes associated with heat transfer at a finite temperature difference (i.e. flowing at a finite speed), with friction, diffusion of gases, expansion of gases into void , release of Joule heat, etc., are irreversible, i.e., they can spontaneously flow only in one direction.

The second law of thermodynamics reflects the direction of natural processes and imposes restrictions on the possible directions of energy transformations in macroscopic systems, indicating which processes in nature are possible and which are not.

The second law of thermodynamics is a postulate that cannot be proven within the framework of thermodynamics. It was created on the basis of a generalization of experimental facts and received numerous experimental confirmations.

Statements of the second law of thermodynamics

1). Carnot formulation: the highest efficiency of a heat engine does not depend on the type of working fluid and is completely determined by the limiting temperatures, between which the machine operates.

2). Clausius formulation: a process is impossible whose only result is the transfer of energy in the form of heat from a less heated body, to a warmer body.

The second law of thermodynamics does not prohibit the transfer of heat from a less heated body to a more heated one. Such a transition takes place in a refrigeration machine, but at the same time external forces carry out work on the system, i.e. this transition is not the only result of the process.

3). Kelvin formulation: circular process is not possible, the only result of which is the conversion of heat, received from the heater, into equivalent work.

At first glance, it may seem that this formulation contradicts the isothermal expansion of an ideal gas. Indeed, all the heat received by an ideal gas from some body is completely converted into work. However, obtaining heat and converting it into work is not the only end result of the process; In addition, as a result of the process, a change in the volume of gas occurs.

P.S.: you need to pay attention to the words “sole result”; the prohibitions of the second principle are lifted if the processes in question are not the only ones.

4). Ostwald's formulation: implementation perpetual motion machine the second kind is impossible.

A perpetual motion machine of the second kind is a periodically operating device, which does work by cooling one heat source.

An example of such an engine would be a ship's engine, which draws heat from the sea and uses it to propel the ship. Such an engine would be practically eternal, because... The supply of energy in the environment is practically limitless.

From the point of view of statistical physics, the second law of thermodynamics is statistical in nature: it is valid for the most probable behavior of the system. The existence of fluctuations prevents its accurate implementation, but the likelihood of any significant violation is extremely small.

Entropy

The concept of “entropy” was introduced into science by R. Clausius in 1862 and is formed from two words: “ en" - energy, " trope- I turn it.

According to the zero law of thermodynamics, an isolated thermodynamic system spontaneously passes into a state of thermodynamic equilibrium over time and remains in it for an indefinitely long time, if external conditions are kept unchanged.

In an equilibrium state, all types of energy in the system are converted into thermal energy of the chaotic movement of atoms and molecules that make up the system. No macroscopic processes are possible in such a system.

Entropy serves as a quantitative measure of the transition of an isolated system to an equilibrium state. As the system transitions to an equilibrium state, its entropy increases and reaches a maximum when the equilibrium state is reached.

Entropy is a function of the state of a thermodynamic system, denoted by: .

Theoretical background: reduced heat,entropy

From the expression for the efficiency of the Carnot cycle: it follows that or , where is the amount of heat given off by the working fluid to the refrigerator, we accept: .

Then the last relation can be written as:

The ratio of the heat received by a body in an isothermal process to the temperature of the heat-releasing body is called reduced amount of heat:

Taking into account formula (2), formula (1) can be represented as:

those. for the Carnot cycle, the algebraic sum of the reduced amounts of heat is equal to zero.

The reduced amount of heat imparted to the body in an infinitesimal portion of the process: .

The given amount of heat for an arbitrary area:

Strict theoretical analysis shows that for any reversible circular process the sum of the reduced amounts of heat is equal to zero:

From the fact that integral (4) is equal to zero, it follows that the integrand is the complete differential of some function, which is determined only by the state of the system and does not depend on the path by which the system came to this state:

Single-valued state function, whose total differential is ,called entropy .

Formula (5) is valid only for reversible processes; in the case of nonequilibrium irreversible processes, such a representation is incorrect.

Properties of entropy

1). Entropy is determined up to an arbitrary constant. The physical meaning is not entropy itself, but the difference between the entropies of two states:

. (6)

Example: if system ( ideal gas) makes an equilibrium transition from state 1 to state 2, then the change in entropy is equal to:

,

Where ; .

those. the change in entropy of an ideal gas during its transition from state 1 to state 2 does not depend on the type of transition process.

In general, in formula (6), the entropy increment does not depend on the path of integration.

2).The absolute value of entropy can be established using the third law of thermodynamics (Nernst’s theorem):

The entropy of any body tends to zero as its temperature tends to absolute zero: .

Thus, the initial reference point for entropy is taken at .

3). Entropy is an additive quantity, i.e. The entropy of a system of several bodies is the sum of the entropies of each body: .

4). Like internal energy, entropy is a function of the parameters of the thermodynamic system .

5), A process occurring at constant entropy is called isentropic.

In equilibrium processes without heat transfer, entropy does not change.

In particular, a reversible adiabatic process is isentropic: for it ; , i.e. .

6). At constant volume, entropy is a monotonically increasing function of the internal energy of the body.

Indeed, from the first law of thermodynamics it follows that when we have: , Then . But the temperature is always there. Therefore, the increments have the same sign, as required to be proved.

Examples of entropy changes in various processes

1). During isobaric expansion of an ideal gas

2). During isochoric expansion of an ideal gas

3). During isothermal expansion of an ideal gas

.

4). During phase transitions

Example: find the change in entropy when a mass of ice at temperature is converted into steam.

Solution

First law of thermodynamics: .

From the Mendeleev–Clapeyron equation it follows: .

Then the expressions for the first law of thermodynamics will take the form:

.

When moving from one state of aggregation in another, the total change in entropy consists of changes in individual processes:

A). Heating ice from temperature to melting point:

,Where - specific heat ice.

B). Melting Ice: ,Where - specific heat melting ice.

IN). Heating water from temperature to boiling point:

, where is the specific heat capacity of water.

G). Water evaporation: , where is the specific heat of vaporization of water.

Then the total entropy change is:

The principle of increasing entropy

Entropy of a closed system for any the processes occurring in it do not decrease:

or for the final process: , therefore: .

The equal sign refers to a reversible process, the inequality sign refers to an irreversible process. The last two formulas are the mathematical expression of the second law of thermodynamics. Thus, the introduction of the concept of “entropy” made it possible to strictly mathematically formulate the second law of thermodynamics.

Irreversible processes lead to the establishment of an equilibrium state. In this state, the entropy of the isolated system reaches its maximum. No macroscopic processes are possible in such a system.

The magnitude of the entropy change is a qualitative characteristic of the degree of irreversibility of the process.

The principle of increasing entropy applies to isolated systems. If the system is not isolated, then its entropy may decrease.

Conclusion: because Since all real processes are irreversible, then all processes in a closed system lead to an increase in its entropy.

Theoretical justification of the principle

Let's consider a closed system consisting of a heater, a refrigerator, a working fluid and a “consumer” of the work performed (a body that exchanges energy with the working fluid only in the form of work), performing a Carnot cycle. This is a reversible process, the change in entropy of which is equal to:

,

where is the change in entropy of the working fluid; – change in heater entropy; – change in the entropy of the refrigerator; – change in the entropy of the “consumer” of the work.

As is known, the first law of thermodynamics reflects the law of conservation of energy in thermodynamic processes, but it does not give an idea of ​​​​the direction of the processes. In addition, you can come up with many thermodynamic processes that will not contradict the first law, but in reality such processes do not exist. The existence of the second law (law) of thermodynamics is caused by the need to establish the possibility of a particular process. This law determines the direction of flow of thermodynamic processes. When formulating the second law of thermodynamics, they use the concepts of entropy and the Clausius inequality. In this case, the second law of thermodynamics is formulated as the law of growth of entropy of a closed system if the process is irreversible.

Statements of the second law of thermodynamics

If a process occurs in a closed system, then the entropy of this system does not decrease. In the form of a formula, the second law of thermodynamics is written as:

where S is entropy; L is the path along which the system moves from one state to another.

In this formulation of the second law of thermodynamics, attention should be paid to the fact that the system under consideration must be closed. In an open system, entropy can behave in any way (it can decrease, increase, or remain constant). Note that entropy does not change in a closed system during reversible processes.

An increase in entropy in a closed system during irreversible processes is a transition of a thermodynamic system from states with a lower probability to states with a higher probability. The famous Boltzmann formula gives a statistical interpretation of the second law of thermodynamics:

where k is Boltzmann's constant; w - thermodynamic probability (the number of ways in which the macrostate of the system under consideration can be realized). Thus, the second law of thermodynamics is a statistical law that is associated with the description of the patterns of thermal (chaotic) movement of molecules that make up a thermodynamic system.

Other formulations of the second law of thermodynamics

There are a number of other formulations of the second law of thermodynamics:

1) Kelvin's formulation: It is impossible to create a circular process, the result of which will be solely the conversion of the heat received from the heater into work. From this formulation of the second law of thermodynamics, they conclude that it is impossible to create a perpetual motion machine of the second kind. This means that periodically acting heat engine must have a heater, working fluid and refrigerator. In this case, the efficiency of an ideal heat engine cannot be greater than the efficiency of the Carnot cycle:

where is the heater temperature; — refrigerator temperature; ( title="Rendered by QuickLaTeX.com" height="15" width="65" style="vertical-align: -3px;">).!}

2) Clausius's formulation: It is impossible to create a circular process as a result of which only heat will be transferred from a body with a lower temperature to a body with a higher temperature.

The second law of thermodynamics notes the essential difference between the two forms of energy transfer (work and heat). From this law it follows that the transition of the ordered movement of the body as a whole into the chaotic movement of the molecules of the body and external environment- is an irreversible process. In this case, ordered movement can turn into chaotic without additional (compensatory) processes. Whereas the transition from disordered motion to ordered motion must be accompanied by a compensating process.

Examples of problem solving

EXAMPLE 1

Exercise

What is the essence of the “Heat Death of the Universe” problem? Why is this problem untenable?

Solution

This problem was formulated in the 19th century. If we consider the Universe a closed system and try to apply the second law of thermodynamics to it, then according to the Clausius hypothesis, the entropy of the Universe will reach a certain maximum. That is, after some time, all forms of motion will become thermal motion. All the heat from bodies with more high temperature will move on to bodies that have more low temperature, that is, the temperatures of all bodies in the Universe will become equal. The universe will come to a state thermal equilibrium, all processes will stop - this is called the thermal death of the Universe. Error this provision about the thermal death of the Universe lies in the fact that the second law of thermodynamics is not applicable to open systems, and the Universe should not be considered closed. Since it is limitless and consists of endless development.

EXAMPLE 2

Exercise

What is the efficiency of the cycle shown in Fig. 1? Consider that an ideal gas is involved in the process (the number of degrees of freedom is i) and its volume changes n times.

Solution

The efficiency of the cycle, which is presented in Fig. 1, is found as: where is the amount of heat that the working fluid receives from the heater in the presented cycle. In adiabatic processes there is no supply or removal of heat; it turns out that heat is supplied only in process 1-2. - the amount of heat that is removed from the gas in process 3-4. Using the first law of thermodynamics, we find the amount of heat received by the gas in process 1-2, which is isochoric: since there is no change in volume in this process. Let us define the change in the internal energy of the gas as: By analogy, for an isochoric process in which heat is removed, we have: Let us substitute the obtained result (2.2 - 2.5) into expression (2.1): We use the adiabatic equation to find temperature differences, and consider Fig. 1. For process 2-3 we write:

How is energy generated, how is it converted from one form to another, and what happens to energy in a closed system? The laws of thermodynamics will help answer all these questions. The second law of thermodynamics will be discussed in more detail today.

Laws in everyday life

Laws govern everyday life. Traffic laws say you must stop at stop signs. Government workers are required to provide a portion of their salaries to the state and federal government. Even scientific ones apply to everyday life. For example, the law of gravity predicts a pretty bad outcome for those trying to fly. Another set of scientific laws that affect everyday life are the laws of thermodynamics. So, a number of examples can be given to see how they affect everyday life.

First law of thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, but it can be converted from one form to another. This is also sometimes called the law of conservation of energy. So how does this relate to everyday life? Well, take for example the computer you are using now. It feeds on energy, but where does this energy come from? The first law of thermodynamics tells us that this energy couldn't come from out of thin air, so it came from somewhere.

You can track this energy. The computer is powered by electricity, but where does this electricity come from? That's right, from a power plant or hydroelectric station. If we consider the second, it will be associated with a dam that holds back the river. A river has a connection with kinetic energy, which means that the river flows. The dam converts this kinetic energy into potential energy.

How does a hydroelectric power plant work? Water is used to rotate the turbine. When the turbine rotates, a generator is driven, which will create electricity. This electricity can be carried entirely in wires from the power plant to your home so that when you plug the power cord into an electrical outlet, electricity flows into your computer so it can operate.

What happened here? There was already a certain amount of energy that was associated with the water in the river like kinetic energy. Then it turned into potential energy. The dam then took that potential energy and turned it into electricity, which could then go into your home and power your computer.

Second law of thermodynamics

By studying this law, one can understand how energy works and why everything moves towards possible chaos and disorder. The second law of thermodynamics is also called the law of entropy. Have you ever wondered how the Universe came into being? According to Theory Big Bang, before everything around was born, a huge amount of energy gathered together. After the Big Bang, the Universe appeared. All this is good, but what kind of energy was that? At the beginning of time, all the energy in the Universe was contained in one relatively small place. This intense concentration represented a huge amount of what is called potential energy. Over time, it spread across the vast expanse of our Universe.

On a much smaller scale, the reservoir of water held by a dam contains potential energy because its location allows it to flow through the dam. In each case, the stored energy, once released, spreads out and does so without any effort being exerted. In other words, the release of potential energy is a spontaneous process that occurs without the need for additional resources. As energy spreads, some of it is converted into useful energy and does some work. The rest is converted into unusable energy, simply called heat.

As the Universe continues to expand, it contains less and less useful energy. If a less useful one is available, less work can be done. As the water flows through the dam, it also contains less useful energy. This decrease in useful energy over time is called entropy, where entropy is the amount of unused energy in a system, and a system is simply the collection of objects that make up the whole.

Entropy can also be referred to as the amount of randomness or chaos in an organization without organization. As useful energy decreases over time, disorganization and chaos increase. Thus, as the accumulated potential energy is released, not all of it is converted into useful energy. All systems experience this increase in entropy over time. This is very important to understand, and this phenomenon is called the second law of thermodynamics.

Entropy: randomness or defect

As you may have guessed, the second law follows the first, which is commonly called the law of conservation of energy, and it states that energy cannot be created and cannot be destroyed. In other words, the amount of energy in the Universe or any system is constant. The second law of thermodynamics is commonly called the law of entropy, and it holds that as time passes, energy becomes less useful and its quality decreases over time. Entropy is the degree of randomness or defects a system has. If a system is very disordered, then it has high entropy. If there are many faults in the system, then the entropy is low.

Speaking in simple words, the second law of thermodynamics states that the entropy of a system cannot decrease over time. This means that in nature things move from a state of order to a state of disorder. And this is irreversible. The system will never become more orderly on its own. In other words, in nature the entropy of a system always increases. One way to think about this is your home. If you never clean and vacuum it, then pretty soon you will have a terrible mess. Entropy has increased! To reduce it, you need to use energy to use a vacuum cleaner and mop to clear the dust from the surface. The house won't clean itself.

What is the second law of thermodynamics? The formulation in simple words states that when energy changes from one form to another form, matter either moves freely or entropy (disorder) in a closed system increases. Differences in temperature, pressure and density tend to level out horizontally after a while. Due to gravity, density and pressure do not equalize vertically. The density and pressure at the bottom will be greater than at the top. Entropy is a measure of the spread of matter and energy wherever it has access. The most common formulation of the second law of thermodynamics is mainly associated with Rudolf Clausius, who said:

It is impossible to construct a device that produces no other effect than the transfer of heat from a body of lower temperature to a body of higher temperature.

In other words, everything tries to maintain the same temperature over time. There are many formulations of the second law of thermodynamics that use different terms, but they all mean the same thing. Another Clausius statement:

Heat by itself does not occur from a colder body to a hotter one.

The second law applies only to large systems. It concerns the likely behavior of a system in which there is no energy or matter. The larger the system, the more likely the second law is.

Another wording of the law:

Total entropy always increases in a spontaneous process.

The increase in entropy ΔS during the process must exceed or be equal to the ratio of the amount of heat Q transferred to the system to the temperature T at which the heat is transferred.

Thermodynamic system

IN in a general sense The statement of the second law of thermodynamics in simple terms states that temperature differences between systems in contact with each other tend to equalize and that work can be obtained from these nonequilibrium differences. But at the same time, thermal energy is lost, and entropy increases. Differences in pressure, density and temperature tend to equalize if given the chance; Density and pressure, but not temperature, depend on gravity. A heat engine is a mechanical device that provides useful work due to the difference in temperature of the two bodies.

A thermodynamic system is one that interacts and exchanges energy with the region around it. Exchange and transfer must occur in at least two ways. One way must be heat transfer. If a thermodynamic system is "in equilibrium", it cannot change its state or status without interacting with its environment. Simply put, if you are in equilibrium, you are a "happy system", there is nothing you can do. If you want to do something, you must interact with the world around you.

Second law of thermodynamics: irreversibility of processes

It is impossible to have a cyclic (repeating) process that completely converts heat into work. It is also impossible to have a process that transfers heat from cold objects to warm objects without using work. Some energy in a reaction is always lost to heating. In addition, the system cannot convert all its energy into work energy. The second part of the law is more obvious.

A cold body cannot heat a warm body. Warm naturally tends to flow from warmer to cooler areas. If heat moves from cooler to warmer temperatures, it goes against what is "natural" so the system has to do some work to make it happen. in nature - the second law of thermodynamics. This is perhaps the most famous (at least among scientists) and important law in all of science. One of his formulations:

The entropy of the Universe tends to its maximum.

In other words, entropy either remains the same or becomes greater; the entropy of the universe can never decrease. The problem is that this is always true. If you take a bottle of perfume and spray it in a room, soon fragrant atoms will fill the entire space, and this process is irreversible.

Relationships in thermodynamics

The laws of thermodynamics describe the relationships between thermal energy, or heat, and other forms of energy, and how energy affects matter. The first law of thermodynamics states that energy cannot be created or destroyed; total quantity energy in the Universe remains unchanged. The second law of thermodynamics deals with the quality of energy. It states that as energy is transferred or converted, more and more useful energy is lost. The Second Law also states that there is a natural tendency for any isolated system to become more disordered.

Even when the order increases in certain place when you take into account the entire system, including environment, there is always an increase in entropy. In another example, crystals may form from a salt solution when water is evaporated. Crystals are more ordered than salt molecules in solution; however, evaporated water is much more messy than liquid water. The process taken as a whole results in a net increase in disorder.

Work and Energy

The second law explains that it is impossible to convert thermal energy into mechanical energy with 100 percent efficiency. You can give an example with a car. After the process of heating the gas to increase its pressure to drive the piston, there is always some heat left in the gas, which cannot be used to perform any additional work. This waste heat must be rejected by transferring it to the radiator. In the case of a car engine, this is done by extracting the spent fuel and air mixture into the atmosphere.

Additionally, any device with moving parts creates friction, which converts mechanical energy into heat, which is usually unusable and must be removed from the system by transferring it to a heat sink. When a hot and cold body come into contact with each other, thermal energy will flow from a hot body to a cold body until they reach thermal equilibrium. However, the heat will never return the other way; the temperature difference between two bodies will never increase spontaneously. Moving heat from a cold body to a hot body requires work, which must be performed by an external energy source such as a heat pump.

Fate of the Universe

The Second Law also predicts the end of the universe. This is the ultimate level of disorder, if there is constant thermal equilibrium everywhere, no work can be done and all energy will end up as random motion of atoms and molecules. According to modern data, the Metagalaxy is an expanding non-stationary system; there can be no talk of the thermal death of the Universe. Heat death is a state of thermal equilibrium in which all processes cease.

This position is erroneous, since the second law of thermodynamics applies only to closed systems. And the Universe, as we know, is limitless. However, the term “heat death of the Universe” itself is sometimes used to designate a scenario for the future development of the Universe, according to which it will continue to expand indefinitely into the darkness of space until it turns into scattered cold dust.