Second Law of Thermodynamics

Thermodynamics is a very important sub-discipline of engineering. We are dealing with thermodynamics to improve and develop new technologies that make our lives much easier. Also, this area has laws that define the general boundaries. One of these laws is the second law of thermodynamics. Here, you can find detailed information about it.

What is the Second Law of Thermodynamics?

The second law of thermodynamics.
Image Source: Wikimedia.

Fİrstly, the first law of thermodynamics deals with the quantity of energy. So, we can not create or destroy the energy completely. Energy must be conserved and changed one to another. And you can understand that the direction of the energy transformation is not the main business of the First law.

But also, the direction of the energy is very important. In nature and space, there is always a direction for energy transformation. we can not change this direction. You can understand it with examples.

For example, we are heating a room with a heater. The temperature of the heater is much higher than the environment. And heat energy flow takes place from the heater to the environment. So, the electrical energy of the heater is converted into the temperature in the room.

But we can not achieve the reverse of this process. Can you think about, the amount of heat in the environment can be reversed to the heater as electricity? No, because the temperature of the heater is higher. And heat flow takes place from higher temperatures to lower temperatures. This directional rule is about the Second Law of Thermodynamics.

The second law of thermodynamics states that the energy flows every time in a direction. This direction is always from high energy sources to low energy sources.

Quality of Energy

Also, the second law of thermodynamics is about the quality of energy. The first law deals with only the quantity. And this quality is about the enthalpy of nature. So, in the second law of thermodynamics, we deal with the energy flow limitations of different devices.

According to the second law of thermodynamics, there is always a degree of completion of thermodynamical processes.

Thermodynamical Cycles

We generally assume the general practical applications such as steam energy plants and refrigerators as devices that are working in thermodynamical cycles. In a thermodynamical cycle, there is an always working fluid. This working fluid takes energy from an energy source and converts this energy to other useful forms in different devices. And then, rejects the remaining energy outside as unusable energy.

So, these are the general cycle of a thermodynamic device. But to understand the other statements of thermodynamics, we can assume that there is a heat engine. A heat engine takes the heat energy and produces mechanical work such as a rotating shaft or propeller. And rejects the unusable remainder energy to the outside. So, this heat engine works according to thermodynamical cycles.

Other Statements of Second Law of Thermodynamics

The second law of thermodynamics is a very important phenomenon that we generally use in practical applications. Because it states the general limitations of the efficiencies and working efficacies of the working devices. We will explain the general working conditions of these devices.

These devices generally work in a thermodynamical cycle. And in these thermodynamical cycles, there are lots of work inputs and outputs and energy inputs and outputs. So, we can easily apply the first and second laws of thermodynamics to these devices to see the general energy efficiencies.

But first of all, we need to understand other statements of the second law of thermodynamics to completely understand the general working principles.

Kelvin-Plank Statement

Generally, all the statements are derived from the same fact of thermodynamics. But, these different statements clarify the general working conditions of devices. So, we need to learn about them.

According to the Kelvin-Plank statement of the second law of thermodynamics, there must be always a heat rejection from thermodynamical devices. No thermodynamic cycle devices can convert all the heat energy to the net work. In other words, no device works with 100% efficiency.

Classius Statement

This is the second important statement of the second law of thermodynamics. According to this statement, we can not achieve a heat transfer or heat energy transfer from lower temperature bodies to higher temperature bodies. We stated in the first section of this article about this statement.

This is a very important restriction of the energy flow from higher temperatures to lower temperatures. And, all the devices must work in this way.

Reversible and Irreversible Processes

These statements and explanations are also very important to understanding the second law of thermodynamics. We stated that a thermodynamical machine can not convert all the energy inputs to the net work. So, there are no machines or devices that work with 100% of efficiency. Because we define the efficiency of machines by the ratio of total heat input to total work output.

But also, there is a limitation to the maximum attainable efficiency. We call this maximum efficiency value as second law efficiency. 

To understand the difference between reversible and irreversible processes, we need to think about heat engines. In a heat engine, ideally, the summation of the work output and heat rejection is equal to the energy input. There are no irreversibilities to surroundings such as heat leakages or other types of irreversible interactions. So, these processes are reversible. 

But in nature and space, there are no reversible processes. All the processes are irreversible. So, in thermodynamical cycles, there must be irreversibilities. We can understand that the net work output plus heat rejection is not equal to the total energy input. There are irreversibilities in these thermodynamical systems. This is another statement of the second law of thermodynamics. There are always irreversibilities.

So, if we consider the irreversibilities, the efficiency drops more. We need to consider the total energy that goes to the environment as irreversibilities and heat rejection in the calculation of the efficiency. And 0 irreversibilities give the maximum efficiency value that we can achieve with that device.

Types of Irreversibilities in Thermodynamical Systems

According to the second law of thermodynamics, there must always be irreversibilities in practical systems. This is the general nature of objects and matter. The general reasons for these irreversibilities;

  • Friction
  • Gas expansion
  • Unintended heat transfer across finite temperature differences
  • Inelastic deformation of solids
  • Unintended chemical reactions


Friction is the most common cause of the irreversibilities of thermodynamic cycles. Between the contacting materials, there are always frictional effects. To overcome these frictional effects, the system must expend an amount of energy. And this frictional energy is irreversible. The energy is dissipated to the environment.

The most important example of friction is, the drag force that cars or airplanes are exposed to. This is a very important frictional effect that applies a reverse force in the motion directions of cars. So, the engine must produce extra power and expense extra energy to overcome this friction effect.

Gas Expansion

Gas expansion.
Gas expansion(Image Source:

Expansion and leakages for gases are other important examples of irreversibilities. When you compress a gas inside a cylinder or container, the gas becomes more dense and hot. But with time, the gas will release this temperature and lose the energy stored in it. This energy that dissipated unintended way into the environment is an irreversible process.

Also, the gas leakages around the mechanical systems such as compressors or other gas transmission systems are irreversible energy losses.

The most important applications are compressors. In compressors, the pressure and the internal energy of gases increases. And lots of irreversibilities take place. The gas will lose energy through the walls of the compressor. Also from the different points of the compressor vanes, there are possible and minimal gas leakages that will lead to irreversibilities.

Unintended Heat Transfer

We mentioned the unintended heat transfers in the gas expansion. In lots of kinds of thermodynamical cycles, we use heat as an energy source. And the temperature rises in different components of the systems. And also there is always a temperature difference between the environment and the system sections.

So, if there is a temperature difference, heat transfer and heat energy transfer take place. And we can understand from it, that we can not avoid the heat transfer between the environment and the system. And there is no 100% insulation.

We can give an example of internal combustion engines. In a normal working condition of it, you can observe that the engine is very hot. So, there is a heat transfer between the engine and the surroundings. This is the irreversible and unintended heat loss from the engine. And the source of this heat is the combustion of gases.

Inelastic Deformation of Solids

Think about a beverage can. If you squeeze it enough, it gets deformed. And the energy that you spent to compress is stored as deformation energy in that can. And you can not reverse this system by taking the stored energy in that can to take back the energy that you spend. So, inelastic deformations or plastic deformations are irreversible processes.

In lots of systems, we can see the plastic deformation’s irreversibilities. For example, think about a machine that takes a garbage heap from a point to the damper of the truck. So, it takes all the garbage and there are lots of plastic deformations that occur on the parts of this garbage. And this is the intended work expense of the machine. The only purpose of this machine is to put the garbage inside the damper.

Unintended Chemical Reactions

There are also lots of unintended chemical reactions that take place in lots of the thermodynamical processes. These reactions are not taking place with 100% efficiency. And also, nearly all the chemical reactions take place to obtain a stable situation. So, chemical reactions are irreversible processes that we can not invert back.

In industry, there are lots of chemical reactions and chemical processes that take place to obtain work or different systems. And also, these chemical reactions are generally irreversible.

Internal and External Irreversibilities

Also, we can classify the irreversibilities as internal and external irreversibilities. We call the irreversibilities internal if they take place within the boundaries of the system. Also, if the irreversibilities take place outside of the boundaries of the system, we call them external irreversibilities.

For example, heat transfer between the system and the surrounding boundaries is the external irreversibilities. And also, the friction between the piston systems is the internal irreversibilities.

Systems that We Use Second Law of Thermodynamics

There are lots of practical systems in which we generally use the second law of thermodynamics. Around these systems, are refrigerators, heat pumps, and steam energy plants.

Steam Power Plants

The first law of thermodynamics is very important for steam power plants.
Steam power plant(Image Source:

These plants are a very common example of thermodynamical cycles. If we take a look at the working principles of these power plants, there is a boiler that consumes energy to obtain steam to generate power.

This very high temperature of steam wents into the turbine system where we obtain the total work output. And this steam reduces into lower energy situation which we can not produce useful work from it. And also, in the condenser, this remainder of the energy is dissipated and we obtain water again. With this dissipated energy, we can not produce any positive work. Because the quality of energy is not good to use. So, this is about the second law of thermodynamics.

And there is a pump system that pumps the liquid in this cycle. So, the steam and the liquid can circulates in the system. And there is a net work input to that pump system.

From the thermodynamical viewpoint, we can calculate the efficiency of this steam power plant as;

Efficiency formula.

According to this equation, if the output works energy of the turbine increases, the efficiency increases. And also, if the work input in the pump and the heat energy input in the boiler increase, the efficiency decreases.

Also, we need to state that, there is an energy rejection in the condenser. So, we can not obtain 100% efficiency where the turbine output energy is equal to the Wpump+Qboiler.

And also, there are irreversibilities in the fluid circulation and the environment. So, the second law efficiency will be much lower.

As you understand, heat energy rejection and irreversibilities are related to the second law of thermodynamics. We need to consider these important parameters in the efficiency calculations for steam power plants.

Second Law of Thermodynamics in Refrigerators

Refrigerators are also very important thermodynamical devices. There is a thermodynamical cycle just as at the steam power plants. But the purpose of these devices is different. And, the efficiency and performance calculations for refrigerators are different from the steam power plants. Furthermore, the second law of thermodynamics is in work for these devices.

If we take a look at the general working principles of refrigerators, there is a compressor that increases the pressure of the liquid. So, there is no work input in this compressor. And the temperature and the pressure of the liquid increase.

After the compressor, the fluid gets into the condenser where the condensation of the fluid takes place. So, fluid in the gas state becomes liquid in the condenser. The pressure does not change and the temperature slightly decreases in the condenser. And there is a heat rejection that takes place.

After the condenser, there is an expansion valve where the fluid expands, and the pressure and temperature of the fluid decrease. And fluid becomes a liquid in here. There is no energy input and output in the expansion valve. So the total energy of the fluid does not change. Only the phase change occurs because of the sudden expansion.

And after the expansion valve, fluid in the liquid state enters the evaporator. In an evaporator, evaporation of the liquid takes place. So, for this evaporation, the fluid uses heat energy inside the refrigerated environment. So, this energy takes is an important parameter for us.

The fluid comes into the compressor again. This is the one cycle of refrigeration.

Coefficient of Performance

We calculate the efficiencies or performance of refrigerators with Coefficient of Performance which is very important to us.

In COP calculations for refrigerators, the desired output is the total heat energy taken from the refrigerated environment. And the total energy expenditure is the work input in the compressor. According to the conservation of energy, the net work input in the compressor is equal to the difference between rejected heat and the taken heat. So;

COP of refrigerators.

According to the second law of thermodynamics, the heat rejection in the condenser must take place. Because this heat is a low quality of energy we can not obtain useful work from it with the conventional methods.

Also, there will be irreversibilities according to the second law of thermodynamics, where the losses of energy will take place. For example, the irreversibilities of compressor and heat loss from it.

Second Law of Thermodynamics in Heat Pump Systems

The working principles of refrigerators and heat pumps are completely the same. But the purposes are different. The purpose of the refrigeration systems, holding cool the environment. But in heat pump systems, we take the heat from the cold environment and reject it to the hot environment to hold the hot environment at a specific temperature.

So the COP calculations for heat pumps will be like this;

COP of heat pumps. The second law of thermodynamics is very important for them.

As you understand, we are making work against the second law of thermodynamics by taking heat from cold environments and rejecting this heat in hot environments. This is against the direction of energy. So, we need to spend much more energy doing this kind of thing. This means we are not violating the second law. Because we create more entropy by expensing much more energy to complete these processes.


As you understand from above, the second law of thermodynamics is very important. So, to explain the energy conservations in nature and the practical engineering systems, the first law of thermodynamics are not quite efficient.

Also, there are different sub-statements of the second law of thermodynamics. These statements are the complementary statements to completely understand this second law.

Furthermore, we gave more important examples to understand the second law. Heat pumps, air conditioners, refrigerators, and steam power plants are the most important practical engineering applications to understand the second law of thermodynamics.

And also, it is very important to understand the irreversibilities in nature and the system. This defines the maximum efficiencies that we can take from the systems.

Do not forget to leave your comments and questions below about the second law of thermodynamics.

Your precious feedbacks are very important to us.

FAQs About Second Law of Thermodynamics

What does the 2nd law of thermodynamics state? 

Above the first law, the second law states that the direction of the energy transformation is defined from a high energy state to a low energy state. So, you can not see any natural heat energy transfer from colder environments to hotter environments.

What is the 2nd law of thermodynamics and give an example?

This law states that there is a quality of energy. From low-quality energy sources, we can not produce any useful work. But from high-quality energy sources, we can produce work and low-quality energy rejection. For example in refrigeration systems, we have an energy input and heat rejection from a cold environment. But also we reject heat in condensers. So this rejected heat energy is low-quality energy that we can not produce any useful work from it.

Why is the 2nd law of thermodynamics so important?

This is very important because it shows us the general limits of thermodynamical systems. And give motivation to engineers and people to obtain more efficient systems. The second law of thermodynamics state that you can produce a specific amount of useful work and you need to reject a specific amount of heat in these processes. So, it is very important that shows us the limitations that we have.





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